The chemistry of 4-mercaptoazetidin-2-ones. Part 1. Preparation and properties of (3R,4R)-4-mercapto-3-phenoxyacetamidoazetidin-2-one

(3R,4R)-4-Mercapto-3-phenoxyacetamidoazetidin-2-one (4) has been prepared in good yield from the penicillin V-derived thiazoline-azetidinone (1). Alkylation and acylation led exclusively to S-substituted azetidinones. Reaction with dimethyl acetylenedicarboxylate in hexamethylphosphoramide gave, in addition to the expected dimethoxycarbonylvinylthioazetidinone (15), the C-4-epimer (18). The latter probably arises via the intermediacy of the azetinone (16).