Synthesis of Spirocyclopropyl γ‐Lactams by a Highly Stereoselective Tandem Intramolecular Azetidine Ring‐Opening/Closing Cascade Reaction

Abstract A new tandem intramolecular azetidine ring‐opening/closing cascade reaction affording spirocyclopropyl γ‐lactams in high regio‐ and stereoselectivity is reported. The key step of the process is an S N 2‐type ring‐opening of TMSOTf‐activated azetidine rings by silyl ketene acetals generated by treatment with TMSOTf and triethylamine. This study is a very rare example of nucleophilic ring opening of azetidines that does not require formation of quaternary azetidinium salts by N ‐alkylation or the use of N ‐electron‐withdrawing groups.