Diastereoisomers of an ‘arsenomethionine’-based structure from Sargassum lacerifolium: the formation of the arsenic–carbon bond in arsenic-containing natural products

Separation and 1H NMR spectra of a pair of arsenic-containing diastereoisomers (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 lipids indicated that they were derived from an analogue of methionine in which the dimethylarsinoyl- group had replaced amino. Although S-adenosylmethionine is probably the source of methyl and 5′-deoxyribos-5′-yl groups in arsenic-containing natural products, the arsenic–carbon bonds in some compounds might be formed by a process in which arsenic replaces nitrogen in amino-acid synthesis.