The course by which valent orbitals of reactants evolve into valent orbitals of product in a Diels-Alder reaction has been discussed by using a three-orbital interaction method, and the evolutive result is in agreement with the principle of conservation of orbital symmetry. The rationality of the evolutive course has been approved by a quantum chemistry calculation, which indicates that the principle of conservation of orbital symmetry is more logical than frontier molecular orbital theory in interpreting a D-A reaction.

Key words: Diels-Alder reaction, principle of conservation of orbital symmetry, mechanism, orbital interaction, frontier molecular orbital theory