Lewis Acid Catalysis Alters the Shapes and Products of Bis-Pericyclic Diels−Alder Transition States

The reactions of cyclopentadiene with α-keto-β,γ-unsaturated phosphonates or with nitroalkenes proceed through an unsymmetrical bis-pericyclic transition state to give both Diels−Alder and hetero-Diels−Alder cycloadducts. The change in periselectivity of the Lewis acid catalyzed reactions can be qualitatively rationalized by the change in the potential energy surface (PES) landscapes, which ultimately affects the branching ratio of these bis-pericyclic reactions.