Indole Alkaloids

The majority of marine indole alkaloids are rather simple compounds. However, some of the indole alkaloids carry unique structural features. Bacteria and algae have yielded halogenated simple indoles, while more complicated structures have been isolated from marine snails, sponges, bryozoans, etc.123126 Strong antibacterial and antiyeast activities were associated with the red alga Laurencia brongniartii.123 The alga yielded the antibacterial agent (71). The compound has been characterized by its 1H and 13C NMR spectra. The proton at C7 in (71) exhibits substantial solvent-dependent downfield shift.

Six unique bisindole alkaloids have been isolated from the blue-green alga Rivularia firma exhibiting antiinflammatory activity. The major alkaloid (72) (m.p. 239240 °C) is responsible for the antiinflammatory effect.127 Sponges have yielded brominated tryptamines, 5,6-dibromotryptamine (73) and Nb-methyl-5,6-dibrmotryptamine (74) are obtained from the sponge Polyfibro spongia australis. The bromoindole derivatives showed in vitro Gram-negative, Gram-positive antibacterial activity, and platelet aggregation inhibitory activity.128

Carboline Alkaloids

Rinehart and others have searched systematically pharmaceutically useful compounds from marine species.129133 Initial efforts have been involved

surveying of antibacterial and antifungal organisms, with bioassays carried out on shipboard. Later on, focus was shifted to antiviral and cytotoxic compounds. The most active antiviral extract in shipboard testing was obtained from a tunicate subsequently identified as Eudistoma olivaceum, but the extract was surprisingly found inactive in the secondary assay. When the tunicate was recollected, its extracts proved very active in both primary and secondary assay against Herpes simplex and other viruses. The Eudistoma extract yielded eudistomins variously substituted P-carbolines. All the compounds isolated had antimicrobial or antiviral activity, but the most potent are the tetracyclic eudistomins with the unique oxathiazepine ring (eudistomins C, E, K, L).130133 Their structures are assigned by spectroscopic techniques (high resolution, FABMS, high resolution EIMS, 13C NMR, UV) and by partial synthesis. Eudistomins C, E, K, and L have been assigned the structure (75), (76), (77) and (78), respectively.129,134 The stereochemistry of eudistomins (76-78) has been determined by NMR,135 and the absolute stereochemistry of eudistomin K (77) was determined by single crystal X-ray structural, analysis of its 4-bromobenzyl derivative.136,137 A new manzamine related tetrahydro-p-carboline alkaloid with a methylene bridge between N-2 and N-27 and named eghedin-A was isolated from an Okinawan marine sponge Amphimedon sp. Its structure was determined on the basis of spectroscopic data. The compound exhibited antibacterial activity against Sarcina lutea (MIC = 2.8 g/mL), and Corynobacterium xerosis (5.7 g/mL) and showed cytotoxicity against murine leukemia L 1210 cells (IC50 = 4.4 g/mL).136

Complex Indole Alkaloids

A deep water Caribbean sponge Dragmacidon sp. has yielded an alkaloid named dragmacidin (79) which inhibits the growth of several cancer cell lines.138 Dragmacidin (79) contains two tryptamine units and an unoxidized piperizene ring which had not been found before in marine natural products. The pacific sponge Hexadella sp.139 collected from the coast of British Columbia afforded dragmacidon A (80) another member of the group, exhibits potent cytotoxicity.

Fascaplysin (81),140143 a blood red pigment obtained from a Fijian sponge Fascaplysinopsis sp. Fascaplysin (81), exhibits antimicrobial and cytotoxic properties.144 Chartelline A (82) a unique ^-lactam indole alkaloid, which exhibits cytotoxicity in vitro against KB, and PS cell lines, has been isolated from a marine bryozoan Chartella papyracea collected in North Britanny waters.145 The biosynthesis of this new class of alkaloid appears quite interesting. It will be worth to know whether chartelline-A (82) is a true bryozoan metabolite or produced by some microorganism.

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Five flustramine derivatives,146-49 dihydroflustramine-C (83), and its Noxide flustramine-D (84) and its N-oxide and isoflustramine have been isolated from bryozoan Flustra foliaceae.146 Flustramines are tricyclic indole alkaloids responsible for the antimicrobial activity of the methylene chloride soluble fraction of aqueous methanolic extract of the bryozoan. Oxidation of dihyro flustramine-C (83) and D (84) with m-chloroperbenzoic acid yielded the corresponding N-oxide, (85) and (86), respectively.

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