A variety of nucleophiles, including amines, thiolates, and alkoxides, were employed to open the aziridinium ions Az. The latter are opened stereospecifically and regioselectively at the C-3 position by a wide range of amines, and thiolate nucleophiles attack predominately at the C-2 position. Poor regioselectivities (ca. 1:1) were observed using nucleophiles derived from phenols, carboxylic acids, and imides. Base-mediated ring closure of the aziridinium opening products, from primary amines, gave beta-lactams and a 1, 5-benzodiazepin-2-one in high yields.