This thesis presents an investigation of the 4-methoxypyrrolic constituents of two South African marine invertebrates, the nudibranch Tambja capensis and the bryozoan Bugula dentata. Three known compounds tambjamine A (7), tambjamine E (13) and the tetrapyrrole (15) were isolated during this investigation. All three compounds were shown to be active against oesophageal cancer in accordance with the general anticancer and immunosuppressive properties observed for 4-methoxypyrrolic natural products. Tambjamine A (7), tambjamine E (13) and the tetrapyrrole (15), together with tambjamine K (21) and L (22) (previously isolated in our laboratory) were used as standards to quantitatively assess the presence of these tambjamines in T. capensis and B. dentata collected from three different sites along the South African coast. This study confirmed that B. dentata is the source of the 4-methoxypyrrolic natural products sequestered by T. capensis and eliminated the closely related bryozoan B. neritina as a source of these metabolites. The paucity of tambjamine L (21) and K (22) obtained in previous investigations of the sequestered chemistry of T. capensis prompted an attempt at the development of synthetic methodology that could be used to synthesize tambjamines in sufficient yield for in depth bioactivity studies. In order to by pass the extensively reported problems associated with the synthesis of this group of compound 3-methoxy-2-formylpyrrole (47), readily accessible from 3-methoxypyridine N-oxide (48), was used as the starting material in a singlet oxygen induced 2,2’ bipyrrole coupling reaction. Although 47 proved unreactive in this coupling reaction, when the N-Boc protected analogue of 47 was used, and the reaction worked up in the dark, the novel methyl 4-aza-5-oxo-6,6-di-(2-pyrrolyl)-2(Z)-hexenoate (57) was obtained in low yield. The physical properties of tambjamine (E) (13) and the tetrapyrrole (15) were investigated to further the understanding of the proposed oxidative DNA cleavage mechanism and to determine the potential of the 4-methoxypyrrolic natural products as photodynamic therapy agents.

This thesis is divided into two parts. The first part (Chapter Two) documents a bioassay guided investigation of the ethyl acetate extracts of four marine invertebrates from Mozambique (an Irciniid sponge, a Haliclona sp. sponge, an ascidian tentatively identified as Diplosoma sp., and the soft coral Cladiella kashmani). Eight known compounds [ilimaquinone (2.1), renierone (2.7), N-formyl-1,2-dihydrorenierone (2.8), 1,6-dimethyl-7-methoxy-5,8-dihydroisoquinoline-5,8-dione (2.9), mimosamycin (2.10) 7Z-allylidene-5-hydroxy-7,7a-dihydro-2H-cyclopenta[b]pyran-6-one (2.11), flaccidoxide (2.18) and 11S,12S-epoxycembra-1Z,3E,7E-trien-14S-ol (2.19)] and a new diterpene [13S,14R-diacetoxy-11S,12R-epoxycembra-1Z,3E,7E-triene (2.20)] were isolated and identified using standard spectroscopic techniques. Anomalies in the published spectral data of 2.1 and 2.8 were exposed and corrected, and the absolute stereochemistry of the cembrane diterpenes 2.18 and 2.20 established using the modified Mosher’s method. The comparative activities of the nine natural products against four cancer cell lines (A549, LOX, OVCAR3, SNB19) are reported. The second part of the thesis (Chapter Three – Chapter Six) is concerned with an ecological, structural and synthetic study of diterpenes from the endemic South African pulmonate limpet Trimusculus costatus. Two new labdane diterpenes [6b,7a-diacetoxylabda-8,13E-dien-15-ol (3.10) and 2a,6b,7a-triacetoxylabda-8,13E-dien-15-ol (3.11)] were isolated from T. costatus and evaluated for anti-feeding activity against the common predatory fish Pomadasys commersonnii. A strategy for the semi-synthesis of 3.10 from rhinocerotinoic acid (4.14), a diterpene reportedly present in the ubiquitous South African shrub Elytropappus rhinocerotis, was devised in order to allow further bioactivity tests to be performed and unequivocally assign the unknown absolute stereochemistry of the T. costatus metabolites. Attempts