Aspects of the chemistry of dehydromethionine
Dehydromethionine [(1R,3S)-S-methylisothiazolidine-3-carboxylate] is shown to be a useful intermediate for the preparation of methyl-labelled methionines via its base-catalysed exchange in [2H4]methanol or methan[2H]ol. However, it is not possible to effect stereoselective exchange of protons at C-5 of dehydromethionine using sodium[2H3]methoxide–[2H4]methanol. A complete analysis of the 1H n.m.r. spectrum of dehydromethionine has been achieved by computer-assisted simulation and by comparison with the spectra of 2H-labelled dehydromethionines. Dehydromethionine is converted, by treatment with aqueous sodium hydroxide, mainly into the (S,S)-sulphoxide of methionine. This result can be rationalised by postulating a trigonal bipyramidal intermediate having in-line OH and NH attached to sulphur. Syntheses of stereochemically distinct [3,4-2H2]methionines are described.