| Marine cyanobacteria are prolific producers of structurally diverse secondary metabolites that possess potent biological activity. Two important structural classes are cyclic peptides, which contain highly modified amino acids and polyketides. A key aspect in structural determination of cyanobacterial secondary metabolites is assignment of absolute stereochemistry. This dissertation focuses on the structural assignment of four families of secondary metabolites with cyanobacterial origin.; The anti-fungal cyclic peptides lobocyclamide A, B and C isolated from the Bahamian cyanobacterial sand of Lyngbya confervoides possess a number of beta-substituted amino acid residues, including beta-hydroxyleucine, beta-hydroxythreonine and aminodecanoic acid (or beta-aminodecanoic acid). To determine the absolute stereochemistry a novel enantioselective synthesis of beta-hydroxyleucine and beta-hydroxythreonine was accomplished by a (-)-sparteine mediated aldol reaction in over 50% ee.; Caylobolide A, a cytotoxic 36-membered ring macrolide isolated from a Bahamian cyanobacterial sand mat of Lyngbya majuscula is reported in chapter three. Caylobolide A has the novel structural fragment of a tetrad of repeating 1,5-diols as well as a 1,3,5-triol fragment. The planar structure of caylobolide A was established by a combination of 2D NMR experiments, while stereochemistry was established using Kishi's Universal NMR database and Mosher's analysis. Due to the lack of methods to determine the absolute stereochemistry of long chain 1,5-diols, such as those encountered in caylobolide A, a novel exciton coupling CD (ECCD) method has been developed for assignment of the relative and absolute configurations of long-chain syn- and anti-1,5, 1,7- and 1,9-glycols of the corresponding bis-5-(4'-carboxyl)-5,10,15,20-tetraphenylporphyrin esters measured in unilamellar liposomes.; Chapter four focuses on the isolation and structural determination of two trichloroleucine containing secondary metabolites, herbacic acid and neodysidenin, isolated from the symbiotic relationship between the sponge Dysidea herbacea and the cyanobacterium Oscillatoria spongeliaea .; Chapter five reports the isolation and determination of stereochemistry of five cytotoxic macrolides, phorbasides A--E. The phorbasides contain the interesting structural elements of a trans-chlorocyclopropane ring as well as a 3-C-methyl sugar residue. The relative stereochemistry of the phorbasides was established based on ROESY correlations and coupling constants. The absolute stereochemistry of the sugar residue was determined by exciton coupling circular dichroism of a derivatized evalose residue. |
