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.