Diastereoisomers of an 'arsenomethionine'-based structure from Sargassum lacerifolium: The formation of the arsenic-carbon bond in arsenic-containingnatural products

Separation and H-1 NMR spectra of a pair of arsenic-containing diastereoiso mers (1a and 1b) isolated from a brown alga has provided support for their structures (proposed on the basis of NMR spectra of the unseparated mixture ). The diastereoisomerism and analogies with nitrogen-containing algal lipi ds indicated that they were derived from an analogue of methionine in which the dimethylarsinoyl- group had replaced amino. Although S-adenosylmethion ine is probably the source of methyl and 5'-deoxyribos-5'-yl groups in arse nic-containing natural products, the arsenic-carbon bonds in some compounds might be formed by a process in which arsenic replaces nitrogen in amino-a cid synthesis. (C) 2000 Elsevier Science Ltd. All rights reserved.