Rare Gold-Catalyzed 4-- Cyclization for Ring Expansion of Propargylic Aziridines toward Stereoselective ()-Alkylidene Azetidines, via Diborylalkyl Homopropargyl Amines.

We report an uncommon 4-- cyclization of -tosyl homopropargyl amines, catalyzed by [AuCl(PEt)]/AgOTf, to prepare stereoselective ()-2-alkylidene-1-tosylazetidine compounds. The reaction outcome contrasts with the gold-catalyzed cyclization of -tosyl homopropargyl amines containing a methyl group at the propargylic position that provides substituted 2,3-dihydropyrroles via a 5-- mechanism. The access to -tosyl homopropargyl amines is possible by the regioselective nucleophilic attack of α-diboryl alkylidene lithium salts to propargylic aziridines.