DOI: 10.1039/B701534H
(Review Article)
Nat. Prod. Rep., 2008, 25, 35-94
Marine natural products
John W.
Blunt
*a,
Brent R.
Copp
b,
Wan-Ping
Hu
a,
Murray H. G.
Munro
a,
Peter T.
Northcote
c and
Michèle R.
Prinsep
d
aDepartment of Chemistry, University of Canterbury, Christchurch, New Zealand. E-mail: john.blunt@canterbury.ac.nz
bDepartment of Chemistry, University of Auckland, Auckland, New Zealand
cSchool of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand
dDepartment of Chemistry, University of Waikato, Hamilton, New Zealand
Received
(in Cambridge, UK)
26th October 2007
First published on 3rd January 2008
Abstract
Covering: 2006. Previous review: Nat. Prod. Rep., 2007, 24, 31
This review covers the literature published in 2006 for marine natural products, with 758 citations (534 for the period January to December 2006) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidaria, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (779 for 2006), together with their relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.
 John W. Blunt | John Blunt obtained his BSc (Hons) and PhD degrees from the University of Canterbury, followed by postdoctoral appointments in Biochemistry at the University of Wisconsin–Madison, and with Sir Ewart Jones at Oxford University. He took up a lectureship at the University of Canterbury in 1970, where he is now a Professor. His research interests are with natural products, and the application of NMR techniques to structural problems. |
 Brent R. Copp | Brent Copp received his BSc (Hons) and PhD degrees from the University of Canterbury, where he studied the isolation, structure elucidation and structure–activity relationships of biologically active marine natural products under the guidance of Professors Blunt and Munro. He undertook postdoctoral research with Jon Clardy at Cornell and Chris Ireland at the University of Utah. 1992–93 was spent working in industry as an isolation chemist with Xenova Plc, before returning to New Zealand to take a lectureship at the University of Auckland, where he is currently an Associate Professor. |
 Wan-Ping Hu | Wan-Ping Hu received her BSc (Hons) and PhD degrees from the University of Canterbury, where she studied molecular beams with Professor Peter Harland. She carried out postdoctoral research with Professor Stephen Price at University College London before returning to New Zealand, where she is currently a research associate at the University of Canterbury working on breath-testing projects using SIFT-MS for clinical studies, in addition to maintenance of the MarinLit database. |
 Murray H. G. Munro | Murray Munro, University of Canterbury, Christchurch, New Zealand, has worked on natural products, mainly of New Zealand origin, right through his career. This started with diterpenoids (PhD), followed by alkaloids during a postdoctoral spell with Alan Battersby at Liverpool. Following a sabbatical with Ken Rinehart at the University of Illinois in 1973, an interest in marine natural products developed with a particular focus on bioactive compounds. In recent years his research interests have widened to include terrestrial and marine fungi and actinomycetes as well as marine invertebrates. |
 Peter T. Northcote | Peter Northcote received his BSc and PhD degrees from the University of British Columbia, Canada, where he was a member of R. J. Andersen's marine natural products research group. He carried out postdoctoral research with Professors Blunt and Munro at the University of Canterbury before taking a position as a senior research scientist at Lederle Laboratories, American Cyanamid Co. He joined the faculty of the Victoria University of Wellington in 1994 where he is currently an Associate Professor in organic chemistry. |
 Michèle R. Prinsep | Michèle Prinsep received her BSc (Hons) and PhD degrees from the University of Canterbury, where she studied the isolation and structural elucidation of biologically active secondary metabolites from sponges and bryozoans under the supervision of Professors Blunt and Munro. She undertook postdoctoral research on cyanobacteria with Richard Moore at the University of Hawaii before returning to New Zealand to take up a lectureship at the University of Waikato, where she is currently a Senior Lecturer. |
1 Introduction
This review is of the literature for 2006 and describes 779 new compounds from 283 articles, a decrease of ∼4% from the number of compounds reported for 2005. As in previous reviews, the structures are shown only for new compounds, or for previously reported compounds where there has been a structural revision or a newly established stereochemistry. Previously reported compounds for which first syntheses or new bioactivities are described are referenced, but separate structures are generally not shown. No reference is made in the text to the establishment of absolute stereochemistry when standard methods (e.g. Mosher, Marfey, X-ray crystallography) have been used, unless there are some unusual features associated with the use of these techniques. However, where such determinations have been made for a compound the identifying diagram number is distinguished by addition of a † symbol. Stereochemistries shown for compounds not labelled with † should be assumed to be relative.It is with great sadness that we note the passing of Professor Richard E. Moore, University of Hawaii, another pioneer in the field of marine natural products, who died 11 December, 2007. Professor Moore will long be remembered for his momentous work on the structure of palytoxin and his extensive investigations on cyanobacteria (blue-green algae) as sources of potential anticancer compounds such as the cryptophycins.
2 Reviews
Two general annual reviews of marine natural products were published, one covering selected papers from 2005,1 the other a comprehensive coverage of the 2004 literature.2 The continued and growing interest in the development of marine natural products as drug candidates has resulted in numerous reviews.3–9 The association of marine natural products with specific types of bioactivity is reviewed in papers on antiplasmodial,10 biofouling,11,12 tumour-promoting,13 antileukaemic,14 anti-inflammatory,15 antitubercular,16 anticancer,17 and antitumour18 activities. Several reviews cover the chemistry and bioactivities of compounds from various organism types including Indian marine algae,19 South China Sea invertebrates,20 bacterial symbionts,21 and marine organism-associated microbes,22 with additional references noted in the appropriate sections that follow. Specific classes of compounds from marine organisms have been reviewed for spongian diterpenoids,23 sesquiterpene hydroquinones,24 heterocycles from cyanobacteria,25 indolocarbazoles,26 astaxanthin,27 bioactive sulfur-containing compounds,28 invertebrate pigments,29 imidazole, oxazole and thiazole alkaloids,30 polyether compounds,31 dysiherbaine,32 terpenoids,33 trisoxazole macrolides,34 bioactive heterocyclic alkaloids,35 diterpenoids,36 and sesquiterpenoids.37 Marine toxins from diverse sources such as cnidaria,38 Conus spp.39,40 and microorganisms41–45 have been reviewed, with additional references noted in the appropriate sections that follow. There has been a review on biosynthetic studies in macroorganisms.46 The second in a companion series providing a broad review of synthetic aspects of marine natural products, covering publications in 2004, has appeared.47 More specific reviews that appeared in 2006 relating to the synthesis of marine natural products will be referenced in the fourth of this broad review series. Aspects of methodology relating to the structure determination48 and aquacultural production49,50 of marine natural products have been reviewed. Nutraceuticals from marine processing byproducts,51 selected studies on marine natural products,52 and marine chemical ecology investigations have also been reviewed.53,54 The MarinLit database55 continues to be updated and has again been used as a literature source for the preparation of this present review.3 Marine microorganisms and phytoplankton
Interest in marine microorganisms continues to grow, with sediment becoming an increasingly popular source of bacteria and macroorganisms a consistent source for fungi. Cultivation of the actinomycete “Marinispora” yielded the macrodiolide antibiotics marinomycins A–D 1–4. The all-(E) isomer marinomycin A 1 photoisomerised slowly to marinomycins B 2 and C 3. In the absence of light, marinomycin A was the predominant metabolite isolated with only small quantities of marinomycins B and C, suggesting that 1 only may be the true natural product, but this could not be proven rigorously. Given the propensity of 1 to photoisomerise, it was postulated that an all-(E) isomer of 4 was possibly the true natural product in that series as well. Despite rigorous determination of absolute stereochemistry of the marinomycins, the optical rotation of marinomycin A was opposite to that of marinomycins B–D, which led the authors to conclude that in marinomycin A 1 the all-(E) geometry of the tetraene led to transannular hydrogen bonding, resulting in a configuration which could facilitate olefinic exciton coupling. All marinomycins exhibited antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), and marinomycin A 1 was also active against vancomycin-resistant Streptococcus faecium (VREF) and Candida albicans (weakly). Marinomycins A–C 1–3 exhibited potent and highly selective activity in the National Cancer Institute's (NCI) 60 tumour cell line panel with six of the eight melanoma cell lines, especially SK-MEL-5, being very sensitive to marinomycin A.56 Photoisomerisation of marinomycin A was utilised in a total synthesis of marinomycins A–C.57 Cyanosporasides A 5 and B 6 are chlorinated cyclopenta[a]indene glycosides, possessing a new 3-ketopyranohexose sugar, and were isolated from the obligate actinomycete “Salinispora pacifica” from sediment (Palau).58 Culture of a new strain of Halomonas sp. isolated from seawater (East Frisian Wadden Sea, Germany) yielded two new hydroxyphenylpyrrole-2,5-dicarboxylic acids, 7 and 8, with syntheses of both compounds completed.59Both compounds were effective tumour promoters, inhibiting 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of Epstein–Barr virus early antigen.59 The same strain of Halomonas, when cultured with the addition of anthranilic acid to the medium, produced a different secondary metabolite pattern including the new 2-aminophenoxazin-3-one derivatives 9–11. Of these, 9 and 10 were growth inhibitors of Gram-positive bacteria and moderately cytotoxic against human tumour cell lines.60 A new marine bacterium related to Streptomyces, isolated from sediment (Guam), was the source of the actinofuranones A 12 and B 13, isolated as C-2 epimeric mixtures.61
Azamerone 14, a novel meroterpenoid, which contains an unprecedented chloropyranophthalazinone core, confirmed by X-ray analysis, was isolated from culture of the same bacterium.62 Four amides, daryamides A–C 15–17 and (2E,4E)-7-methylocta-2,4-dienoic acid amide 18, were isolated from culture of a Streptomyces sp. obtained from marine sediment (San Diego, California).
Compounds 15–18 were weak to moderately cytotoxic to HCT-116 and weakly active against Candida albicans.63 A staurosporine 19 and a sesquiterpene 20 were isolated from the culture broth of a Streptomyces sp. derived from sediment (Jiaozhou Bay, China) together with a new alkaloid, 5,7-dihydroxy-5,6,7,8-tetrahydro-1H-azocin-2-one 21.64,65
Staurosporine 19 was selectively active against a number of solid tumour derived human tumour cell lines and weakly active against Streptomyces viridochromogenes.64 A degraded sesquiterpene 22 was isolated from a Streptomyces sp. (source not known).66 Sediments were the source of several Streptomyces spp. producing new metabolites. The pyrrolizidine alkaloids bohemamine B 23, bohemamine C 24 and 5-chlorobohemamine C 25 were isolated from culture of Streptomyces sp. from sediment (Guam),67 and urauchimycin C 26 and urauchimycin D 27 were isolated from culture of a Streptomyces strain from sediment (Shetland Islands).68
A deep-sea sediment (Ayu Trough, western Pacific Ocean) Streptomyces sp. yielded the cytotoxic streptokordin 28,69 while anthraquinones 29 and 30, which displayed moderate activity against Staphylococcus aureus and Streptomyces viridochromogenes, came from culture of a Streptomyces strain separated from sediment (Laguna de Terminos, Gulf of Mexico).70 The marine plant Salicornia herbacea (Qingdao, China) was the source of two other Streptomyces spp. which produced the same chromone 31 seen from a sediment sample, and two anthraquinones 3071 and 32.
Anthraquinone 32 was moderately active against HL-60, BCTC-823 and MDA-MB-435 cell lines.72 Culture of Roseivirga echinicomitans, separated from the gonads of Strongylocentrotus intermedius (sea urchin; Troitsa Bay, Sea of Japan), yielded N,N-diacetyltryptamine 33, which was cytotoxic to mammalian cell lines, sperm and egg cells of S. intermedius, and was synthesised from tryptamine.73 Culture of a Streptomyces sp. isolated from a cyanobacterium associated with the ascidian Ecteinascidia turbinata (La Parguera, Puerto Rico) led first to the isolation of two bisanthraquinone compounds 34 and 35, and then 36 and 37.
Both 34 and 35 were potent inhibitors of MRSA, but less potent against vancomycin-resistant Enterococcus faecalis (VCE).74 The planar structure of compound 34 matched that of compounds previously reported from a Streptomyces sp. in a Japanese patent,75 but no stereochemical details had been given.76 The structure of vanchrobactin 38, a siderophore produced by the fish pathogen Vibrio anguillarum,77 was not originally determined but has now been re-isolated from iron-deficient cultures and characterised as N-[N′-(2,3-dihydroxybenzoyl)arginyl]serine.78 Marine actinomycete bacteria as a new resource for drug discovery have been reviewed,79 and there have been two reviews on metabolites from marine-derived fungi.80,81 The mycelium extract of a cultured unidentified endophytic fungus from the brown alga Sargassum sp. (Zhanjiang Sea, China) yielded two 12-membered ring lactones, 39 and 40.
Three known compounds, lasiodiplodin82 and two derivatives,83 were co-isolated from the fungus and exhibited varying levels of antibacterial activity.84 Myrothecium species were the sources for four macrocyclic trichothecenes: roridin R 41 from an unidentified marine sponge (Manado, Indonesia), while Myrothecium roridum from submerged wood in Palau was the source of 12′-hydroxyroridin E 42, roridin Q 43 and 2′,3′-deoxyroritoxin D 44.
