The preparation and antimicrobial activity of a series of beta-lactams (3a-f) are described. These compounds were prepared from the 2+2 cycloaddition of beta,beta-disubstituted enamines with aryl isocyanates; compounds 3a-f underwent facile beta-lactam ring fission between aminal carbon atom C4 and the lactam nitrogen N1. The resisting formylacetanilide derivatives were devoid of antibiotic activity.