tert-Butyl hypoiodite (t-BuOI) was found to be a powerful reagent for the cyclization of N- alkenylamides leading to a variety of N-heterocycles under extremely mild conditions. When N-alkenylsulfonamides were employed in the reaction, three-to six-membered saturated N- heterocycles were obtained in good to excellent yields with complete stereoselectivity. The method was applicable to the cyclization of alkenylbenzamide derivatives to afford N-, O- ...