N-ALKENYL NITRONE DIPOLAR CYCLOADDITION ROUTES TO PIPERIDINES AND INDOLIZIDINES .6. ALLYLIC STEREOCONTROL IN THE INTRAMOLECULAR CYCLIZATION OF MONOSUBSTITUTED NITRONES

The intramolecular, thermal dipolar cycloadditions of the (Z)-N-alk-4-enyl nitrones 18-21, 34 and 35 bearing a single, allylic substituent were investigated. Certain alkoxy substituted nitrones 18-21 showed a remarkable preference for the formation of axially substituted isoxazolidines 22a-24a, whereas the propyl and trifluoromethyl substituted nitrones 35 and 34 gave the equatorially substituted cycloadducts 37 and 36a respectively, consistent with the involvement of 'chair-like' transition states 38.