Importance of the C(3) substituent of the penam derivative in interconversion reactions of penam and cepham systems

Abstract

The azetidinone disulphide (1b), a structural analogue of Kamiya's disulphide (1a), has been synthesized. Some cyclization reactions of the disulphides (1)(Br₂ in CH₂Cl₂ and AcOAg in ClCH₂CO₂H–CH₂Cl₂) leading to penam and cepham derivatives through episulphonium ions (2), have been studied. The data obtained show that changing the substituent X in (1a) and (1b) brings about changes in the distribution of the positive charge in the intermediate episulphonium ions (2a) and (2b), and thus affects to some extent the regioselectivity of the episulphonium ring opening and the chemical behaviour of (1a) and (1b).