Reexamination of orbital interactions in Diels–Alder reactions

An application of the perturbation theory to the [4+2] addition of butadiene has demonstrated that the effect of secondary orbital interactions should be much less significant than has been assumed within the frontier orbital scheme. This has been confirmed by a numerical analysis with respect to the endo transition state of the Diels–Alder reaction between butadiene and maleic anhydride.