Rare Gold-Catalyzed\n4-<i>exo</i>-<i>dig</i> Cyclization for Ring Expansion\nof Propargylic Aziridines\ntoward Stereoselective (<i>Z</i>)‑Alkylidene Azetidines,\nvia Diborylalkyl Homopropargyl Amines

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