Studies related to penicillins and cephalosporins. Part I. The preparation of 4-alkylthio-β-lactams in which the ring nitrogen atom is part of an α-amino-acid ester system

Abstract

Alkyl thiobenzimidates in which the nitrogen atom is derived from the methyl ester of glycine (7), DL-valine (8), or didehydrovaline [(14) and (19)] undergo cycloaddition with diphenylketen to give the respective N-substituted 4-alkylthio-3,3,4-triphenylazetidin-2-ones (20)–(23). The methyl thiobenzimidate derivative (14) of didehydrovaline was conveniently prepared by ring cleavage of methyl 5,5-dimethyl-2-phenyl-Δ²-thiazoline-4-carboxylate (16) followed by S-alkylation of the intermediate thioenolate ion (13). The 2-methoxycarbonylethyl thiobenzimidate (19) was obtained in an exchange reaction with (14). The interconversion of the thiazoline ester (16) and 4-isopropylidene-2-phenyl-Δ²-thiazolin-5-one (12) and the base-induced degradation of 4-(2-methoxycarbonyl-ethylthio)-1-(1-methoxycarbonyl-2-methylprop-1-enyl)-3,3,4-triphenylazetidin-2-one (23) are described.