The stereochemistries of 41 and 44 were only partially determined. All but 44 were cytotoxic to L1210 cells, while 44 was active against Saccharomyces cerevisiae.85 4,5-Di(tridecyl)octanedioic acid 45 came from a Myrothecium sp. isolated from the spermary of Argyrosomus argentatus (white croaker fish; Yellow Sea near Lvsi Port, China).86 Culture of Acremonium sp. from sea mud (Awajishima Is., Japan) produced awajanomycin 46, an inhibitor of the human lung adenocarcinoma cell line A549,87 and the dihydrobenzofuran derivative 47, which also inhibited growth of A549 cells in addition to having moderate antibacterial and antifungal activity.88
Two highly N-methylated linear octapeptides 48 and 49, weakly cytotoxic to L1210 cells, were isolated from culture of Acremonium sp. isolated from a Teichaxinella sponge (Papua New Guinea). Octapeptide 48 was also active against Staphylococcus epidermidis.89 Efrapeptin G,90 a well-known terrestrial antibacterial metabolite,91 was isolated from the same culture and is the first report of efrapeptin G from a marine source. Three diterpene glycosides, virescenosides V–X 50–52, were isolated from a culture of Acremonium striatisporum isolated from Eupentacta fraudatrix92 (holothurian; Kitovoe Rebro Bay, Sea of Japan).93 Four meroterpenoids, tropolactones A–D 53–56, were isolated from an Aspergillus sp. culture originating from an unidentified sponge (Manele Bay, Hawaii), with 53–55 being weakly cytotoxic to HCT-116 cells.94
The endophytic fungus Aspergillus sydowii has been isolated from the red alga Acanthophora spicifera (Bay of Bengal, India) and produced two new cyclopentanoids, sydowins A 57 and B 58.95 From another Aspergillus sp., isolated from sediment (Behai Bay, China), dimethyl 2,3′-dimethylosoate 59 was characterised and was cytotoxic to the human chronic myelogenous leukaemia cell line K5672 through S-phase blockage and apoptosis.96 The moderately cytotoxic (A375-S2 melanoma cell line) cyclotetrapeptide trichoderide A 60 was isolated from culture of Trichoderma reesei separated from sea mud (Lianyungang, China).97 A series of short peptaibols, the trichobrachins A I–IV 61–64 and trichobrachins B I–IV 65–68, each comprising eleven amino acid residues, were isolated from culture of Trichoderma longibrachiatum isolated from the shellfish Mytilus edulis (Tharon, France).98
Cyclic depsipeptides, guangomides A 69 and B 70 and a destruxin derivative, homodescartin 71, were isolated from culture of an unidentified fungus derived from the sponge Ianthella sp. (Guango, Papua New Guinea). Guangomides A and B displayed weak antibacterial activity against Staphylococcus epidermidis and Enterococcus durans.99 Two new destruxins, [β-Me-Pro] destruxin E chlorohydrin 72 and pseudodestruxin C 73 were isolated from culture of Beauveria felina obtained from a Caulerpa species (São Paulo, Brazil).
A number of known cyclic depsipeptides were also isolated, of which roseotoxin B100 was moderately cytotoxic to several human tumour cell lines.101 Bioactivity/HPLC/microtitre plate profiling led to the isolation of the cyclodepsipeptide gliotide 74, along with the known compounds 4-ketoclonostachydiol 75102 and clonostachydiol,103 from a culture of Gliocladium sp., obtained from the alga Durvillaea antarctica (Tauranga Bay, New Zealand). Clonostachydiol was modestly active against the P388 cell line, and the absolute configuration of 75 was determined.104 The filamentous fungus Clonostachys sp., isolated from an unidentified Japanese sponge, was the source of the cyclodepsipeptide 76, which had potent activity against several human tumour cell lines including LN-caP (prostrate), SK-BR3 (breast), HT29 (colon) and HELA (cervix) cell lines.105
A solid-phase synthesis of 76 was completed.106 A cyclic pentadepsipeptide, zygosporamide 77, from culture of Zygosporium masonii separated from a cyanobacterium (Maui, Hawaii), exhibited highly selective cytotoxicity in the NCI cell line assay, inhibiting the SF-268 cell line (CNS) and the RXF 393 cell line (renal) cancer by at least three orders of magnitude more than the mean value.107 Culture of Spicaria elegans from marine sediment (Jiaozhou Bay, China) gave the modestly active cytochalasins Z7–Z9 78–80.
The absolute configuration of cytochalasin Z9 80 was assigned on biogenetic grounds through comparison with cytochalasins Z7 78 and cytochalasin Z8 79 and an X-ray analysis of 78.108 From a culture of Microsporum sp., isolated from the red alga Lomentaria catenata (Ulsan City, Korea), the anthracene glycoside, asperflavin ribofuranoside 81, was characterised. Asperflavin ribofuranoside 81 exhibited radical scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and moderate activity against MRSA and multidrug-resistant Staphylococcus aureus (MDRSA).109 Culture of an endophytic fungus isolated from an estuarine mangrove (South China Sea coast) produced the carboxylic acid 82.110 A cytotoxic (human A375-S2 and HELA cell lines) isocoumarin 83 was isolated from the culture broth of Alternaria tenuis isolated from an alga (Zhoushan Is., China) and the stereochemistry determined for all but one stereocentre.111
Culture of the endophytic Emericella nidulans isolated from a green alga (Sardinia) produced the prenylated polyketides arugosins G 84 and H 85. Arugosin H 85 was active against the fungus Mycotypha microspora and the green alga Chlorella fusca, while sterigmatocystin,112 a co-occuring metabolite, was reported as antialgal for the first time.113 Culture of Chaetomium globosum, an endophytic fungus from the red alga Polysiphonia urceolata, gave chaetopyranin 86, which was modestly active against several human tumour cell lines in addition to having DPPH radical-scavenging properties.
Known compounds were also isolated, including a benzaldehyde congener,114 isotetrahydroauroglaucin115 and an anthraquinone erythroglaucin,116 isolated for the first time from the marine environment and found to have DPPH activity.117 A dioxopiperazine alkaloid 87 was isolated from the culture broth of Pseudallescheria sp. separated from the surface of the brown alga Agarum cribosum (Uljin, Korea). Two known compounds, bisdethiobis(methylthio)gliotoxin118 and gliotoxin,119,120 were also isolated. All three compounds were active against MRSA and MDRSA, and gliotoxin also exhibited radical scavenging activity against DPPH.121 Culture of Ascochyta salicorniae isolated from Ulva sp. (Tönning, Germany) led to the epimers ascolactone A 88 and ascolactone B 89, and the known polyketide ascochitine,122 which was inhibitory against mycobacterial protein tyrosine phospatase B (MPtpB).123
Gymnascella dankaliensis, originally isolated from Halichondria japonica (Osaka Bay, Japan), was the source of gymnastatins F–H 90–92 and gymnamide 93. Gymnastatin H 92 was synthesised from gymnamide, and gymnastatins F 90 and G 91 were inhibitors of the P388 cell line.124 The quinone griseusin C 94 was isolated from culture of a Penicillium sp. collected from the mangrove Kandelia candel (Hainan Is., South China) and had modest activity against 3α-hydroxysteroid dehydrogenase (3α-HSD).125
P. janthinellum, isolated from the soft coral Dendronephthya sp. (Hainan Is., South China Sea), was the source of the 2,5-piperazinedione alkaloids, janthinolides A 95 and B 96.126 Chromocleista sp. from sediment (Gulf of Mexico), gave p-hydroxyphenopyrrozin 97 and the diketopiperazines 98–100, but 100 was identified as a decomposition product of 98 and 99.
The phenopyrrozin 97 displayed moderate activity against C. albicans.127 Peribysins H 101 and I 102 were isolated from Periconia byssoides separated from the sea hare Aplysia kurodai (source not given) and inhibited adhesion of HL-60 cells to human-umbilical-vein endothelial cells (HUVECs),128 while shimalactone B 103, isolated from culture of Emericella variecolor, separated from sediment (Gokasyo Gulf, Japan), induced neuritogenesis in neuroblastoma neuro 2a cells at a low concentration.129 Culture of Fusarium sp. separated from driftwood (Oga Peninsula, Japan) gave three pyrones, neofusapyrone 104, fusapyrone 105 and deoxyfusapyrone 106.
Fusapyrone and deoxyfusapyrone were originally isolated from F. semitectum130 and reported as α-pyrone derivatives, but NMR spectroscopic data comparison led to the corrected structures 105 and 106 respectively. All three compounds were moderately active against Aspergillus clavatus.131 Xyloketal H 107 was isolated from culture of the mangrove endophytic fungus Xylaria sp. (Mai Po, Hong Kong) and despite X-ray analysis the absolute configuration remains undetermined.132 Culture of Paecilomyces marquandii, obtained from intertidal sediment (Miramar, Buenos Aires), gave sorbicillinol urea 108.133
Discovery of a gene cluster for biosynthesis of a cyclic peptide in a culture of the cyanobacterium Trichodesmium erythraeum (Woods Hole Oceanographic Institution) led to prediction of the structure of the peptide as trichamide 109. FTMS confirmed the structure, with the stereochemistry as shown being inferred from the gene sequence.134
The trungapeptins A–C 110–112 were isolated from Lyngbya majuscula (Ratchamonkol Beach, Thailand). Trungapeptin A 110 exhibited mild icthyotoxicity and weak toxicity to brine shrimp.135 Two cyclic peptides, aurilides B 113 and C 114, were isolated from L. majuscula (Alotau Bay, Papua New Guinea) and were cytotoxic to NCI-H460 human lung tumour and neuro-2a mouse neuroblastoma cell lines.
Aurilide B 113 displayed net tumour cell killing activity in the NCI hollow fibre assay and induced loss of the microfilament network in the microfilament disruption assay.136 Aurilide was originally isolated from the sea hare Dolabella auricularia,137 but the isolation of aurilides B and C from L. majuscula indicates that the sea hare accumulates aurilide from its cyanobacterial diet.136 A Symploca sp. (Salmedina Reef, Panama) was the source of the highly methylated tetrapeptide belamide A 115.
Belamide A, which bears some structural analogy to dolastatins 10138 and 15,139 disrupted the microtubule network in A-10 cells, and was cytotoxic to the MCF7 breast cancer and HCT-116 cell lines.140 Oscillatoria sp. (Florida Keys) was the source of eight cyclic peptides, largamides A–H 116–123. The largamides comprise three different structural classes of peptides with largamides A–C 116–118 containing a senecioic acid unit and largamides B 117 and C 118 the unusual 2-amino-5-(4′-hydroxyphenyl)pentanoic acid (Ahppa) and novel 2-amino-6-(4′-hydroxyphenyl)hexanoic acid (Ahpha) units respectively. Largamides D–G 119–122 are 3-amino-6-hydroxy-2-piperidone acid (Ahp)-containing peptides that also contain the Ahppa moiety, while largamide H 123 is a unique cyclic peptide containing non-standard amino acids. Largamides D–G 119–122 exhibited moderate chymotrypsin inhibitory activity.141 Two further amphidinols designated AM14 124 and AM15 125 have been isolated from culture of the dinoflagellate Amphidinium klebsii originally isolated from surface wash of several seaweed species (Aburatsubo Bay, Japan).142
Culture of Amphidinium sp., originally isolated from an Okinawan flatworm Amphiscolops sp.,143 gave the polyhydroxylated metabolite amphezonol A 126, a modest inhibitor of DNA polymerase α,144 while culture of a different Amphidinium sp. strain, also from Amphiscolops sp. (Zanpa, Okinawa), yielded the polyketide amphidinin B 127.145 A modular total synthesis of amphidinolide Y146 has been completed.147 Culture of the symbiotic dinoflagellate Amphidinium sp. isolated from an unidentified marine acoel flatworm (Karatung Is., Indonesia), yielded two polyols karatungiols A 128 and B 129, of which 128 exhibited potent antifungal activity against Aspergillus niger and antiprotozoan activity against Trichomonas foetus.148
Progress on the chemistry and bioactivities of Amphidinium spp. has been reviewed.149 Culture of Protoceratium reticulatum isolate CAWD40150 from the Cawthron Institute Culture Collection yielded the diglycoside yessotoxin 32-O-[β-L-arabinofuranosyl-(5′→1″)-β-L-arabinofuranoside] 130,151 while a new yessotoxin analogue, 45,46,47-trinorhomoyessotoxin 131, has been isolated from culture of P. reticulatum (Yamada Bay, Japan).152
Two new spirolides, 13,19-didesmethylspirolide C 132 and spirolide G 133, have been isolated from culture of Alexandrium ostenfeldii (Limfjorden, Denmark),153 while two new pectenotoxins PTX-13 134 and PTX-14 135, oxidised analogues of pectenotoxin-2, were isolated from Dinophysis acuta (Westport, New Zealand).
PTX-13 ((32R)-hydroxy-PTX-2) 134 corresponds to an unidentified analogue PTX-11x previously reported.154 As the 32,36-dehydration product of PTX-13, PTX-14 135 may be an artefact.155 D. acuta (Buller Bay, New Zealand) yielded a new pectenotoxin PTX-11 136, which was determined as (34S)-hydroxypectenotoxin-2,156 while D. acuta (Westport, New Zealand) also yielded a cis-C8-diol ester of okadaic acid, 137.157 From cultures of Prorocentrum belizeanum (IEO Vigo collection) a sulfated water-soluble diol ester of okadaic acid, DTX-5c 138, was isolated.158
Maristentorin 139, the photoreceptor pigment of Maristentor dinoferus, has been isolated from a ciliate (Apra Harbour, Guam). The structure as proposed is cis, but the trans possibility should not be excluded.159 Four butenolides, previously isolated from marine Streptomyces spp.,160,161 were isolated from Streptoverticillium luteoverticillatum derived from sediment (Qingdao, China) and were moderately cytotoxic to human leukaemia K562 cells and murine lymphoma P388 cell lines. The absolute configuration of butenolide 140 has been determined,162 as has that of lagunapyrone B 141, a cytotoxic polyketide isolated from an actinomycete,163 by preparation of the four candidate stereoisomers utilising fluorous mixture synthesis.164
The absolute configuration of varitriol 142, a metabolite of Emericella variecolor,165 was established by a convergent total synthesis of (−)-varitriol.166 Thallusin, isolated from culture of a bacterium obtained from the green alga Monostroma oxyspermum,167 was a potent differentiation inducer of M. oxyspermum and induced germination in other green macroalgae.168 Attempted synthesis of thallusin from sclareol oxide resulted in ent-thallusin, thus establishing the stereochemistry of thallusin as 143.169
The total synthesis of fellutamide B, a neurotrophic lipopeptide aldehyde isolated from Penicillium fellutanum,170 has been completed by a combination of solid phase and solution phase synthesis.171 A toluhydroquinone, originally isolated from a marine Penicillium species,172,173 has been synthesised from 2-methyl-1,4-benzoquinone.174 Gymnasterone B, a metabolite of the fungus Gymnascella dankaliensis,175 has been synthesised from cholic acid,176 and a total synthesis of (−)-xyloketal A177 has been accomplished from phloroglucinol and a chiral alcohol.178 A total synthesis of the Cladisporium sp. metabolite sporiolide A179 has been achieved which utilises D-glucal as a chiral template,180 while sporiolide B from the same source179 has been synthesised utilising D-xylose as a chiral template.181 Two total syntheses of symbioimine, a tricyclic imine alkaloid isolated from the dinoflagellate Symbiodinium sp.,182 have been reported; the first utilised 3,5-dihydroxybenzoic acid as a starting material183 and the second employed an intramolecular Diels–Alder reaction.184 The polyether brevenal was recently isolated from culture of Karenia brevis185,186 and a total synthesis of the proposed structure completed.187 A mismatch between the NMR data of the synthetic and the natural compound resulted in revision of the structure to the C-26 epimer 144.
