Syntheses based on 1,2-secopenicillins. Part 5. Penicillin analogues with electron-withdrawing 2-substituents

6-Acylaminopenam-3-carboxylic esters substituted at C-2 by one or two alkoxycarbonyl groups or by an acetyl group have been prepared from penicillins. The azetidinone ring remained intact throughout the reaction sequence, and the thiazolidine ring was completed by intramolecular alkylation at an activated methylene or methine group adjacent to the sulphur atom. Only in the case of the 2-alkoxycarbonylpenams could stable carboxylic acids be liberated from the esters.