An Efficient Stereoselective Synthesis of 4,5-<i>trans</i>-1,5-<i>cis</i>-3-Oxabicyclo[3.1.0]hexan-2-ones via the Iodolactonization of Alkylidenecyclopropyl Esters

A highly stereoselective iodolactonization of alkylidenecyclopropyl esters with iodine or N-iodosuccinimide (NIS) in aqueous CH3CN to afford 4,5-trans-1,5-cis-3-oxabicyclo-[3.1.0]hexane-2-ones is described. The reaction is very general, accommodating a wide range of substituents. A plausible mechanism that explains the essential role of water in this reaction is proposed.