THEORETICAL-STUDY ON THE REGIOSELECTIVITY OF INTRAMOLECULAR N-ALKENYLNITRONE CYCLOADDITION .4. THE REGIOSELECTIVITY OF INTRAMOLECULAR N-4-HEXENYL NITRONE CYCLOADDITION

The regioselectivity of intramolecular N-4-hexenyl nitrone cycloadditi on has been investigated by AM1 MO method and transition state theory. Two isomeric products and corresponding transition states were locate d by energy gradient technique. The ratio of the rate constants of the se two parallel reaction (k(b)/k(a)) is calculated to be 25.46, which is in good consistence with experiments. In comparison of this ratio w ith that of N-4-pentenyl nitrone cycloaddition, which is 1.21, it is r ealized that N-5-methyl substituent changes the regioselectivity notab ly. The main reason is that the electronic effect of N-5-methyl makes the activation enthalpy of reaction b reduced and the configuration ch ange by introducing an N-5-methyl group makes the activation entropy o f reaction b increased.