The regioselectivity of intramol. N-4-hexenyl nitrone cycloaddn. has been investigated by AM1 MO method and transition state theory. Two isomeric products and corresponding transition states were located by energy gradient technique. The ratio of the rate constants of these two parallel reaction (kb/ka) is calculated to be 25.46, which is in good agreement with experiments In comparison of this ratio with that of N-4-pentenyl nitrone cycloaddn., which is 1.21, it is realized that N-5-Me substituent changes the regioselectivity notably. The main reason is that the electronic effect of N-5-Me makes the activation enthalpy of reaction b reduced and the configuration change by introducing an N-5-Me group makes the activation entropy of reaction b increased.

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