Computational study of intermolecular formal oxa-[3Â +Â 3] cycloaddition reactions

Density functional theory (DFT) calculations employing B3LYP/6-31G(d,p) method have been carried out to characterize the mechanisms of the title reactions. The obtained results indicate that the mechanisms of oxa-[3 + 3] cycloaddition reactions include three typical steps, namely the addition, the elimination and the cyclization, similar to those of the aza-[3 + 3] cycloaddition reactions between vinylogous amides and α,β-unsaturated imine cations reported previously. And the differences between these two kinds of reactions involved in different steps have been elucidated in this paper.