Direct introduction of a benzoyloxy substituent at the ring junction of fused β-lactam rings

Treatment of the 7-phthalimido-3-methyl-ceph-3-em-4-carboxylate (1) and 6-phthalimido-penicillins (4) with t-butyl perbenzoate in the presence of copper(I) chlride gave compounds bearing a benzoyloxy substitutent at the ring junction of the fused β-lactam rings, (2) and (5) respectively.