3,3′‐diaryl‐5‐morpholino‐4,5,4′,5′‐tetrahydro‐4,5′‐spirobi[isoxazoles]. Synthesis by cycloaddition reactions and substituent effect on the cycloaddition kinetics

3‐Aryl‐4‐methylene‐5‐morpholino‐4,5‐dihydroisoxazoles 1a‐e were synthesized; fifteen different 3,3′‐diaryl‐5‐morpholino‐4,5,4′,5′‐tetrahydro‐4,5′‐spirobi[isoxazoles] 3 were obtained by their reaction with some stable aryl nitrile oxides. The spiro‐derivatives were characterized by their nmr spectra. Kinetic measurements showed that substituents on the nitrile oxide have a weak effect on the cycloaddition rate (Hammett ' p = ca 0.3), while substituents on the dipolarophile have no effect at all.