A total synthesis of the revised structure established the absolute configuration.188 Ciguatoxin was first isolated from the skin of the moray eel (Gymnothorax javanicus)189–191 but is actually produced by the epiphytic dinoflagellate Gambierdiscus toxicus. An analogue, 51-hydroxy-CTX3C, was subsequently isolated.192 Total syntheses of both ciguatoxin and 51-hydroxy-CTX3C have been achieved in 10 and 9 steps respectively.193 Total synthesis of the proposed structure of obyanamide, a cyclic depsipeptide isolated from Lyngbya confervoides,194 indicated the structure should be revised.195 A total synthesis of the revised structure 145 has modified the stereochemistry from (3S) to (3R).196
Hermatimides A and B, toxic metabolites of the cyanobacterium Lyngbya majuscula,197 have been synthesised via a cross-metathesis reaction.198 A Streptomyces sp. from marine sediment (Jiaozhou Bay, China) yielded known compounds of which N-(4-hydroxyphenethyl) acetamide199 and uridine displayed potent cytotoxicity against the murine cell line tsFT210.200 (−)-Hexylitaconic acid, isolated from the marine endophytic fungus Apiospora montagnei,201 has been identified as an inhibitor of the interaction of human double minute 2 (HDM2), an ubiquitin-protein ligase (E3) with p53 protein.202 3-Chloro-2,5-dihydroxybenzyl alcohol (chlorogentisyl alcohol), first isolated from a terrestrial Phoma sp.,203 was characterised from an Ampelomyces sp. isolated from biofilm off a glass slide (waters off Hong Kong), and found to be a potent inhibitor of larval and marine bacteria settlement.204 The known synthetic compound 1-methyl-1,4-dihydroquinoline205 was isolated from a natural source for the first time from Pseudomonas aeruginosa isolated from ribbonfish (Baluchistan coast, Pakistan) and was active against a range of Gram-positive and Gram-negative bacteria.206 Culture of an unidentified mangrove fungus (South China Sea coast) yielded (−)-byssochlamic acid. Byssochlamic acid has been synthesised as a racemate207 but this is the first reported isolation from nature.208 Secalonic acid D, previously reported as a metabolite of Penicillium oxalicum,209 has now been isolated from a marine source (a Gliocladium sp. separated from a lichen-derived fungus; South Pole).210 Culture of a marine actinomycete (Jiaozhou Bay, China) gave 1,6-dihydroxy-8-(hydroxymethyl)anthraquinone, previously isolated from a recombinant Streptomyces lividans,211 but this is the first report of isolation from a wild-type strain.212 Phylogenetic prediction was utilised in the isolation of rifomycin B213 and SV214 from a cultured Salinispora sp. isolated from Pseudoceratina clavata (Great Barrier Reef). These compounds have previously only been reported from the soil actinobacterium Amycolatopsis mediterranei.215 The biosynthetic gene cluster for thiocoraline, an octathiodepsipeptide produced by a marine Micromonospora sp.,216,217 has been isolated and utilised in a proposal for the biosynthesis of the unusual starter unit 3-hydroxyquinaldic acid.218 Examination of mixed cultures of microbial populations obtained from Oahu, Hawaii, revealed that Pseudomonas aeruginosa acted synergistically with other microbial species to produce pyocyanin, a known metabolite of P. aeruginosa.219 Furthermore, pyocyanin had transcriptional effects consistent with its purported role as an inducer of oxidative damage.220 In the biosynthesis of barbamide, a trichlorinated mixed polypeptide–polyketide metabolite of Lyngbya majuscula,221 the triple chlorination of the L-leucine substrate was mediated by tandem action of two nonheme FeII halogenases,222 and a separate study implicated a mixed NPRS–PKS route incorporating a unique trichloroisovaleryl starter unit with the dichloro- and trichloro-, but not the monochloroleucine, being incorporated.223 During the course of the feeding experiments a new didechloro analogue of barbaleucamide B 146 was isolated.223
Biosynthetic experiments on cultures of the dinoflagellate Alexandrium ostenfeldii utilising stable isotopically labelled acetate and glycine indicated that the cyclic imine toxin 13-desmethylspirolide C224 was polyketide in origin and that glycine was incorporated intact into the cyclic imine.225 A series of experiments on the dinoflagellate symbionts of the Caribbean gorgonians Pseudopterogorgia elisabethae and Eunicea fusca has demonstrated that biosynthesis of pseudopterosins226 and fuscol227 was markedly increased through addition of the plant bioactive substances methyl jasmonate, salicylic acid and gibberellic acid, and that the terpene content of P. elisabethae increased in response to decreased light due primarily to increased dinoflagellate concentration.228 Biosynthetic studies of the diatom Thalassiosira rotula utilising labelled precursors and cell preparations demonstrated that T. rotula can enzymatically transform various fatty acids to (2E,4Z)-octadienal or (2E,4Z,7Z)-decatrienal,229 while enzymatic preparations were utilised in a study that demonstrated that chloroplast-derived glycolipids were the main substrates for the biosynthesis of polyunsaturated aldehydes in T. rotula and that the process was mainly dependent on glycolipid hydrolytic activity.230
4 Green algae
In 2006 relatively few studies on compounds from green algae were reported. Bryopsis sp. (Oahu, Hawaii) was the source of the cyclic depsipeptides kahalalides P 147 and Q 148, modest inhibitors of the HL-60 cell line.Carboxypeptidase hydrolysis was employed to determine the sequential positions of DL-antipodal amino acids.231 Two sesquiterpenes, caulerpals A 149 and B 150, were isolated from Caulerpa taxifolia (Nanji Is., China) in addition to the known caulerpin,232 which was shown to be a potent inhibitor of human protein tyrosine phosphatase 1B (hPTP1B).233 Capisterones A and B, originally isolated from Penicillus capitatus,234 were re-isolated and absolute stereochemistry assigned as 151 and 152 using electronic CD.
In addition, the capisterones have been shown to significantly enhance fluconazole activity in Saccharomyces cerevisiae.235 Six known cycloartane compounds, 24-hydroperoxycycloart-25-en-3β-ol, cycloart-25-en-3β,24-diol, 25-hydroperoxycycloart-23-en-3β-ol, cycloart-23-en-3β,25-diol, cycloart-23,25-dien-3β-ol and cycloart-24-en-3β-ol, have been reported for the first time from a green alga, Cladophora fascicularis.236 These findings represent a significant increase in the number of cycloartane sterol derivatives from marine sources and illustrates the linkages between terrestrial plants and Chlorophyta.
5 Brown algae
Terpenes continue to be the predominant metabolites isolated from brown algae. The tetraprenyltoluquinols, thunbergols A 153 and B 154, were isolated from Sargassum thunbergii (Busan, Korea) and were scavengers of the DPPH radical and of ONOO− from morpholinosydnonimine (SIN-1).237 Cystophora fibrosa (De Hoop Nature Reserve, South Africa) yielded a number of cyclised tetraprenyltoluquinols, of which 155–159 were new.
A previously reported methylation product of cytosketal238 was also isolated from a natural source for the first time. Interestingly, 155 appeared in the Scifinder Scholar database prior to this report,239 but was apparently incorporated in error and is in fact a new compound.240 Further study of Dictyopteris divaricata (Qingdao, China), previously the source of three bisnorsesquiterpenes and a norsesquiterpene,241 has led to the isolation of five sesquiterpenes 160–164 with novel carbon skeletons and an oplopane sesquiterpene 165.242
A chlorine-containing perhydroazulene diterpene, dictyol J 166, was isolated from Dictyota dichotoma (source not given) along with two known diterpenes, dictyolactone243 and sanadaol.244 All three metabolites were algicidal to the bloom-forming species Heterosigma akashiwo and Karenia mikimotoi, and dictyolactone also displayed moderate activity against the dinoflagellate Alexandrium catenella.245 From D. dichotoma (Troitsa Bay, Sea of Japan) two new diterpenes, ent-erogorgiaene 167 and (+)-1,5-cyclo-5,8,9,10-tetrahydroerogorgiaene 168, were characterised,246 while from Eisenia arborea (Mugizaki coast, Japan) the anti-allergic phlorotannin phlorofucofuroeckol-B 169, an inhibitor of histamine release from rat basophile leukaemia (RBL) cells, was isolated.247
Caulocystis cephalornithos (Lady Bay, Tasmania) produced (E)-nonadec-3-en-2-one248 in addition to a range of other known compounds, and was the first report of its isolation as a natural product.249 The known terrestrial compound, 5′-deoxy-5′-(methylamino)adenosine (DMAA),250,251 has been isolated from the marine environment for the first time from Laminaria japonica as a growth stimulant for arbuscular mycorrhizal fungi.252 Some known phlorotannins isolated from Ecklonia stonifera, namely eckol,253 phlorofucofuroeckol A254 and dieckol,253 were shown to have marked inhibitory activity against angiotensin-converting enzyme (ACE).255
6 Red algae
The number of reports from 2006 of red algal metabolites is less than in the previous review and as for brown algae, terpenes continue to be the predominant class of metabolites isolated. A tetracyclic bromobenzaldehyde dimer, urceolatol 170, was isolated from Polysiphonia urceolata (Qingdao coast, China).256 The 2,7-naphthyridine alkaloids lophocladines A 171 and B 172 were isolated from Lophocladia sp. (Savusavu, Fiji).Lophocladine A 171 displayed affinity for N-methyl-D-aspartate (NMDA) receptors and was also a δ-opioid receptor antagonist, whilst lophocladine B 172 was moderately active against NCI-H460 human lung tumour and MDA-MB-435 breast cancer cell lines and shown to be an inhibitor of microtubules.257 Haraldiophyllum sp. (Almadies, Senegal) yielded almazolone 173, an indole alkaloid containing an oxazolone ring as a mixture of (Z/E) stereoisomers, the ratio of which could be altered via photochemical and thermal isomerisation. Almazolone 173 was synthesised by condensation of indole-3-carboxaldehyde with an N-acyl glycine.258 Three halogenated monoterpenes 174–176 were isolated from Portiera hornemannii (Tolagniaro, Madagascar) along with the known compound halomon.259
Both halomon and 176 were moderate inhibitors of DNA methyl transferase-1.260 Bromophycolides C–I 177–183 are diterpene-benzoate macrolides isolated from Callophycus serratus (Fiji) with modest activity against a range of human tumour cell lines, while two of them, 180 and 183, were weakly active against Candida albicans.261
A laurane sesquiterpenoid, 3β-hydroxyaplysin 184, and two rearranged sesquiterpenes, laurokamurenes A 185 and B 186, were isolated from Laurencia okamurai (Nanji Is., China),262 while five new sesquiterpenes 187–191 have been isolated from L. luzonensis (Sesoko Is., Okinawa).263
Laurencia scoparia (São Paulo, Brazil) was the source of halogenated β-bisabolene sesquiterpenes 192–194. The absolute configuration of C-10 in 193 and 194 was not unambiguously determined due to lack of material.264 Weak in vitro anthelmintic activity against Nippostrongylus brasiliensis was exhibited by 192. Aldingenins B–D 195–197 are brominated sesquiterpene derivatives isolated from Laurencia aldingensis (Castelhanos, Brazil).265
Laurencia obtusa (Koutalas Bay, Greece) was the source of sesquiterpenes 3,7-dihydroxydihydrolaurene 198, perforenol B 199 and 200, while L. microcladia (Vroulidia Bay, Greece) yielded a dimeric sesquiterpene 201.
Compounds 198–200 were tested against five human tumour cell lines and the Chinese hamster ovary (CHO) cell line. Perforenol B 199 exhibited strong activity while sesquiterpenes 198 and 200 exhibited weak activity. Sesquiterpene 201 was moderately cytotoxic to NSCLC-N6 and A549 lung cancer cell lines.266 Rhodomela confervoides (Qingdao, China) was the source of eight bromophenols 202–209, of which 202–205 also feature a urea substructure.
