N-Alkenyl nitrone dipolar cycloaddition routes to piperidines and indolizidines. Part 6. Allylic stereocontrol in the intramolecular cyclisation 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.