Cycloadditions of mesitonitrile oxide with amino- and nitrostilbenes†

2-Amino, 4-amino- and 2-nitrostilbenes react with mesitonitrile oxide affording the 5-substituted phenyldihydroisoxazole as the more abundant regioisomer, while 3-amino and 3-, 4-nitro derivatives give comparable amounts of the two regioisomers. Reactions of 2-acylamino derivatives show a lower regioselectivity and furthermore no solvent effect was found for the regiochemistry of 2-aminostilbene and its N-acetyl derivative. These experimental results indicate that reactions of 2-aminostilbene and its N-acyl derivatives are not governed by hydrogen bonding or steric effects. They are not explainable by the frontier molecular orbital theory, but semiempirical PM3 calculations performed on regioisomeric transition structures gave values of regioisomeric ratios well in accord with those experimentally observed.