to isolate rhinocerotinoic acid from local specimens of Elytropappus rhinocerotis were unsuccessful, and as the repetition of a published synthesis of 4.14 from (-)-sclareol (4.15) gave rhinocerotinoic acid in unacceptably low yields with poor stereoselectivity, an improved synthesis of 4.14 is presented. Comprehensive studies using hispanone (5.1) as a model compound showed that 6,7-dioxygenated labda-8-enes could be prepared from compounds possessing a 7-oxo-labda-8-ene skeleton with some degree of stereocontrol. In the process, fourteen new hispanone analogues were prepared and most of these were tested for activity in a suite of ten agro-chemical assays. The novel compound 7b-hydroxy-9a-carbonitrile-15,16-epoxylabda-13(16),14-dien-6-one (5.34) exhibited significant activity against the crop fungus Phytophthora infestans and is currently being subjected to further agro-chemical tests. Unfortunately, the results from the oxygenation study performed on the model compound 5.1 could not be directly extrapolated to rhinocerotinoic acid. Attempts to prepare the naturally occurring 3.10 from 4.14 via an alternative route were unsuccessful but yielded an analogue of 3.10 in which the substituents at C-6 and C-7 are in a diequatorial rather than a diaxial configuration.

This thesis describes the chemical and biological investigation of the extracts of six different marine invertebrate organisms collected along the South African coastline. The work on these extracts has resulted in the isolation and structural elucidation of twenty-one previously undescribed secondary metabolites; The history of marine natural product chemistry in South Africa has not previously been reviewed and so a comprehensive review covering the literature from the 1940's up until the end of 1995 is presented here. The marine ascidian Pseudodistoma species collected in the Tsitsikamma Marine Reserve was shown to contain four new unsaturated amino alcohols [47], [48], [49] and [50] which were isolated as their acetyl derivatives. These compounds exhibited strong antimicrobial activity. Four new pyrroloiminoquinone alkaloids, the tsitsikammamines A [90] to D [93],were isolated from a new genus of Latrunculid sponge collected in the Tsitsikamma Marine Reserve. These highly pigmented compounds also possessed strong antimicrobial activity. An investigation of two phenotypic colour variants of the soft coral Capnella thyrsoidea resulted in the isolation of the known steroid 5α-pregna-1, 20-dien-3-one [97] and an additional six new metabolites, 16β-hydroxy-5α-pregna-1 ,20-dien-3-one 16-acetate [98], 3α,16β-dihydroxy-5α-pregna-1, 20-diene 3,16-diacetate [99] and four xenicane diterpenes, the tsitsixenicins A [100] to D [103]. This is the first reported isolation of xenicane diterpenes from the soft coral family Nephtheiidae. Tsitsixenicin A and B showed good anti-inflammatory activity by inhibiting superoxide production in both rabbit and human cell neutrophils. A further four new metabolites were isolated from two soft corals which could only be identified to the genus level and were designated Alcyonium species A and species B. Alcyonium species A was collected in the Tsitsikamma Marine Reserve and yielded two new polyhydroxysterols, cholest-5-ene-3β, 7β, 19-triol 19-acetate [121] and cholest-5,24-diene-3β, 7β, 19-triol 19-acetate [122]. The soft coral Alcyonium species B was collected off Aliwal Shoal and was found to contain two known xenicane diterpenes, 9-deacetoxy-14, 15-deepoxyxeniculin [110] and zahavin A [16], and two new xenicane diterpenes, 7 -epoxyzahavin A [123] and xeniolide C [124]. Compounds [110], [16] and [123] exhibited strong anti-inflammatory activity and compounds [110] and [16] showed good antithrombotic activity. The endemic soft coral A/cyanium fauri collected at Riet Point near Port Alfred yielded the new sesquiterpene hydroquinone rietone [141] in high yierd, fogether with the minor compounds 8'-acetoxyrietone [142] and 8'-desoxyrietone [143]. Rietone exhibited moderate activity in the NCl's in-vitro anti-HIV bioassays.

