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

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