conveniently prepared in moderate or good yields by direct 1,3-dipolarcycloaddition. Isoxazolidines 5-7 derived from fluorinated N-vinyl nucleobases and N-alkyl-methylenenitrones were obtained with complete regiocontrol and stereocontrol and the reactions proceeded with full transfer of the dipolarophile geometry to the cycloadduct. The cycloadditionreactions of N-alkyl-trifluoromethylnitrones with N-vinyl