A chemical investigation of six different marine invertebrates, collected along the South African coastline, resulted in the isolation and structural elucidation of fifteen previously undescribed secondary metabolites along with seven known compounds. The structures of the new metabolites were determined by a combination of spectroscopic and chemical methods. The endemic false limpet Siphonaria capensis was shown to contain two unusual polypropionate metabolites capensinone (162) and capensifuranone (163) as well as 2,4,6,8-tetramethyl-2-undecenoic acid (164) and the known polypropionates (E)- and (Z)siphonarienfuranone (149 and 161). Capensinone is the first example of a marine polypropionate containing a cyc1opentenone moiety. An investigation of the endemic South African soft coral Pieterfaurea unilobata yielded six new, highly oxygenated, pregnadiene sterols (180-185) and the known metabolite (169). Compounds 180-185 are the first pregnadienes obtained from the marine environment containing a C-7 substituent. An alternative procedure for the quick assignment of the absolute configuration at C-3 in this series of compounds was proposed. A companson of the pyrroloiminoquinone alkaloids of three undescribed l'}trunculid sponges resulted in the isolation of 3-dih¥drodiscorhabdin C (243), 3-dihydrodiscorhabdin B (244), discorhabdin H (197) and the previously reported alkaloids discorhabdin A (189) and discorhabdin D (192). While all three sponges were found to be morphologically different they all contained discorhabdin A as the major metabolite and discorhabdin H as one of their minor metabolites. It was found that a feature common to most of the South African latrunculid sponges is the reduction of the C-3 carbonyl gr,o up in some of the minor metabolites. The indole alkaloids, dilemmaones A-C (261-263), containing an unusual cyc1opentanone-indole skeleton, were isolated in trace amounts by bioassay guided fractionation of an extract obtained from a mixed collection of sponges collected near Cape Town. In an attempt to acquire more of these novel compounds for further investigation of their biological activity, several synthetic strategies towards their total synthesis were explored. A key feature of these approaches was the exploitation of the regioselective Gassman's artha-alkylation procedure for the introduction of an aromatic methyl substituent.

One of the major impediments in the development of marine natural products is the provision of biologically active natural products in sufficient quantity for complete pharmacological evaluation, clinical trials and eventual commercial production. Marine microorganisms show great promise in providing a renewable source of biologically active natural products. The main aim of this study was to develop and evaluate methods for the isolation, identification and cultivation of marine fungi from the South African marine environment for the production of biologically active secondary metabolites. Twenty-four species of fungi were isolated from marine algae collected from the intertidal zone near Port Alfred, South Africa. The fungi were cultivated in small-scale under static and agitated conditions and their crude intra- and extracellular organic extracts were screened by ¹H NMR and a series of bioassays. Using this as a basis, one isolate was selected for further study. By analyses of the lTS1 region of the ribosomal DNA, the fungal isolate was identified as a marine-derived isolate of Eurotium rubrum (Aspergillus ruber). Although E. rubrum has been isolated from the marine environment, no investigations have been undertaken to determine the adaptation of these isolates to the marine environment. In order to optimise productivity, creativity and incubation time, the fungus was cultivated in small-scale using a variety of carbon (glucose, fructose, lactose, sucrose, marmitol and maltose) and nitrogen sources (ammonium tartrate, urea, peptone and yeast extract). An HPLC-DAD method was developed to assess the metabolic creativity and productivity under different fermentation conditions. Distinctive variations in the range and yield of metabolites produced as well as morphology and growth time were observed. The crude extracts from all fermentations were combined and six known compounds were isolated by reversed-phase chromatography and their structures elucidated by spectroscopic techniques. The known compounds were fIavoglaucin, aspergin, isodihydroauroglaucin, isotetrahydroauroglaucin, neoechinuline A and physcion. Neoechinuline A, isodihydroauroglaucin and isotetrahydroauroglaucin showed activity against oesophageal and cervical cancer cell lines.