Biosynthetic pathway and origin of the chlorinated methyl group in barbamide and dechlorobarbamide, metabolites from the marine cyanobacterium Lyngbya majuscula

Structural and biosynthetic studies have been conducted on the barbamide cl ass of molluscicidal agent. Dechlorobarbamide was isolated from a Curacao c ollection of the marine cyanobacterium Lyngbya majuscula and its structure determined through spectroscopic analysis and comparisons with barbamide. T he absolute stereochemistry of the dolaphenine moiety of barbamide was dete rmined to be S, defining the absolute configuration of barbamide as 2S,7S. Stable isotope feeding experiments conducted with cultured L. majuscula hav e provided clear evidence that barbamide biosynthesis involves chlorination of the unactivated pro-R methyl group of leucine. Experiments with L-[H-2( 10)]leucine demonstrated that chlorination of the pro-R methyl occurs witho ut detectable activation via the leucine-catabolic pathway. Moreover, an ex tremely high level of incorporation of fed [2-C-13]-5,5,5-trichloroleucine into barbamide indicates that leucine is the probable substrate for the chl orination reaction. Incorporations of [1,2-C-13(2)]acetate and [1-C-13, 1-O -18]acetate confirmed the origins of C-5 and C-6 whereas incorporation of L -[3-C-13]phenylalanine supported the hypothesis that the phenyl group and i ts 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 aris e from cysteine through a [2-C-13, N-15]glycine feeding experiment. Detecti on of intact C-13-N-15 bond was observed by application of a new GHNMBC NMR experiment. Results from this latter feeding experiment also indicated tha t the N-CH3 and O-CH3 groups of 1 originate from the CI pool; this was supp orted by enrichment in these methyl groups when cultures were provided with L-[methyl-13C]methionine. (C) 2000 Elsevier Science Ltd. All rights reserv ed.