Bioactive natural products from marine algae

This thesis is an account of my investigation on the natural products deriving from various marine algae and has resulted in the discovery of thirteen novel bioactive metabolites. Isolation and characterization of these unique molecules were carried out using different chromatographic techniques and by careful analyses of 1D and 2D NMR data, respectively.; Two new thiazole-containing cyclic heptapeptides, cis,cis- and trans,trans-ceratospongamide, were identified from an Indonesian red alga, Ceratodictyon spongiosum Zanardini. These are a pair of stable conformers differing only in the conformation of two proline amide bonds. The trans,trans-form was revealed to be a potent anti-inflammatory agent in the inhibition of sPLA₂ expression in a cell-based model.; The hermitamides are new malyngamide-type molecules isolated from a collection of the marine cyanobacterium Lyngbya majuscula Gomont made in Papua New Guinea. The semi-syntheses of these natural metabolites were achieved by coupling the free-occurring C-14 lyngbic acid isolated from the microalga and the commercially available aromatic amines. Two new semi-synthetic molecules, the N-methylated derivatives of the hermitamides, were also prepared. The biological properties of the hermitamides and synthetic derivatives were evaluated in several bioassays.; The organic extract of another collection of L. majuscula Gomont from Papua New Guinea was a prolific source of several novel cyclic depsipeptides, given the trivial names clairamides A and B, wewakamide, carliamide, and guinamides A and B. These molecules were characterized by having unique β-amino or β-hydroxy acid moieties, one of which is being reported for the first time from the marine environment. The absolute stereochemistry in clairamide A of one such unit, 2-methyl-3-amino-pentanoic acid, is discussed. Clairamide B and carliamide exhibited moderate cytotoxic activities.; Investigation of another species of filamentous marine cyanobacterium, L. bouillonii Hoffmann and Demoulin from Papua New Guinea yielded three novel cytotoxic metabolites. These were two cyclic depsipeptides, bouillonamides A and B, and a glycosidic macrolide, lyngbouilloside. The cytotoxic activities of these molecules were evaluated in the neuro-2a mouse neuroblastoma cell assay. The isolation of these metabolites demonstrated that L. bouillonii is a rich source of novel and biologically active natural products.