Structural and biosynthetic studies have been conducted on the barbamide class of molluscicidal agent. Dechlorobarbamide was isolated from a CuracËao collection of the marine cyanobacterium Lyngbya majuscula and its structure determined through spectroscopic analysis and comparisons with barbamide. The absolute stereochemistry of the dolaphenine moiety of barbamide was determined to be S, de®ning the absolute con®guration of barbamide as 2S,7S. Stable isotope feeding experiments conducted with cultured L. majuscula have provided clear evidence that barbamide biosynthesis involves chlorination of the unactivated pro-R methyl group of leucine. Experiments with l-[2 H10]leucine demonstrated that chlorination of the pro-R methyl occurs without detectable activation via the leucine-catabolic pathway. Moreover, an extremely high level of incorporation of fed [2-13C]-5,5,5-trichloroleucine into barbamide indicates that leucine is the probable substrate for the chlorination reaction. Incorporations of [1,2-13C2]acetate and [1-13C, 1-18O]acetate con®rmed the origins of C-5 and C-6 whereas incorporation of l-[3-13C]phenylalanine supported the hypothesis that the phenyl group and its three carbon side-chain in barbamide (C-7, C-8 and C-10±C-16) arise from phenylalanine. The thiazole ring (C-17±C-18) of 1 was shown to likely arise from cysteine through a [2-13C, 15N]glycine feeding experiment. Detection of intact 13C±15N bond was observed by application of a new GHNMBC NMR experiment. Results from this latter feeding experiment also indicated that the N±CH3 and O±CH3 groups of 1 originate from the C1 pool; this was supported by enrichment in these methyl groups when cultures were provided with l-[methyl-13C]methionine. q 2000 Elsevier Science Ltd. All rights reserved.