The phenylethanol and the phenylethanol sulfate bromophenols 206–209 exhibited moderate cytotoxicity against several human cancer cell lines.267 Two new sulfated oligobromophenols, 210 and 211 from the red alga Osmundaria obtusiloba (Parati beach, Espírito Santo), have been described.268 The brominated allenes nipponallene 212 and neonipponallene 213 were isolated from Laurencia nipponica (Troitsa Bay, Sea of Japan).269
Majapolene B, a brominated sesquiterpene, was originally isolated from Laurencia majuscula.270 Reisolation of majapolene B 214 from Laurencia sp. (Teluk Juara, Malaysia), along with the isolation of the previously unreported acetylmajapolene B 215, led to the determination of the absolute configuration of both compounds by vibrational circular dichroism (VCD).271
Gracilaria asiatica (Indonesia) was the source of three cyclopropyl derivatives, the cerebroside gracilarioside 216 and the ceramides gracilamides A 217 and B 218, which were mildly cytotoxic to the human A375-S2 melanoma cell line.272 Two new norisoprenoids, 219 and 220, were isolated from Gymnogongrus flabelliformis (Qingdao, China).273
Laurencenone C, originally isolated from Laurencia obtusa and L. scoparia (Jamaica),274 has been synthesised,275 while synthesis of 2,3,5-tribromo-N-methylindole, originally isolated from Laurencia brongniartii,276 has also been achieved via bromination of N-carbomethoxy-2,3-dihydroindole.277 16β-Hydroxy-5α-cholestane-3,6-dione, a cytotoxic oxysterol from Jania rubens,278 has been synthesised utilising diosgenin as a starting material.279 Synthesis of the proposed structures of the halogenated compounds elatenyne, originally isolated from Laurencia elata,280 and an enyne originally isolated from Laurencia majuscula281 established that the proposed structures are incorrect, and structural revisions to 221 and 222, with a 2,2′-bifuranyl structural core, have been tentatively suggested.282
5′-Hydroxyisoavrainvilleol was originally isolated from the tropical green alga Avrainvillea nigricans,283 but has now been isolated from Polysiphonia urceolata (source not given) as a protein tyrosine phosphatase 1B (PTP1B) inhibitor.284 Lanosol enol ether, originally isolated from the brown alga Fucus vesiculosus,285 has been shown to be an antibacterial and antifungal component of Osmundaria serrata.286 Culture of Asparagopsis armata (Bare Is., Sydney) without bromide in an artificial seawater medium stopped the production of halogenated metabolites in this organism.287
7 Sponges
For 2006 there has been a continuing list of new metabolites from all classes, but with more alkaloids, particularly in the nitrogenous terpene area, which are becoming a feature of sponge metabolite chemistry. With this comes increased use of nitrogen NMR spectroscopy as an elucidation tool. In the review period there were specific reviews on aspects of bioactive compounds from sponges.288,289 Six glycolipids, clathrosides A–C 223–225 and isoclathrosides A–C 226–228, were isolated from Agelas clathrodes (Bahamas). The positions of the methyl branches were revealed by very long range TOCSY correlations (through 13 vicinal couplings).290 A Sarcotragus species (China Sea) contained three cyclitol derivatives 229–231.291Two naturally occurring congeners of agelagalastatin (Agelas sp.),292 have been synthesised, confirming stereochemistry.293 A 1,3-diglyceride ester, corticaglyceride 232, and a sphingolipid, corticaceramide 233, were isolated from Negombata corticata (Red Sea, Egypt).294 A fatty acid analysis of over twenty calcareous sponges has been carried out and the chemotaxonomic relevance discussed.295 From Sarcotragus sp. (Cheju Is., Korea) two cytotoxic glycerolipids 234 and 235 were isolated.296 In a separate study the ω-hydroxy methyl ester 236 was reported from the same sponge.297
Irciniasulfonic acids B1 237 and B2 238, isolated from Ircinia sp. (Tsuzumi Is., Japan), were found to reverse MDR in mammalian cancer cells.298
Seven macrocyclic oxylipins, topsentolides A1, A2, B1–B3, C1 and C2 239–245, were isolated from Topsentia sp. (Cheju Is., Korea).299 Three new inhibitors of type III bacterial secretion system, caminosides B–D 246–248, were obtained from Caminus sphaeroconia (Dominica),300 the same sponge from which caminoside A was previously reported.301 The revised structure of plakevulin A 249 has been synthesised.302
Plakevulin A (Plakortis sp.) was first reported as an ester of livulinic acid,303 and the structure was revised in a racemic synthesis as the free alcohol.304 The compound was subsequently synthesised asymmetrically, establishing the absolute stereochemistry.305 The polyacetylene, callyberyne C (Callyspongia sp.)306 has been synthesised.307 Four melophlins, P–S 250–253, isolated from two species of Melophlus (Palau) as epimeric mixtures around the tetramic acid moiety, were inhibitory against L1210 cells.308
(S)-Plakolide A, synthesised from (S)-lactic acid,309 was the antipode of the natural product, (R)-plakolide A 254, originally isolated from Plakortis sp.310 The natural product, (−)-plakolide A, was assigned (S) on the basis of CD curve and chiral exciton coupling analysis, and a further synthesis of the natural product from (R)-lactic acid confirmed the stereochemistry.311 The peroxylipid plakinic acid A 255 (Plakinidae sp.)312 and three stereoisomers have been synthesised. Comparison of spectral data to related compounds suggested that the relative and absolute stereochemistries of plakinic acid A 255 are as drawn.313
Two antifungal and antileishmanial compounds, ent-3,6-epidioxy-4,6,8,10-tetraethyltetradeca-7,11-dienoic acid and ent-[3,5-diethyl-5-(2-ethylhex-3-enyl)-5H-furan-2-ylidene]acetic acid, were isolated from Plakortis sp. (Cheju Is., Korea).314 The antipodes of both compounds have been isolated previously.315,316 The absolute stereochemistry of (−)-plakortone E 256 (Plakortis simplex)317 was established by asymmetric synthesis.318 Similarly, the configuration of plakortone B 257 (Plakortis halichondrioides)319 has been established by synthesis.320 A biomimetic synthesis of spiculoic acid A 258 (Plakortis angulospiculatus)321 employing a Diels–Alder cycloaddition established the absolute configuration.322 5-Hydroxy-3-methyl-5-pentyl-2,5-dihydrofuran-2-one 259, known as a synthetic product, was isolated from Callyspongia vaginalis (St. Michael, Barbados).323
Halichondria rugosa (South China Sea) yielded the HIV-1 active halichondria sulfonic acid 260 known previously as a synthetic product.324 The unprecedented poly-arsenic compound, arsenicin A 261, was isolated from Enchinochalina bargibanti (New Caledonia) and the structure determined by spectral comparison to model compounds.325 The antifungal latrunculin T 262 was isolated from Negombata magnifica (Red Sea, Egypt).326
Okadaic acid, originally isolated from Halichondria sp.,327 was detected in intracellular vesicles of Suberites domuncula and found to be an apoptogenic toxin to symbiotic/parasitic annelids of the sponge.328 A synthesis has confirmed the structure and established the configuration of chlorodysinosin A 263, isolated from a Dysideidae sponge previously reported in a patent.329
Plakoridine B (Plakortis sp.)330 has been synthesised.331 A series of human histone deacetylase inhibitors, the azumamides A–E 264–268, were isolated from Mycale izuensis (Japan).332 Azumamides A and E have been synthesised, confirming structures and configurations.333 A series of depsipeptides, the seragamides A–F 269–274, were obtained from Suberites japonicus (Okinawa, Japan).
All but F were cytotoxic and caused multi-nucleation in mammalian cells while A was found to promote the polymerisation of G-actin and stabilised F-actin filaments.334 Stylissa caribica (Jamaica) was found to contain stylisins 1 275 and 2 276.335
Four new antifungal callipeltins, F–I 277–280, were isolated from Latrunculia sp. (Vanuatu, S. Pacific).336 The recent reassignment of callipeltin A was consistent with the stereoselective synthesis of callipeltin E 281 (Latrunculia sp.)337 and callipeltin B 282 (Callipelta sp.),338 with confirmation of the D-allo-threonine residue.339,340
The cytotoxic peptide theopapuamide 283 was obtained from Theonella swinhoei (Papua New Guinea).341 Halipeptins B and C (Haliclona sp.)342 have been synthesised, confirming absolute stereochemistry.343 Mycothiazole-4,19-diol 284 was isolated from Cacospongia mycofijiensis (previously Spongia) (Vanuatu, S. Pacific) along with the previously isolated mycothiazole 285 (Spongia mycofijiensis)344 which was re-assigned as the (Z) isomer.345
This compound had previously been synthesised as the (E) isomer,346,347 but a new racemic synthesis has confirmed the (Z) nature of the natural product.348 The macrolide exiguolide 286, isolated from Geodia exigua (Amami-Oshima, Japan), was found to inhibit the fertilisation of sea urchin eggs but not their embryogenesis.349 The dimeric macrolide clavosolide A 287 has attracted considerable attention from the synthetic community.
Originally isolated independently by two groups from the same Philippines sponge Myriastra clavosa,350,351 the C-9, C-10, C-9′, C-10′ relative stereochemistries were assigned from 1H–1H coupling constants and NOE measurements. Syntheses of the purported structure revealed that the structure of 287 was a diastereomer of that originally proposed.352,353 More recently, the revised structure (diastereomeric around C-9, C-10, C-9′, C-10′) was synthesised using D-xylose and proved this revised relative stereochemistry. Surprisingly, the reported rotation of the synthetic material was positive, opposite to the natural product, suggesting the opposite configuration.354 Subsequent syntheses of the same structure, however, gave negative rotations identical to the natural product, establishing the configuration as consistent with D-sugars.355–357 A deepwater Leiodermatium sp. (Palau), collected by manned submersible at over 200 m depth, yielded two cytotoxic macrolides, leiodolides A 288 and B 289.358,359 Tedanolide C 290, isolated from Ircinia sp. (Papua New Guinea), was found to be potently cytotoxic, causing S phase arrest suggestive of protein synthesis inhibition.360
Pachastrissa nux (Gulf of Thailand) yielded the cytotoxic kabiramides F–I 291–294.361 Swinholide I 295 and the related hurghadolide A 296 were isolated from Theonella swinhoei (Hurghada, Egypt) and were cytotoxic to human colon cancer cells.362
Tedanolide (Tedania ignis)363 has been synthesised asymmetrically, confirming the absolute stereochemistry.364 Quantitation of the levels of latrunculins A and B in organic extracts of Negombata (Latrunculia) magnifica (Red Sea)365 were determined by HPLC with seasonal and locational differences noted.366 A bromophenol ether, 2-(3′,5′-dibromo-2′-hydroxyphenoxy)-3,5-dibromophenol 297, was isolated from Dysidea herbacea (Great Barrier Reef).367 (−)-(5S)-2-Imino-1-methylpyrrolidine-5-carboxylic acid 298 was isolated from Cliona tenuis (Rosario Is., Caribbean Sea) using a coral allelopathic assay and the structure confirmed by synthesis.368
This structure, 298, was previously assigned to pyrostatin B (reported from Streptomyces sp.369), but the spectroscopic data did not match that of the synthetic product. NMR data comparisons suggested that pyrostatins A and B were identical with 5-hydroxyectoine370,371 and ectoine370,371 respectively.368 (+)-Dysamide B (Lamellodysidea (Dysidea) fragilis)372 has been synthesised, confirming its structure and configuration,373 and an asymmetric synthesis of (−)-dysibetaine PP (Dysidea herbacea)374 was also reported.375 Two known synthetic α-glucosidase-inhibitory phosphorylated aminosugars, 299 and 300, were isolated from Lendenfeldia chondrodes (Yap, Micronesia).376 9-Methyl-8-oxoadenine was isolated as a natural product from an unidentified Thorectidae sponge (Kurile Islands) together with the known 8-oxoadenine and two known sterols.377 The weakly cytotoxic cinachyramine 301, isolated from Cinachyrella sp. (Okinawa), is the first representative of a new alkaloid skeleton,378 and a cytotoxic thioether ircinamine B 302 has been characterised from Dactylia sp. (Cape Sada, Japan).379
The macrocyclic alkaloid (−)-sarain A (Reniera sarai)380 has been synthesised.381 The azoxy-containing alkylpyridine alkaloid, pyrinadine A 303, was isolated from Cribrochalina sp. (Okinawa),382 and in a separate report by the same authors, six further congeners, pyranadines B–G 304–309, were reported from the same sponge.383 A series of cytotoxic aminosugar-linked alkylpyridines, amphimedosides A–E 310–314, were isolated from Amphimedon sp. (Hachijo Is., Japan).384
Oligomeric macrocyclic alkylpyridinium salts have been a recent feature of sponge metabolites, and a Haliclona sp. (Okinawa) yielded a series of cytotoxic cyclohaliclonamines, 315–319.385 The trimeric viscosamine (Haliclona viscosa)386 has been synthesised,387 as have a series of dimeric congeners, cyclostellettamines H–L (Pachychalina sp.).388,389 Indole-3-methylethanoate 320, previously described from Camellia sinensis (tea),390 has been isolated from Sarcotragus sp. (Cheju Is., Korea).391
Barettin and 8,9-dihydrobarettin (Geodia barretti)392,393 were found to bind to a variety of human 5-hydroxytryptamine (serotonin) receptors.394 A potent inhibitor of indolamine-2,3-dioxygenase, exiguamine A 321, was isolated from Neopetrosia exigua (Papua New Guinea) as a natural racemate. Indolamine-2,3-dioxygenase is involved in the degradation of tryptophan and is upregulated in many tumours.395 Hyrtios erectus (Red Sea, Egypt) yielded hyrtiazepine 322 and the (synthetically) known 5-hydroxy-1H-indole-3-carboxylic acid methyl ester 323.396
Manzamine-type alkaloids, 12,28-oxamanzamine E 324, 12,34-oxa-6-hydroxymanzamine E 325, 8-hydroxymanzamine B 326 and 12,28-oxaircinal A 327, were obtained from Acanthostrongylophora sp. (Manado, Indonesia).397
The role of makaluvamine-type alkaloids from Zyzzya fuliginosa as sunscreens was assessed by measuring the protection offered to sea urchin eggs from mercury vapour UV radiation.398 Discorhabdins P399 and U400 (Batzella sp.) have been made semi-synthetically from discorhabdins C and D respectively.401 Discorhabdins S, T and U 328–330, mistakenly omitted from the 2003 review, were isolated from a deep-water Batzella sp. (Bimini, Bahamas).400
Aaptamine (Aaptos aaptos)402 activated the p21 promoter in a p53-independent manner which thereby regulates cell cycle progression.403 The bromopyrroles 331 and 332 were isolated from Stylissa caribica (Little San Salvador, Bahamas) and were synthesised, establishing absolute stereochemistry.404 A diffusion-edited DOSY experiment was used to detect the presence of new compounds in an extract of Agelas sp. (Okinawa) and led to the isolation of the agesamides A 333 and B 334.405
Four bromopyrroles with neuroprotective activity against agonists serotonin and glutamate in primary neurons, 335–338, were obtained from Axinella verrucosa (Corsica, France).406 Of the netamines A–G 339–345 that were isolated from Biemna laboutei (Madagascar), 341 and 342 were found to be cytotoxic.407
Stylissa caribica (Little San Salvador, Bahamas) yielded oxocyclostylidol 346, an oroidin derivative.408 The verpacamides A–D 347–350, isolated from Axinella vaceleti (Mediterranean Sea), are thought to represent possible intermediates in an alternative biogenetic pathway to pyrrole-2-aminoimidazole alkaloids such as oroidin.409
Verpacamide A is known as a synthetic product.410 Two tetrameric pyrrole-imidazole alkaloids with mild antimicrobial activity, stylissadines A 351 and B 352, were isolated from Stylissa caribica (Little San Salvador, Bahamas).411
An asymmetric synthesis of (S)-(−)-cyclooroidin (Agelas oroides)412 has been accomplished and confirmed the absolute configuration.413 A biomimetic Diels–Alder cyclisation of oroidin to cyclooroidin has been demonstrated.414 Ageladine A (Agelas nakamurai)415 was synthesised independently by two groups.416,417 Attempts at the synthesis of the 5,5-trans-fused bicyclic ring system reported for spiroleucettadine (Leucetta sp.)418 failed, suggesting that a 5,5-trans ring junction may be incorrect.419 The itampolins A 353 and B 354 were isolated from Iotrochota purpurea (Itampolo, Madagascar).420 The structure of zamamistatin 355 has been revised after re-isolation from the original Okinawan Pseudoceratina sp.421 and comparison of spectral data from synthetic analogues.422
The agelocaissarines A1 356, A2 357, B1 358, B2 359 and caissarine C 360 were isolated from Aplysina caissara (Brazil).423 Two cytotoxic macrocycles, bastadins-22 361 and -23 362, were isolated from Dendrilla cactos (Mandapam, India).424
Bromotyrosine alkaloids have attracted the attention of synthetic chemists. An asymmetric synthesis of (+)-calafianin (Aplysina gerardogreeni)425 has confirmed the absolute configuration.426 Purealin, the related compounds lipopurealins A–C and purealidin A, (Psammaplysilla purea)427–429 have been synthesised and are inhibitors of cytoplasmic dynein heavy chain activity (membrane and microtubule transport).430 An asymmetric synthesis of aerothionin (Verongia aerophoba and V. thiona)431 has confirmed that the original assignment of absolute stereochemistry of the natural compound432 was correct.433 Bastadin-6 (Ianthella basta),434 an inhibitor of VEGF- and bFGF-dependent proliferation of human umbilical vein endothelial cells in vitro, also had in vivo anti-angiogenesis activity in mice.435 Psammaplin A (Psammaplysilla sp.),436 with a diverse range of reported activities, was also found to activate peroxisome proliferator activated receptor-γ (PPARγ) and induce apoptosis in human breast tumour cells.437 The ecological role of brominated isoxazoline alkaloids in sponges of the genus Aplysina was investigated. These complex alkaloids were found to be converted to the simpler compounds aeroplysinin-1 and dienone in live sponges as a response to wounding or upon incubating lyophilised sponge material in sea water. A possible role suggested was to inhibit microbial infection.438 The antipode of (+)-agelasidine A (Agelas clathrodes)439 was obtained from a Caribbean specimen of the same species.440 Agelasine D (Agelas sp.)441 was synthesised and found to be active against aerobic and anaerobic Gram-positive and Gram-negative bacteria including Mycobacterium tuberculosis.442 A quite remarkable compound of mixed biogenetic origins (terpenoid, shikimate, purine and sugar composition), avinosol 363, was isolated along with the related 3′-aminoavarone 364 and 3′-phenethylaminoavarone 365 from Dysidea sp. (Papua New Guinea).
