Cycloadditions of mesitonitrile oxide with amino- and nitrostilbenes

2-Amino, 4-amino- and 2-nitrostilbenes react with mesitonitrile oxide affor ding the 5-substituted phenyldihydroisoxazole as the more abundant regioiso mer, while 3-amino and 3-, 4-nitro derivatives give comparable amounts of t he two regioisomers. Reactions of 2-acylamino derivatives show a lower regi oselectivity and furthermore no solvent effect was found for the regiochemi stry of 2-aminostilbene and its N-acetyl derivative. These experimental res ults indicate that reactions of 2-aminostilbene and its N-acyl derivatives are not governed by hydrogen bonding or steric effects. They are not explai nable by the frontier molecular orbital theory, but semiempirical PM3 calcu lations performed on regioisomeric transition structures gave values of reg ioisomeric ratios well in accord with those experimentally observed.