The regioselectivity of intramol. N-4-methyl-4-pentenylnitrone 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 reactions is calculated to be 5.04 (at 383.75 K), which is in good agreement with experiments In comparison of this ratio with that of N-4-pentenylnitrone cycloaddn., which is 0.83, it is realized that N-4-Me substituent changes the regioselectivity notably. The main reason is that the difference between the activation entropy of the two regioisomers is enhanced by introducing an N-4-Me group.

Key words: REACTION MECHANISM, COMPUTER APPLICATIONS, THERMODYNAMIC FUNCTION, CYCLOADDITION REACTION, PENTENE P, REACTION RATE CONSTANT, BRIDGE COMPOUNDS, GEOMETRICAL ISOMERISM, NITRONE, REGIOSELECTIVITY