Avinosol 363 was synthesised from avarone and 2′-deoxyinosine and showed antiinvasion activity in a cell-based assay.443 A Spongia sp. (Okinawa) contained the cytotoxic metachromins J 366 and K 367.444 Liphagal 368, isolated from Aka coralliphaga (Dominican Republic, Caribbean Sea), was a selective phosphatidylinositol-3-kinase (PI3K) inhibitor.445 An inhibitor of Leishmania sp. adenosine phosphoribosyl transferase, isoakaterpin 369, and the related meroterpenoids ilhabelanol 370 and ilhabrene 371 were isolated from Callyspongia sp. (Ilhabela Is., Brazil).446
A Haliclona (aka Adocia) species (Palau) yielded the merotriterpenoids adociaquinol 372, adociasulfate 11 373 and adociasulfate 12 374.447 Axinyssa aplysinoides (Madang, Papua New Guinea) was found to contain two N-phenylethyl-2-formamido-6-axenes 375 and 376 along with the previously described 2-formamido-6-axene 377 (Amorphinopsis (syn. Trachyopsis) aplysinoides).448,449
The relative stereochemistry of 377 has been revised to (2R*,5R*,10S*).449 In the same report, a specimen of Amorphinopsis foetida (Papua New Guinea) was also found to contain 375 and 376 along with 378 and the previously reported 379 (Amorphinopsis (syn. Trachyopsis) aplysinoides).448 Again the relative stereochemistry was revised to (2R*,5R*,10S*).449 3-Oxoaxisonitrile-3 380 was isolated from Acanthella sp. (Hainan, China).450 Dysidea fragilis (Hainan, China) yielded spirofragilin 381.451 The haterumadysins A–D 382–385, inhibitors of the first cell division of fertilised sea urchin eggs, were isolated from Dysidea chlorea (Okinawa).452
An inhibitor of human protein tyrosine phosphatase 1B, O-methylnakafuran-8-lactone 386, was isolated from Dysidea sp. (Hainan, China).453 A synthetic conversion of isotwistinol to tricyclo[5.2.1.04,8]decane suggests a plausible biogenesis of the allopupukeananes from the pupukeananes.454 Coelodiol 387 and coeloic acid 388 were obtained from Coelocarteria cfr. singaporensis (Sulawesi, Indonesia) and were cytotoxic to human gastric adenocarcinoma cells.455 Cyanthiwigins AB–AD 389–391 were isolated from Myrmekioderma styx (Jamaica) in 2003 but were omitted from that year's review.456
(+)-Cyanthiwigin AC 390 has been synthesised asymmetrically,457 but due to the minute quantities originally isolated, the optical rotation of the natural product remains unknown. Oculatolide 392 was isolated from Haliclona oculata (Hainan, China).458 Ircinia formosana (E. Taiwan) yielded the norsesterterpenoids irciformonins A–D 393–396, of which 395 and 396 were mildly cytotoxic.459
Fasciospongia cavernosa (Croatia, N. Adriatic Sea) was found to contain two sesterterpenoids 397 and 398 related to luffarins.460 The cytotoxic 24-n-propyl-O-manoalide 399 was isolated from Luffariella sp. (Pohnpei, Micronesia).
The authors suggested that it may be an artefact of isolation, but no use of n-propanol was given in the extraction procedure.461 A Spongia species (Tong-Kong, Korea) yielded 12,24-diacetoxydeoxoscalarin 400, 12-O-deacetoxyl-24-hydroxyldeoxoscalarin 401 and 12-O-deacetoxyl-19-O-methyldeoxoscalarin 402, which were inhibitors of human farnesoid X-activated receptor.462 Hyatella intestinalis (S. E. Queensland, Australia) yielded the cytotoxic norsesterterpene mooloolabenes A–E 403–407 and sesterterpene mooloolaldehyde 408.463
A Gulf of California (Mexico) collection of Hyatella intestinalis was found to contain the hyatelones A–C 409–411, the hyatolides A–E 412–416, hyatelactam 417, 12-O-deacetyl-19-epi-scalarin 418 and 12-O-deacetylnorscalaral 419, of which 409, 412, 417 and 419 were mildly cytotoxic.464 Deacetoxyscalarin 420 was isolated from Spongia sp. (Toyama Bay, Japan).465
Sesterstatin 7 from Hyrtios erecta (Red Sea)466 was previously reported467 as a semisynthetic intermediate from 16-hydroxyscalarolide.468 Oculatol 421, a C15 terpenoid, thought to be a nor-ergosterol compound, was isolated from Haliclona oculata (Hainan, China) along with a series of ring-A-contracted steroids 422–427.458
The cytotoxic homaxisterols B1 428, B2 429, C1 430 and C2 431 were obtained from Homaxinella sp. (Cheju Is., Korea).469 Geodinella robusta (Kuril Is., N. W. Pacific) was found to contain 5α-ergost-24(28)-ene-3,6-dione 432.470
A series of steroids, topsentisterols A1–A3 433–435, B1–B5436–440, C1–C4 441–444, D1–D3 445–447 and E1 448, were isolated from Topsentia sp. (Cheju Is., Korea).471 Ophirasterol 449 (Topsentia ophiraphidites)472 has been synthesised asymmetrically, establishing the absolute configuration.473
Erylus placenta (Hachijo Is., Japan) yielded the steroidal glycosides sokodosides A 450 and B 451, which were found to be mildly cytotoxic to P388 cells and growth inhibitors of yeast.474 The sponge Melophlus sarasinorum (Scott Reef, N. W. Australia) has yielded two further sarasinosides, A4 452 and A5 453.475
Fortisterol 454, with a seven-membered lactone ring B, was isolated from Biemna fortis (Lingshui Bay, S. China).476
The steroidal isoquinoline alkaloids cortistatins A–D 455–458 were isolated from Corticium simplex (Flores Is., Indonesia). Cortistatin A 455 was found to be antiangiogenic.477 A series of isomalabaricane triterpenoids and nortriterpenoids, the jaspolides A–F 459–464, were obtained from Jaspis sp. (Hainan, China).478 Rhabdastrella globostellata (Fiji) was found to contain stelletins J 465 and K 466 along with 3-epi-29-acetoxystelliferin E 467, known previously as a semi-synthetic derivative.479 Compounds 465 and 467 were found to stabilise the binding of DNA to DNA polymerase β.480 Rhabdastrella globostellata (Kapoposang Is. Indonesia) yielded globostelletin 468, globostellatic acids F–M 469–476, (13E)-stelliferin riboside 477 and 3-O-deacetyl-(13Z)-stelliferin riboside 478.
All compounds were selectively cytotoxic to mouse lymphoma cells.481 (12R)-Hydroxyyardenone 479, 3-epi-sodwanone K 480, 3-epi-sodwanone K 3-acetate 481, 10,11-dihydrosodwanone B 482 and sodwanones T–W 483–486 were isolated from Axinella sp. (S. E. Coast of South Africa); it was speculated that these and other related sodwanones may have been degradation artefacts.482
A mutation of yeast lanosterol synthase was found to cyclise (S)-2,3-oxidosqualene to (13αH)-isomalabarica-14(27),(17E),21-triene-3β-ol, suggesting a biosynthetic route to the isomalabaricanes isolated from sponges.483
8 Cnidaria (coelenterates)
The observant reader will have noticed the name change in this section of the review. The reason is that the phylum Cnidaria encompasses not only coelenterates (soft corals, gorgonians, sea pens etc.) but also hydroids, jellyfish and sea anemones, the secondary metabolites of which are covered in this section. The last two years have seen a plateau in the number of new metabolites reported from cnidarians. Chemical constituents of soft corals of the genus Cladiella have been reviewed.484 In addition to other metabolites, thymidine analogues 487 and 488 and p-vinylbenzyl alcohol 489, previously known as synthetic compounds, were reported from extracts of C. australis (Taiwan).485Mild cytotoxicity was exhibited by 488 and 489. The isolation of γ-lactones 490–494 from Pterogorgia spp. (Caribbean) provided the opportunity to define a set of 13C NMR-based empirical rules that allow determination of the relative stereochemistry of 3-long chain alkyl-4-hydroxy-5-methyldihydrofuranones.486 Combined with the use of a chiral solvating agent (Pirkle), absolute configurations of the known C2-symmetric homoancepsenolide 495,487 dilactones 490 and 491, acids 494 and 496488 and monobutenolide 493 were also established.
Malonganenones A–C 497–499 are mildly cytotoxic tetraprenylated alkaloids isolated from Leptogorgia gilchristi (Ponto Malongane, Mozambique).489
Facile deuterium exchange of the formamide proton present in 498 in MeOH-d4 NMR solvent was noted. The syntheses of dialkyne lipids montiporyne C490 and E 500, both isolated from the stony coral Montipora sp.,491 have been reported, with some discrepancies being noted between the 1H and 13C NMR data observed for synthetic 500 versus that reported in the literature.492 A diverse array of compounds was isolated from Muricea austera (Pacific Coast, Panama) including tyramines 501 and 502.493
Aspects of the structure–antiplasmodial activity of 501 and 502 were explored, indicating the importance of fatty acid chain length and the influence of side chain polarity and the presence or absence of halogen atoms on observed activity. Two acylated tryptamines, granulatamides A 503 and B 504, were isolated as mildly cytotoxic components of Eunicella granulata (Madeleine Islands, Senegal).494 Long chain alkyl glycosides firmacosides A 505 and B 506 were isolated from a collection of Sinularia firma (Ramnathpuram, India).495
The first synthesis of the polar pyrazine alkaloid clavulazine, isolated from Clavularia viridis,496 has been reported.497 The search for new inhibitors of indoleamine-2,3-dioxygenase has led to the isolation of a number of known metabolites in addition to the new natural products annulin C 507, 2-hydroxygarveatin E 508, garveatin E 509 and garvin C 510 from the hydroid Garveia annulata (Barkley Sound, British Columbia).498
Annulin C 507 and related compounds were the most potent inhibitors (sub-micromolar) identified in the study. In two separate accounts, sesquiterpene peroxides sinularioperoxides A–D 511–514499 and sinularianins A 515 and B 516500 were isolated from Sinularia sp. (Taiwan). Soft coral specimens of the same genus (Hainan Is., China) were the source of 517 and 518.501
Both sesquiterpenes exhibited antioxidant and antiproliferative activities. Clavularia inflata var. luzoniana (Green Is., Taiwan) was the source of two new aromadendrane sesquiterpenes 519 and 520, and the epimer 521 of the known (−)-ent-3β-hydroxyspathulenol (from the liverwort Lipicolea ochraleuca),502 in addition to 522, which possesses a highly unusual carbon skeleton.503
Paralemnanone 523, isoparalemnanone 524 and paralemnanol 525 were isolated as mildly bioactive components of extracts of Paralemnalia thyrsoides (Green Is., Taiwan).504 The absolute configuration of 523 was implied by biogenetic reasoning from 524 of known absolute configuration. Biogenetic reasoning was also used to suggest the absolute configuration of 525 and was based upon that determined for 524. The absolute configurations of the gorgonian-derived sesquiterpenes suberosenone 526,505 suberosanone 527 and suberosenol A acetate 528506 were determined from time-dependent density functional theory calculations of the experimentally reported [α]D values.507 The absolute stereochemistries of preclavulone-A methyl ester 529 and the diastereomer 530, isolated from Clavularia viridis,508 were determined by enantioselective synthesis.509
A total synthesis of racemic clavubicyclone, a prostanoid-related oxylipin also isolated from C. viridis,510 has been reported.511 The new pterosin diterpenes 531–535 were reported from Pseudopterogorgia elisabethae (San Andrés and Providencia Islands, Colombian Caribbean).512 Elisabethatrienol 531 was proposed to be an intermediate in the biosynthesis of erogorgiaene, while 532 and 533 were isolated as an interconverting mixture. Enantioselective syntheses of (+)-elisabethadione 536513 and benzoquinone 537,514 both isolated from P. elisabethae, have been achieved.515
Whilst the structure and configuration of 537 was confirmed, differences observed in the NMR data of synthetic 536 and those reported for the natural product suggested that either the structure of the natural product was incorrect or that the NMR data had been mis-reported. A new member of the mildly antimycobacterial diterpene alkaloid family, ileabethoxazole 538, was also isolated from P. elisabethae (Providencia Is., Caribbean).516 The dolabellane-skeleton diterpenes 539–541 were identified as mildly cytotoxic components of Clavularia inflata var. luzoniana,503 while the related epoxydolabellane derivative 542 was isolated from Eunicea laciniata.517
The soft coral Cespitularia taeniata (Taiwan) was the source of cespitulactones A 543 and B 544; 543 was modestly cytotoxic towards two tumour cell lines.518 A number of new metabolites were reported from C. hypotentaculata (Green Is., Taiwan) including diterpenes cespitularins I–L 545–548, norditerpenes cespitularins M 549 and N 550, verticillane diterpenes cespitularins O 551 and P 552 and norditerpene cespitularin Q 543.519
The structure established for cespitularin Q is the same as that of cespitulactone A. Identical 13C and very similar 1H NMR data were reported for both compounds, with the only major difference being chiroptical: [α]D +42 observed for the former metabolite, and [α]D −122 for the latter. Cespitularin O 551 exhibited mild cytotoxicity. Cembrane diterpenes sarcrassins A–E 553–557 were isolated as mildly cytotoxic components of Sarcophyton crassocaule (Hainan Is., China).520 Extracts of soft corals of the same species (Kenting, Taiwan) yielded the trans-fused γ-lactone-containing crassocolides A–F 558–563.521
The absolute configuration of 558 was determined and mapped to the others on biogenetic grounds. Mild cytotoxicity was observed for 558, 559, 561 and 563 towards selected cell lines. 12-epi-Eunicin 564, 4-epi-jeunicin 565 and 13-epi-eupalmerin 566 were isolated as mildly cytotoxic components of Eunicea mammosa (Sweetings Cay, Bahamas).522 Incubation of cell-free extracts of E. mammosa with 3H-GGPP afforded radioactive labelling of all three metabolites, with significantly higher specific activity observed in 564, indicating a possible central role of 564 in the biosynthesis of 566. A limited number of semisynthetic analogues of 564 and 566 were prepared and evaluated for cytotoxicity towards a panel of human tumour cell lines. Sinularia polydactyla (Gulf of Suez, Egypt) was the source of (+)-polydactylide 567 and (+)-7α,8β-dihydroxydeepoxysarcophine 568, a known biotransformation product of sarcophine.523 The same publication also noted the isolation of 569 from the soft coral Sarcophyton trocheliophorum (Egypt).
