Selective C3-Allylation and Formal [3 + 2]-Annulation of Spiro-Aziridine Oxindoles: Synthesis of 5′-Substituted Spiro[pyrrolidine-3,3′-oxindoles] and Coerulescine

Brønsted acid- and/or Lewis acid-catalyzed selective C3-allylation and formal [3 + 2]-annulation of spiro-aziridine oxindoles with allylsilanes have been demonstrated to deliver direct access to 3-allyl-3-aminomethyl oxindoles and 5-silyl methyl spiro­[pyrrolidine-3,3′-oxindoles], respectively. The acid-catalyzed methods do not provide any stereoselectivity when chiral spiroaziridines are used. However, the reaction of nonracemic sprioaziridines with allyl-Grignard reagent under catalyst-free conditions afforded 3-allyl-3-aminomethyl oxindoles with good stereoselectivity (ee up to 80%). The allylation protocol is utilized for the short synthesis of coerulescine and various 5′-substituted spiro­[pyrrolidine-3,3′-oxindoles].