The regiospecificity of intramol. N-3-butenyl nitrone cycloaddn. has been investigated by semiempirical MO AM1 method and energy gradient technique. The calculated results show that the cycloaddn. consts of two similar parallel reactions in which the conformers (Ra and Rb) of N-3-butenyl nitrone generate the regioisomers Pa and Pb via transition states TSa and TSb resp. The geometries of these reactants, TS and products were optimized on the potential energy surface, the regiospecificity is determine by the ratio of the rate constants of the two reactions. The value is 124.1, which is in good agreement with experiments The determing factors of the rate ratio come from the joint contribution of the activation enthalpy and activation entropy.

Key words: REACTION MECHANISM, CYCLOADDITION REACTION, TRANSITION STATE THEORY