This compound has previously been reported as an acid-catalysed methanolysis product of sarcophine,524 suggesting that the occurrence in S. trocheliophorum may be artefactual. Also of potentially artefactual origin was 2-hydroperoxysarcophine 570 which, along with the two previously reported semisynthetically derived cembranoids 571 and 572, was isolated from Lobophytum crassum (Hainan Is., China).525
A number of semi-synthetic and biotransformation products of sarcophine have been investigated for antimetastatic, antiproliferative and anti-inflammatory properties526,527 and as cancer chemopreventive agents.528 Sarcophytonolides E–H 573–576 were isolated from Sarcophyton latum (Ximao Is., China).529
The absolute configuration of 576 was established (Mosher) and transferred on biogenetic arguments to the other metabolites, whose lack of mass had prevented the direct determination of absolute configuration. Investigations of Pseudopterogorgia bipinnata and P. kallos (Providencia Is., Colombia) afforded kallolide D 577, kallolide C acetate 578, and kallolides E–I 579–583.530
The structures and relative stereochemistries of 579 (as the diacetate), and 581–583 were established by X-ray analysis. Modest antimycobacterial and antiplasmodial activities were observed for some of these compounds. Acerolide 584 was isolated as a cytotoxic component of extracts of P. acerosa (Saint Pierre, Martinique),531 while the structurally-related cembranoid 11β,12β-epoxypukalide532 was found to be the mussel larvae antifouling component of Phyllogorgia dilatata (Brazil).533 As well as a number of related compounds, seven new cembrane diterpenes, lophodienone 585, lophodiol A 586, lophodiol B 587, 17-acetoxylophotoxin 588, 15,16-epoxylophotoxin 589, 17-acetoxy-15,16-lophotoxin 590 and isoepoxylophodione 591, were reported from extracts of Lophogorgia peruana (Gulf of California, Mexico).534
Assessed for cytotoxicity towards a panel of tumour cell lines, 585 and 586 were deemed inactive, 587 and 588 more active and 589–591 the most active (TGI <10 µM). In addition to two racemic syntheses,535,536 two syntheses of (−)-bipinnatin J 592 have been reported537,538 and had established the absolute configuration of the furanocembranoid originally isolated from Pseudopterogorgia bipinnata.539 The subsequent conversion of (−)-bipinnatin J to (+)-intricarene 593,537,538 (+)-rubifolide538 and (+)-isoepilophodione B 594538 highlights the close biosynthetic relationship between these gorgonian natural products and established the absolute configurations of 593540 and 594.541 The norcembrane sinularectin 595, isolated from Sinularia erecta (Kenya), was unusual in the degree of oxygenation and the presence of a chloromethyl group.542
The δ-lactone-containing cembranoids manaarenolides A–I 596–604 were isolated from S. manaarensis (Pingtung, Taiwan).543 Absolute configurations were proposed based solely upon the co-occurrence of sinularin 605 and dihydrosinularin 606, both with defined configurations,544 in the same organism. Hydroperoxides 596, 597, 600 and 601 probably derive from singlet oxygen reaction with 606, with alcohols 598 and 599 being reductive products of 596 and 597 respectively.
Allylic alcohols 602–604 could well derive from epoxide 605. Of the compounds isolated, only 602 and 603 exhibited cytotoxicity (mild). The biscembranes bisglaucumlides A–D 607–610 were isolated from Sarcophyton glaucum (Amami Oshima, Japan).545
Absolute stereochemistry was proposed by analysis of CD spectra; mild cytotoxicity was observed for 609 and 610. Junceella juncea (Sanya Bay, China) was the source of briarane-skeleton diterpenes juncins R–ZI 611–620.546 All ten compounds exhibited non-toxic settlement inhibition of larvae of the barnacle Balanus amphitrite and conclusions were drawn on structure–activity requirements. Junceellolides J–L 621–623 and the known briarane 624547 were isolated from Junceella fragilis (Taiwan).548
Chemical conversion of 624 to 621 by reaction with mCPBA indicated that the originally proposed structure of 624 should be corrected to that having a C-11 β-oxirane as shown. A similar oxirane configurational change for junceellolide F549 was also necessitated by comparison of 13C NMR shift data. Mild human neutrophil elastase inhibitory activities were observed for 622 and 624. A Papua New Guinea collection of the same species of soft coral was the source of (−)-2-deacetyljunceellin 625 and (−)-3-deacetyljunceellin 626.550 The absolute configuration of 626 was determined by a combination of chemical shift simulation and molecular dynamics calculations that were compared to NMR data acquired on a chiral auxillary derivative of 626. Pachyclavularia violacea (Ishigaki Is., Japan) was the source of pachyclavulides A–D 627–630.551
The absolute configuration of 627 was determined by X-ray analysis of the p-bromobenzoate derivative, and biogenetic arguments were used to assign the absolute configurations of the remaining compounds. In three separate accounts, eight new briarane diterpenes briaexcavatins A–H 631–638 were reported from a collection of Briareum excavatum (Taiwan).552–554 Briaexcavatin A 631 bears an unusual ε-lactone which may well be biosynthetically derived via Baeyer–Villiger oxidation of briaexcavatin B 632.552 The absolute configurations of 631 and 632 were assigned based on biogenetic grounds to the co-occurring excavatolide B 639,555 of assigned absolute configuration (Mosher). The absolute configurations of briaexcavatins F 636 and G 637 were also related to 639 via semi-synthesis.553,554 In similar fashion, the absolute configuration of excavatolide C 640555 was determined (Mosher) and related to briaexcavatin H 638 via semi-synthesis.554 Errors in the structural assignments of briaexcavatolides X and Y, previously isolated from the same organism,556 were noted and have been corrected to 641 and 642 respectively.553
Mild inhibition of the release of human neutrophil elastase was observed for 635 and 637. The absolute configuration of violide E 643, isolated from Briareum sp.,557 has been secured by X-ray analysis of the bis-MTPA ester.558 Xenia novaebrittanniae (Kitagamwa, Kenya) was the source of seven new Xenia-class diterpenes novaxenicins A–D 644–647 and xeniolides I–K 648–650.559
Antibacterial activity was observed for 648, and 645 was found to induce apoptosis in transformed mammalian cells. In addition to a number of verticillane diterpenes (see earlier) and secosteroids (see later), the xenicane diterpene cespitolide 651 was isolated from Cespitularia hypotentaculata (Taiwan).519 Florxenilides A–C 652–654 were isolated from X. florida (Green Is., Taiwan).
Mild cytotoxicity was observed for both 652 and 653.560 Xenicane 655 was isolated as a mildly cytotoxic component of Clavularia inflata var. luzoniana (Taiwan),503 and umbellacins A–I 656–664, and 14,15-epoxyxeniolide H 665 and the acetate derivative 666 were also mildly cytotoxic (P388) components of Xenia umbellata (Green Is., Taiwan).561
While the majority of Xenia diterpenes contain dihydropyran-cyclononane or δ-lactone-cyclononane skeletons, a further subdivision includes those compounds that bear a caryophyllene skeleton, the so-called xeniaphyllanes. Seven new members of this family, 667 and gibberosins A–F 668–673, were isolated from Sinularia gibberosa (Taiwan).562 The same species of soft coral also afforded the unusual cyclic peroxyhemiketals, sinugibberosides A–E 674–678.563
It is conceivable that the biogenesis of these compounds derives from intramolecular cyclisation of a hydroperoxide structurally related to gibberosin B 669. Total synthesis of antheliolide A 679, isolated in 1988 from the soft coral Anthelia glauca,564,565 has been achieved; the synthetic (−)-enantiomer co-eluted with an authentic sample of the xeniaphyllane on chiral HPLC.566 It is interesting to note that the absolute configuration of 679 is opposite to that of β-caryophyllene. With the aid of tandem mass spectrometry, eunicellin-skeleton diterpene 680 and an inseparable homologous series 681 were identified in extracts of Acalycigorgia sp. (Thailand).567 The structures and absolute configurations of the related diterpenes ophirin B 682568 and astrogorgin 683568,569 were secured by a stereoselective synthesis that made use of an intramolecular Diels–Alder reaction to install four of the stereocentres.570
Stereoselective synthesis afforded (−)-cladiella-6,11-dien-3-ol 684,571 establishing the absolute configuration.572 The (E)-cladiellin diterpene was then converted to (+)-polyanthellin A 685, originally isolated from Briareum sp.,573,574 and triol 686 isolated from Cladiella sp.575
Detailed investigation of lipid extracts of B. polyanthes (Puerto Rico) yielded the eunicellin-type diterpenes briarellin Q 687, R 688, seco-briarellin R 689, the asbestinane-skeletoned diterpenes asbestinins-24–26 690–692, seco-asbestinin B 693 and nor-asbestinin A 694, and briarane 695.576
During the course of the study the known diterpenes asbestinin-10,577 -20, -21 and 11-acetoxy-4-deacetoxyasbestinin F578 were also re-isolated and the NMR spectroscopic data re-analysed. Structures attributed to these compounds have now been corrected to 696, 697, 698 and 699 respectively.
In most cases the correction requires placement of oxygenation at C-6 rather than C-4 as proposed earlier. In the case of 699, the structural reassessment now places a hydroperoxide, rather than hydroxyl, functional group at C-6. These structural corrections also call into question the original structures proposed for asbestinin-7, -9, -15, -22, -23 and 4-deoxyasbestinin G.577,578,576 The sesquiterpene metabolites cladocorans A and B, isolated from Cladocora cespitosa,579 and structural analogues have been found to inhibit secretory phospholipase A2 with micromolar potency.580 Taiwanese collections of Sinularia lochmodes afforded secosterol 700,581 S. leptoclados yielded 701,581 while 702 and 703 were isolated from Cespitularia hypotentaculata.519
Mild cytotoxicity was exhibited by 701 and 702. Secosterols present in Pseudopterogorgia americana provide chemical defence in the form of fish-feeding deterrence.582 Three known 5β sterol bile acids were reported as marine natural products for the first time. Alcyonium sp. (Taiwan) was the source of 3α,7α,12α-triacetoxy-5β-cholanic acid,583 while 3α,12α-diacetoxy-5β-deoxycholic acid and the methyl ester were isolated from Junceella fragilis (Taiwan).584 A new ergostenoic acid derivative 704 was isolated as a mildly cytotoxic component of Cladiella australis (Kenting, Taiwan).485 Sinularia gibberosa (Taiwan) was the source of the sterols gibberoketosterols B 705 and C 706 and mildly cytotoxic gibberoepoxysterol 707, which contain conjugated dienes in the side chain or a 5α,6α-epoxide in ring B.585
In addition to known sterols, 708 and 709 were isolated from lipid extracts of Sinularia sp. (Ximao Is., China).586 During the course of this study, the structure previously reported for the sponge metabolite hyrtiosterol587 was corrected to the C-2/C-3 epimer 710.
