B(C₆F₅)₃: A New Class of Strong and Bulky Lewis Acid for <i>Exo</i>-Selective Intermolecular Diels–Alder Reactions of Unreactive Acyclic Dienes with α,β-Enals

Lewis acid B­(C₆F₅)₃ catalyzed the Diels–Alder reactions of multisubstituted open-chain dienes and α,β-enals to afford the desired products with high <i>exo</i>-selectivities are reported. The substituent effect of the dienes and dienophiles on the product’s stereoselectivity was thoroughly investigated, and it was found that most of the desired <i>exo</i>-Diels–Alder products could be obtained in good yields and with high <i>exo</i>-stereoselectivities