Substituent Effect of Dipole and Dipolarophile in the Reaction of Nitrones with N-(Substituted phenyl)maleimides

The reactions of C-(p-substituted phenyl)-N-(p-substituted phenyl) nitrones with N-(p-substituted phenyl)maleimides were carried out in THF to give both trans- and cis-isoxazolidines. The formation ratios of trans- to cis-isoxazolidines (IT⁄Ic) were determined by the NMR spectra of the cycloadduct mixtures. Though the values of IT⁄Ic varied with the electronic characters of the substituents, R1, R2, and R3, they were always found to be larger than unity.