Nucleophilic attack on 4-aminomethyleneoxazol-5(4H)-ones, a rationalisation of penicillin carcinogenicity

4-Aminomethyleneoxazol-5(4H)-ones are shown to undergo nucleophilic attack at the methylene carbon on the side chain rather than at the ring carbonyl group. Penicillenic acid, formerly reported to react exclusively at the carbonyl, follows this general pattern when intramolecular nucleophilic attack by the thiol group is inhibited, a reaction of possible importance in penicillin carcinogenicity.