Six new polyoxygenated cholestanes were reported from South China Sea gorgonians. Echinogorgia aurantiaca was the source of sterols 711–713,588 while muricesteroid 714 was isolated from Muricella flexuosa, and menellsteroids A 715 and B 716 from Menella verrucosa.589
Muricesteroid 714 was observed to rapidly eliminate water and acetic acid in the NMR tube (CDCl3). The sidechain-oxidised sterol muriceanol 717 and pregnane 718, previously known as a synthetic compound, were isolated from Muricea sp. (Pacheca Is., Gulf of Panama),590 while the deep-water scleractinian coral Dendrophyllia cornigera (Island of Serifos, Greece) yielded the new cholestadiene-3,6-dione 719.591
Sinularia flexibilis (Hainan Is., China) afforded two new members of the 5α,8α-epidioxygorgostane family of metabolites 720 and 721, as well as the 22α,28-epidioxycholestane C-22 epimers 722 and 723.592
Two pregnane and seven pregnane monosaccharide steroids, stereonsteroids A–I 724–732, were isolated from Stereonephthya crystalliana (Green Is., Taiwan) and all were mildly cytotoxic.593 Coincidentally, the deacetyl derivatives of stereonsteroids G and H i.e. 733 and 734, were isolated from Cladiella krempfi (Hainan Is., China),594 and the corresponding β-D-arabinopyranosides 735 and 736 were identified as mildly antimalarial and antitrypanosomal components of Muricea austera (Panama).493
The same collection of C. krempfi (Hainan Is., China) also yielded four additional pregnane steroids, krempenes A–D 737–740.595
Krempene A was proposed to contain a highly unusual 1,3,4-oxadithiino ring. Structural and functional aspects of sea anemone toxins have been reviewed,596 while a more specific review covered voltage-gated ion channel targeting toxins produced by cnidarians.597 CgNa is a 47 amino acid residue toxin, isolated from the sea anemone Condylactis gigantea (Havana, Cuba), which belongs to the type I sodium channel toxin family that delay sodium channel inactivation leading to neurotoxic and cardiotoxic effects.598 The 41 amino acid toxin BcIV, isolated from the venom of the sea anemone Bunodosoma caissarum (São Paulo State, Brazil), was weakly paralysing to crustacea.599 Anemonia erythraea (Banda, Japan) was the source of the type I potassium channel-targeting toxin AETX K.600 The amino acid sequence of the high molecular weight cytolysin RTX-A, previously reported from Radianthus macrodactylus (Seychelles),601 has been reported.602 A 40-residue antimicrobial peptide, aurelin, was isolated from the jellyfish Aurelia aurita (Sredniy Is., Russia) and sequenced.603
9 Bryozoans
Once again, no new metabolites have been reported from bryozoans but several syntheses have been accomplished. Pterocellin A, an alkaloid isolated from Pterocella vesiculosa (New Zealand),604 has been synthesised from kojic acid and 2-bromo-3-pyridinol by a convergent method.605 Chartelline C, a halogenated β-lactam alkaloid from the marine bryozoan Chartella papyracea606 has also been synthesised,607 and synthesis of convolutamydine B 741 (a metabolite of Amathia convoluta608) via a vinylogous Mukaiyama aldol reaction has allowed determination of the absolute configuration.609
10 Molluscs
The past two years has seen a somewhat consistent number of new metabolites reported from molluscs. Also, a number of reviews specific to mollusc chemistry have been published covering the chemical and ecological studies of Australian and New Zealand molluscs,610 Antarctic, southern Africa and South American molluscs,611 the chemistry of pulmonate gastropods,612 and bioactive molecules from sea hares.613 The ecological roles of sterols in molluscs have also been reviewed.614 8,13-Dihydroxyeicosapentaenoic acid 742 was identified as a muscle stimulatory factor in the barnacle Balanus balanus.615 The pulmonate mollusc Siphonaria lessoni (Las Cruces, Chile) was the source of 743, a nor-homologue of the well known metabolite siphonarienolone.616Eight isomers of one enantiomeric series of (−)-maurenone 744, a polypropionate isolated from Siphonaria maura,617 were synthesised, leading to determination of the relative stereochemistry as shown in 744.618 Unfortunately, optical rotation data were not reported for the natural product and so the absolute configuration remains unknown. Two new thioarsenosugars, 745 and 746, were identified in extracts of the clam Venus verrucosa and Canadian and Icelandic kelp.619
The structures and absolute configurations of pteriatoxins A–C 747–749, isolated from the Okinawan bivalve Pteria penguin,620 were established by synthesis.621,622 In each case, all four sidechain stereoisomers were synthesised and careful comparison with published 1H NMR data established the stereochemistry of the toxins. In a similar fashion, the absolute configurations of pinnatoxins B 750 and C 751, isolated from the bivalve Pinna muricata,623 were established as shown.624
A full account of the total synthesis of the correct structure of azaspiracid-1 has appeared,625 and revised structures of the related azaspiracid-2 752 and -3 753 toxins, isolated from Mytilus edulis,626 have been confirmed by total synthesis.627
Ostrerine A 754 was isolated from extracts of the mollusc Ostrea rivularis, which is a food source and traditional Chinese medicine.628 A new oxazinin alkaloid, oxazinin-4 755, was reported from extracts of toxic mussels Mytilus galloprovincialis (Northern Adriatic Sea).629 Synthesis of model compounds, interpretation of ROESY NMR data and comparison of [α]D values established the absolute stereochemistry of 755 and led to the reassignment of stereochemistry of the related toxins oxazinin-1 756 and -2 757.630
The structure of the cytotoxic alkaloid lamellarin N (Lamellaria sp.)631 has been confirmed by total synthesis.632 Further structure–activity relationship studies on lamellarin D have been reported,633 while the mitochondria have been identified as a direct proapoptotic target of the alkaloid.634 The sacoglossan mollusc Elysia cf. expansa (Mandapam, India), yielded the caulerpenyne-related metabolites dihydrocaulerpenyne 758 and expansinol 759.635
New congeners in the kahalalide family of cyclic depsipeptides, kahalalides R 760 and S 761, were reported from an Indian collection of E. grandifolia.636 The stereochemistry of the individual valine units of the compounds were not determined. Kahalalide R exhibited comparable in vitro cytotoxic potency towards MCF-7 cells to that observed for the current clinical drug candidate kahalalide F. The prosobranch mollusc Thais clavigera (Hiraiso, Japan) contains the excitatory pentadecapeptides TEP-1 762 and TEP-2 763.637
Peptides isolated from carnivorous snails of the genus Conus are well known for their potent biological activities. The therapeutic potential of these compounds in general has been reviewed,638 while specific examples of voltage-gated calcium channel inhibitors have also been reviewed.639,640 A new member of the M-superfamily of conus toxins 764, isolated from Conus vexillum (South China Sea), is a 21 amino acid peptide containing three disulfide bonds, with confirmation of structure by synthesis.641
Exact disulfide connectivities were not deduced. The peptide failed to exhibit any effect on Na+, K+ and Ca+ ion channel function in locust neurons. The same species of Conus collected from the Great Barrier Reef yielded the approximately 11 kDa toxins αD-VxXIIA, αD-VxXIIB and αD-VxXIIC.642 Substantially larger than other conus toxins, these homodimeric peptides exhibited potent inhibition of nicotinic acetylcholine receptors. The D-tryptophan-containing (contryphan) toxin Lo959 765 is an octapeptide isolated from Conus loroisii (India) that targets the high voltage-activated Ca2+ channel, leading to increased Ca2+ current.643
Comparative biological evaluation of the peptide contryphan-P, which differs at one amino acid residue and inhibits Ca2+ channel function, indicated that dramatic effects on biological activity can arise from subtle peptide sequence changes. The chemistry of sea hares and nudibranchs has been reviewed.613,644,645 Bisabolene-type sesquiterpenes 766–769 were isolated from extracts of the sea hare Aplysia dactylomela (Canary Islands).646
The unfortunate duplicate use of the trivial name aplysiadiol, attributed to 766, has more recently been changed to aplysiadactydiol.647 Confirmation of the structure and determination of absolute configuration of the moderately cytotoxic Dolabella auricularia metabolite auripyrone A 770648 has been achieved by total synthesis.649 The revised stereochemical assignments650 of (+)-dolastatin 19 771,651 and the originally proposed structure652 for dolabelide D 772653 (also D. auricularia metabolites), were confirmed by total synthesis.
The antitumour effects of aplyronine A, isolated from the sea hare Aplysia kurodai,654 were attributed to binding to actin which was investigated at the molecular level by X-ray analysis.655 The chemistry of nudibranchs from the west coast of North America has been reviewed.656 An anatomically guided chemical investigation of the Australian nudibranch Glossodoris atromarginata led to the isolation of 773 and 774 as natural products for the first time.657
Differential localisation of metabolites highlighted the ability of nudibranchs to sequester and modify chemicals obtained from dietary prey. Fluorescent analogues of norrisolide, originally isolated from the nudibranch Chromodoris norrisi,658 have been used to study Golgi vesiculation properties of norrisolide.659 Analogues of (−)-jorumycin have been prepared660 but were found to exhibit substantially less potent cytotoxicity than the parent natural product derived from the Pacific nudibranch Jorunna funebris.661 Chloro- and dichloro-lissoclimides, isolated from the nudibranch Pleurobranchus forskalii662 and the ascidian prey Lissoclinum voeltzkowi,663–665 inhibit protein synthesis, possibly by inhibition of translocation.666
11 Tunicates (ascidians)
While there was continued interest in the synthesis and biological investigations of ascidian-derived natural products in 2006, a steep decline in the number of new metabolites was noted. Palmerolide A 775 is a cytotoxic macrolide isolated from Synoicum adareanum (Palmer Station, Anvers Is., Antarctica).667 The compound inhibits vacuolar ATPase with an IC50 of 2 nM and was active in the NCI hollow-fibre assay. Four new members of the haterumaimide family of labdane alkaloids, N–Q 776–779, were reported from Lissoclinum sp. (Hateruma Is., Okinawa).668
Potent cytotoxicity was observed for 776 and 779, supporting the requirement of chlorine at C-2 and/or a free hydroxyl at C-7 for enhanced biological activity. The absolute configuration of the long chain polyacetoxy acyl amino acid sagittamide A, isolated from an unidentified ascidian (Pohnpei),669 has been studied by two groups who have come to different conclusions. Molinski's group, utilising combinations of model synthesis, Murata's J-based analysis, application of Kishi's universal database and CD spectrophotometry, concluded the absolute configuration was represented by 780.670 An alternative configuration 781 was proposed by Kishi, again utilising J-based coupling analysis, preparation of model compounds and conducting mixed doping NMR experiments with the natural product.671
Clearly, further studies are required to resolve this issue. Corrections to the structures and assignment of absolute configuration of iejimalides A 782, C 783 and D 784, cytotoxic macrolides isolated from Eudistoma cf. rigida (Okinawa),672,673 were achieved by analysis of 1H–1H NMR coupling data and preparation of a degradation product.674 The absolute configuration of iejimalide B 785, proposed earlier by application of similar techniques,675 has been confirmed by total synthesis.676
X-Ray analysis of a semi-synthetic derivative has afforded the absolute configuration of bistramide D 786,677 while the atomic resolution interaction of bistramide A with actin has also been studied by X-ray crystallography.678 The advanced Mosher method was used to secure the absolute configuration of the mildly cytotoxic prenylated quinone longithorone J 787,679 originally isolated from Aplidium longithorax.680 Biomimetic reasoning was then applied to propose a corresponding (16R) configuration for longithorone K 788.
The structure of the alkaloid rigidin E, isolated from Eudistoma sp. (Papua New Guinea),681 has been confirmed by total synthesis,682 as has the structure of lamellarin α 20-sulfate,683 isolated as an inhibitor of HIV-1 integrase from an unidentified ascidian.684 Racemic syntheses of haouamine A,685 originally sourced from Aplidium haouarianum (Tarifa Is., Spain),686 and floresolide B,687 isolated from Aplidium sp.,688 have been reported. The effects of sulfur-bridged alkaloids isolated from Lissoclinum cf. badium on interleukin-8 production by PMA stimulated HL-60 cells have been investigated, but wide variation of results precluded determination of any structure–activity correlation.689 An additional contributing factor to the antitumour activity exhibited by the ascidian depsipeptide aplidine was the observation that it oxidised and inactivated low molecular weight protein tyrosine phosphatase.690 Aspects of the structure–activity relationships of ecteinascidin alkaloids continue to be investigated, with studies reporting the preparation of simplified analogues691 and acyl derivatives of ET-770.692 The soft-tissue sarcoma activity of the parent clinical compound ET-743 has been reviewed.693 The search for new classes of inhibitors of aldose reductase has identified a number of ascidian-derived scaffolds as being active, including imidazole- and pyrazine-containing alkaloids, lukianol B and members of the rubrolide family.694 A structure–activity study of the cancer-preventive properties of 3-demethylubiquinone Q2 (glabruquinone A) isolated from Aplidium glabrum,695 has identified structural features required for inhibiting cell transformation and induction of p53-independent apoptosis.696 Two new congeners of lepadiformine (now designated A),697 lepadiformines B 789 and C 790, were isolated from Clavelina moluccensis (Musha Islands, Djibouti).698
All three alkaloids were found to prolong the action potential in frog atrial myocytes by blocking the background inward rectifying K+ current; potency varied with alkaloid structure. An analogue of the antimicrobial peptide halocidin, isolated from hemocytes of Halocynthia aurantium,699 demonstrates potent antifungal activity via formation of ion channels.700 Carotenoids derived from the related ascidian H. roretzi induced apoptosis in a number of tumour cell lines.701 One of the noted abilities of ascidians is to sequester Prochloron cyanobacterial cells, which are now implicated in the biosynthesis of biologically active cyclic peptides. The combinatorial biosynthetic capability of these symbionts has been reviewed.702 The epimerisation and macrocylisation steps required in the biosynthesis of the Prochloron-derived patellamide cyclic peptides are proposed to occur spontaneously from the mature linear octapeptide.703 Ascidians also exhibit proficiency in the biosynthesis of polycyclic aromatic alkaloids, many of which exhibit potent biological activities. A putative ecological role played by these compounds in Cystodytes sp. is to deter fish predation.704 A study of the temporal variation of pyridoacridine alkaloid production by Cystodytes sp. failed to identify any seasonal variation, but a minimum was observed in late summer, perhaps associated with reproductive exhaustion.705
12 Echinoderms
The number of new metabolites reported annually from echinoderms has remained relatively constant over the 2002–2006 period. Three amine salts of sulfated alkenes 791–793 were reported from extracts of the urchin Temnopleurus hardwickii (Qingdao, China).706 The absolute configuration of 791 was established by oxidative degradation to (2R)-2-methylpentanedioic acid, and the configurations of 792 and 793 were assumed based upon observation of similar magnitude and sign of optical rotations. Extracts of the starfish Luidia maculata (Hakata Bay, Japan) yielded two ceramide molecular species that were further analysed to identify ceramides LMCer-1-1 794, LMCer-2-1 795, LMCer-2-6 796 and LMCer-2-7 797 as the major components.707Asteriaceramide A 798 and asteriacerebroside G 799, isolated from the starfish Asterias amurensis (Hokkaido, Japan), promoted plant growth in sprouts of Brassica campestris.708 The galactocerebroside molecular species CNC-2 800 was isolated from a lipid extract of the starfish Culcita novaeguineae.709
In two separate accounts, trisaccharide-containing gangliosides CEG-3 801, CEG-4 802 and CEG-5 803710 and tetrasaccharides CEG-6 804, CEG-8 805 and CEG-9 806,711 were isolated from the sea cucumber Cucumaria echinata (Sea of Genkai, Japan).
