Stereoselective Lewis acid mediated (3+2) cycloadditions of N-H- and N-sulfonylaziridines with heterocumulenes

Alkyl and aryl isothiocyanates and carbodiimides are effective substrates in (3+2) cycloadditions with N-sulfonyl-2-substituted aziridines and 2-phenylaziridine for the synthesis of iminothiazolidines and iminoimidazolidines. Additionally, the stereoselective (3+2) cycloaddition of N-H- and N-sulfonylaziridines with isothiocyanates can be accomplished, allowing for the synthesis of highly enantioenriched iminothiazolidines. Evidence for an intimate ion-pair mechanism is presented herein in the context of these chemo-, regio-, and diastereoselective transformations. The demonstrated ability to remove the sulfonyl group from the heterocyclic products displays the utility of these compounds for further derivatization and application. Coordinate, release, and expand: A Lewis acid coordinated process by which the release of strain energy drives the ring expansion of aziridines to five-membered heterocycles through a stereoselective (3+2) cycloaddition with alkyl and aryl isothiocyanates and carbodiimides is presented. These zinc(II)-mediated reactions exhibit broad substrate scope, high yields, and well-defined chemo- and regioselectivity.