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 CC bond isomerization followed by electrocyclic ring opening of the formed azacyclobutenes (=1,2‐dihydroazetes; cf. Scheme 3 ).