Synthesis of isoxazoline N-oxides and its application in the formal synthesis of dehydroclausenamide

A protocol for the synthesis of isoxazoline N-oxides and its application in the formal synthesis of dehydroclausenamide are described. Sulfonium salt 1 reacts with substituted nitroalkenes smoothly to generate isoxazoline N-oxides in high to excellent yields with dr higher than 99/1. Its asymmetric version has been developed by using cinchona alkaloid-derived ammonium salts 6a and 6b instead of sulfonium salt 1 and higher than 96% ee values are achieved. This method has also been successfully applied to the formal synthesis of dehydroclausenamide.