The geometries of three conformers of N-4-pentenylnitrone (Ra, Rb, and Rc) were optimized with the aid of the energy gradient technique. The geometries of two transition states (Ta and Tb) were obtained by Komornicki's method. The reaction paths were also determine The conformers Ra, Rb and Rc are all stable, but only the first two are active in the intramol. cycloaddn., generating regioisomers Pa and Pb via transition states Ta and Tb, resp. Actually, intramol. cycloaddn. of N-4-pentenylnitrone consists of two parallel reactions. Because the activation barrier of the reaction from Rb to Pb is lower than that from Ra to Pa, the higher yield of the former is predicted. This is consistent with experiments The mechanism of the two reactions are similar. All the calcns. were performed on semiempirical MINDO/3 MO method.

Key words: QUANTUM CHEMISTRY, SELECTIVITY, CYCLOADDITION REACTION, TRANSITION STATE THEORY, CONFIGURATION, INTERMEDIATE NEGLECT OF DIFFERENTIAL OVERLAP APPROXIMATION (IND, NITROKETONE, PENTENYL GROUP