Biosynthetic Pathway and Origin of the Chlorinated Methyl Group in Barbamide and Dechlorobarbamide, Metabolites from the Marine Cyanobacterium Lyngbya majuscula

• Published in: Tetrahedron Vol. 56; no. 46; pp. 9103 - 9113
• Main Authors: Sitachitta, Namthip, Márquez, Brian L., Thomas Williamson, R., Rossi, James, Ann Roberts, Mary, Gerwick, William H., Nguyen, Viet-Anh, Willis, Christine L.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 10-11-2000
• Subjects: biological chlorination; biosynthesis; peptide; polyketide; trichloromethyl group; biological chlorination; peptide; trichloromethyl group; polyketide; biosynthesis
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(00)00763-8

Structural and biosynthetic studies have been conducted on the barbamide class of molluscicidal agent. Dechlorobarbamide was isolated from a Curaçao 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, defining the absolute configuration of barbamide as 2
S,7
S. 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-[
2H
10]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-
13C
2]acetate and [1-
13C, 1-
18O]acetate confirmed 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–CH
3 and O–CH
3 groups of
1 originate from the C
1 pool; this was supported by enrichment in these methyl groups when cultures were provided with
l-[methyl-
13C]methionine.