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 exo-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 exo-Diels–Alder products could be obtained in good yields and with high exo-stereoselectivities.