Conversion reactions of cepham into penam systems. A route to determine the relative configurations of two diastereoisomeric 3-bromo-4-methoxycepham derivatives

The p-nitrobenzoates I were stereospecifically transformed with AgOAc-AcOH into the penam derivs. II. Oxidn. of II yielded the (S)- and (R)-sulfoxides. The (R)-sulfoxide of the α-acetoxymethyl isomer was thermally converted into the (S)-sulfoxide of the β-acetoxymethyl isomer. The configurations of II and the sulfoxides were assigned on the basis of 1H NMR studies and mechanistic considerations. The exclusive formation of II from I implies the intermediate formation of the diastereoisomeric α- and β-episulfonium ions, which undergo regiospecific attack of the nucleophile on the methylene carbon