Generation, electrocyclic ring opening, and unprecedented conversion of a 3-acylaminoazetinone into cis-3,4-disubstituted azetidinones

An attempt to synthesize the nucleus of cefaclor by insertion of an appropr iately functionalized rhodium carbene into a beta -lactam N-H bond leads in stead to attack at sulfur, followed by fragmentation into a four-membered t hietanone and a four-membered 3-acylaminoazetinone. The azetinone undergoes electrocyclic ring opening, with a calculated half-life of 11 s at 40 degr eesC, to a trans-disubstituted vinyl isocyanate, but can be trapped by satu rated, unsaturated allylic and benzylic alcohols to form 3,4-disubstituted azetidinones in which the cis- isomer predominates.