Mechanistic Investigation of the Ring Opening in the Staudinger Cycloaddition Involving Ketenes with Electron‐Withdrawing Substituents

The cycloaddition of ketenes and imines (Staudinger cycloaddition) is a general method for the synthesis of various β‐lactams. However, reactions of imines and ketenes with electron‐withdrawing substituents produce α,β‐unsaturated alkenamides, ring‐opening products of the intermediates generated from imines and the ketenes, even as sole products, besides the desired β‐lactams. The mechanism of the formation of α,β‐unsaturated alkenamides was investigated. The results indicate that the α,β‐unsaturated alkenamides are generated via a base‐induced C?C bond isomerization followed by electrocyclic ring opening of the formed azacyclobutenes (=1,2‐dihydroazetes; cf. Scheme 3).