All six compounds exhibited neuritogenic activity towards the PC-12 cell line in the presence of nerve growth factor (NGF). GP-3 807, an unusual ganglioside molecular species isolated from the starfish Asterina pectinifera (Japan), contains two moles of sialic acid in combination with galactose and arabinose sugars.712 Mild activity towards PC-12 cell neurite growth in the presence of NGF was observed. The same starfish species (Qingdao, China) afforded a pyrrole tetrasaccharide 808.713
The utility of FAB–CID–MS/MS analysis in the structure determination of saponin mixtures has been discussed.714 Four steroids 809–812 and a steroid glycoside leviusculoside J 813 were isolated from the starfish Henricia leviuscula (Sea of Okhotsk, Russia).715
Sidechain absolute stereochemistries of 809 and 810 were determined (Mosher). Both leviusculoside J 813 and 810 induced haemolysis of erythrocytes; the former was more potent. Linckosides F–K 814–819, isolated from Linckia laevigata (Okinawa, Japan), all enhanced the neuritogenic activity of NGF with PC-12 cells.716
Interestingly, only 814, 817 and 819 were observed to act as NGF-mimics in neurite outgrowth of PC-12 cells. Steroidal disaccharides milleporosides A 820 and B 821 were isolated from the starfish Fromia milleporella (Seychelles) and were moderately cytostatic towards fertilised sea urchin eggs.717 Mild cytotoxicity was observed for the steroidal pentasaccharide asterosaponin 822 isolated from the starfish Culcita novaeguineae (Sanya Bay, China).718
Triterpene glycosides nobilisides A–C 823–825, isolated from Holothuria nobilis (Dongsham Is., China), exhibited cytotoxicity towards a panel of tumour cell lines.719 In a very confusing development, the same authors have filed a patent for glycoside 826 as a potential antitumour agent, again using the trivial name nobiliside A.720 H. hilla (Zhao'an Gulf, China) was the source of the triterpene glycoside hillaside C 827,721 moderately cytotoxic towards a panel of tumour cell lines. The structurally related tetrasaccharides fuscocinerosides A–C 828–830 were reported as cytotoxic components of extracts of H. fuscocinerea (Leizhou Bay, China).722
Triterpene 830, the 17-dehydroxy derivative of the well known sea cucumber metabolite holothurin A,723 was also reported in a separate account by the same group as being isolated from H. impatiens.724 In the previous review in this series725 the structure of a new bioactive triterpene glycoside philinopside A726 (Pentacta quadrangularis) was shown, though the reader was cautioned that no spectroscopic data had been reported to support the structure. A full account of the isolation and structure elucidation of philinopsides A 831 and B 832 has now appeared,727 but the structure of 831 now bears a 10β methyl group as opposed to the earlier report bearing a 10α methyl. In both cases the steroidal aglycon has a 9β orientation. Two new members of the series, philinopsides E 833 and F 834, were also reported from the same organism.728 Pseudocolochirus violaceus (Sanya Bay, China) was the source of two glycosides that were ascribed the structures 835 and 836 and the trivial names violaceusides A and B, respectively.729 The structures presented are 9α isomers of philinopsides A 831 and F 834 and yet the 1H and 13C NMR data reported for the 9α/9β pairs of metabolites are essentially identical. The non-sulfated triterpene tetrasaccharides rollentosides A 837 and B 838 were isolated from extracts of Asterias rollentoni (Lianyungang, China).730
The structures of the aminated hydroxynaphthazarin sea urchin metabolites echinamines A and B731 have been confirmed by synthesis.732,733 The known sea cucumber triterpene glycoside metabolites neothyosides A734 and B735 and pervicoside B736 display leishmanicidal activity towards Leishmania mexicana as well as antifungal and cytotoxic properties.737
13 Miscellaneous
The structural basis of the interaction between tetrodotoxin and voltage-gated sodium channels has been reviewed.738 Apoptotic induction by cephalostatin 1, a bis-steroidal product of the marine worm Cephalodiscus gilchristi,739 involves an endoplasmic reticulum stress response accompanied by caspase-4 activation.740 The pharmacological utility of anabaseine, a nicotinic acetylcholine receptor activator isolated from the carnivorous marine worm Paranemertes peregrina, has been reviewed.741 Three water-borne, reproductive behaviour-modulating peptides released by the cuttlefish Sepia officinalis have been identified by the classical bioassay-directed isolation method and by MS/MS peptide screening.742 A new carotenoid, 2,3′-dihydroxycanthaxanthin 839, was isolated as a mixture of stereoisomers from the hermit clam Paralithodes brevipes.743 A number of new natural products were reported from various parts of marine mangrove plants. The Δ14-mexicanolide liminoids 840–843, isolated from the fruit of Xylocarpus granatum (Hainan Is., China), were given the trivial names of xylogranatins A–D respectively.744 Unfortunately, the same trivial names were attributed to 844–847, isolated from the seeds of the same species (Hainan Is., China) of mangrove.745
Mild cytotoxicity was observed for all four compounds. The sixth example of a 3β,8β-epoxymexicanolide-skeletoned limonoid, xyloccensin W 848, was identified in extracts of the fruit of X. granatum (Wenchang, China).746 The study also led to the reassignment of NMR data described for the desacetyl co-metabolite xyloccensin K. In three separate accounts, a further six limonoids were reported from mangroves of the genus Xylocarpus, unfortunately with some duplication in assignment of trivial names. Xyloccensins X1 849, X2 850, Y 851, Z1 852 and Z2 853 were isolated from the fruits of X. granatum (Wenchang, China),747,748 while metabolites 854 and 855, isolated from X. molluccensis (India), were given the trivial names xyloccensin X and Y respectively.749
Two triterpene coumaroyl esters, 856 and 857, were reported from stem bark extracts of Barringtonia racemosa (China).750 In addition to known iridoids and flavones, iridoid glycosides 858 and 859 were isolated from aerial parts of the mangrove plant Avicennia marina (Wenchang, China).751,752
Both compounds exhibited weak antioxidant activity in the DPPH radical-scavenging assay. Deoxyrhizophorin A 860 was identified as a new secolabdane diterpene constituent of Avicennia officinalis Linn. (Andhra Pradesh, India).753 In addition to the well known dithiolane sulfoxides brugierol and isobrugeriol, the dithiosulfonate bruguiesulfurol 861 was isolated from a chloroform extract of the flowers of Bruguiera gymnorrhiza (Samutsongkram Province, Thailand).754
In a panel of bioassays, including COX-1, COX-2, antioxidant response element and NF-κB, designed to discover cancer preventative agents, 861 was found to be less active than the sulfoxide congeners. In a separate account, the same source material yielded the dammarane-skeletoned triterpenes bruguierins A–C 862–864.755
All three natural products activated antioxidant response element with micromolar potency, while only 862 inhibited NK-κB activation and COX-2 activity.
14 Conclusion
The database used for the compilation of this review, MarinLit,55 includes the taxonomic origins of the source organisms. The 60–70 years of activity in the marine natural products field has resulted in 4796 distinct genera/species being recorded in the database. Table 1 shows the number of articles published for compounds from each phylum. At the domain level <5% of the species examined originated from the Bacteria, while >95% are Eukaryota in origin (Table 1). It is noteworthy that just three phyla, Porifera, Cnidaria and Chromophyta, constitute ∼60% of the species examined, and that if a further five phyla are added (Mollusca, Rhodophycota, Chordata, Cyanobacteria, Echinodermata), nearly 94% of all of the species recorded are accounted for. The remaining 6% are scattered across another 14 phyla (Table 1). The phyla that have attracted the most attention are predominantly sessile benthic organisms. Members of these phyla are often conspicuous due to size, abundance or colour. How important is this in considering the “popularity” of these phyla? Certainly size and abundance are important when the amount of sample needed for analysis is high, and colour is an advantage at the time of collection. If the search for biological activity from marine organisms is a factor then the focus on these phyla is relevant. Since the 1980s the NCI has collected over 13![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
500 marine invertebrates and 6000 plants across the Pacific, South Pacific and Indo-Pacific regions.756 An analysis of about 50% of this collection was performed in the early 1990s by Dr Peter Murphy using cytotoxicity data provided from the NCI.757 This analysis established that marine invertebrates were a superior source of bioactive extracts. However, other specific marine phyla [Phaeophyta (Pha), Rhodophycota (Rho), Chlorophyta (Chl)], in addition to members of the Animalia [Bryozoa (Bry), Chordata (Cho), Cnidaria (Cni), Echinoderma (Ech), Mollusca (Mol), Porifera (Por)], also showed a higher probability of returning bioactive extracts (see Fig. 1).756,757 It is likely that it is a combination of biological potential in combination with factors such as abundance, size and colour has driven the selection process, leading to the current distribution of phyla sampled. If these sampling data are combined with the estimated numbers of known species on Earth the current preference for certain phyla becomes even more skewed (see Table 1). A global estimate of around 1.74 million appears to be the accepted figure for the total number of known species.758 A breakdown of the individual phyla and the percentage of each phylum that has been sampled to date are shown in Table 1. There are just five phyla where >5% of the estimated known species have been examined to date. From this it is immediately obvious that in the sampling of the marine phyla over the decades there are many “forgotten” phyla. Indeed, many phyla that have been sampled regularly in the past now also appear to have become “forgotten”; for example, bryozoa and green and brown algae, as noted earlier in this review. In the past these phyla have been well sampled and been a source of bioactive extracts (Fig. 1). In considering “forgotten” phyla it is also worth noting that of the 30 phyla in the kingdom Animalia with marine habitats there are records in MarinLit for representatives of only 12 phyla. It is obvious that in the oceans there are a large number of named organisms whose chemistry has yet to appear in the marine literature. Perhaps representatives of these “forgotten” phyla have been sampled and not reported because the chemistry was not novel, or no bioactive compounds were isolated? However, it is more likely that those phyla have yet to be sampled, possibly due to preoccupation with the “popular” phyla. For groups seeking new marine chemistry, a productive search strategy might be to target the “forgotten” phyla.
Table 1 Analysis of the distribution of articles on marine phyla recorded in MarinLit
| Domain | Kingdom | Phylum | No. of articles in MLita | % of Articles in MLitb | No. of compounds in MLit | No. of species listed in MLitc | Est. no. of known species on Earthd | Est. % of species examinede |
|---|---|---|---|---|---|---|---|---|
| a Number of articles for each phylum recorded in MarinLit.55 b Number of articles for each phylum in MarinLit expressed as a percentage of all articles. c Actual number of species noted in MarinLit for each phylum. d Estimated number of known species on Earth for each phylum.758 e Percentage of species for each phylum examined based on estimated total number for that phylum.758 | ||||||||
| Eukaryota | Animalia | Porifera | 5681 | 33.37% | 6668 | 1,185 | 5500 | 21.55% |
| Eukaryota | Animalia | Chordata | 1374 | 8.07% | 977 | 356 | 3000 | 11.87% |
| Eukaryota | Plantae | Rhodophycota | 1431 | 8.41% | 1548 | 563 | 6000 | 9.38% |
| Eukaryota | Animalia | Cnidaria | 2307 | 13.55% | 3469 | 642 | 10000 |
6.42% |
| Eukaryota | Animalia | Hemichordata | 66 | 0.39% | 26 | 17 | 300 | 5.67% |
| Eukaryota | Animalia | Echinodermata | 764 | 4.49% | 1074 | 301 | 7000 | 4.30% |
| Eukaryota | Plantae | Chlorophyta | 465 | 2.73% | 315 | 287 | 8000 | 3.59% |
| Bacteria | Eubacteria | Cyanobacteria | 833 | 4.89% | 602 | 118 | 4000 | 2.95% |
| Eukaryota | Animalia | Bryozoa | 309 | 1.82% | 180 | 63 | 5000 | 1.26% |
| Eukaryota | Protista | Chromophyta | 1934 | 11.36% | 1473 | 641 | 80000 |
0.80% |
| Eukaryota | Animalia | Mollusca | 1568 | 9.21% | 1179 | 445 | 70000 |
0.64% |
| Bacteria | Eubacteria | Proteobacteria | 7 | 0.04% | 3 | 29 | 4760 | 0.61% |
| Eukaryota | Animalia | Brachiopoda | 2 | 0.01% | 0 | 2 | 400 | 0.50% |
| Eukaryota | Animalia | Annelida | 89 | 0.52% | 49 | 40 | 15000 |
0.27% |
| Eukaryota | Protozoa | Euglenozoa | 25 | 0.15% | 36 | 15 | 40000 |
0.04% |
| Eukaryota | Animalia | Platyhelminthes | 60 | 0.35% | 33 | 7 | 25000 |
0.03% |
| Eukaryota | Animalia | Nematoda | 9 | 0.05% | 2 | 7 | 25000 |
0.03% |
| Eukaryota | Fungi | Ascomycota | 25 | 0.15% | 48 | 19 | 80000 |
0.02% |
| Eukaryota | Animalia | Arthropoda | 72 | 0.42% | 23 | 57 | 1065000 |
0.01% |
| Eukaryota | Plantae | Trachaeophyta | 0 | 0.00% | 0 | 0 | 272655 |
0.00% |
| Eukaryota | Plantae | Bryophyta | 0 | 0.00% | 0 | 0 | 15000 |
0.00% |
| Archaea | — | — | 0 | 0.00% | 0 | 0 | 259 | 0.00% |
| Eukaryota | Chromista | Labyrinthulomycota | 2 | 0.01% | 3 | 2 | — | — |
| Bacteria | Eubacteria | Bacteroidetes | 1 | 0.01% | 4 | 1 | — | — |
19![[thin space (1/6-em)]](https://www.rsc.org/images/entities/b_char_2009.gif) 178
|
18248
|
4796 |
1741874
|
|||||
 | ||
| Fig. 1 Percentage of species for a range of marine phyla whose extracts have bioactivities in the 1–5 µg mL−1 range (data extracted from the NCI preclinical antitumour drug discovery screen).756,757 | ||
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