Switchable Diastereoselectivity in Enantioselective [4+2] Cycloadditions with Simple Olefins by Asymmetric Binary Acid Catalysis

The exchange of the metal ion from InIII to ScIII in a binary acid catalyst leads to a switch in the diastereoselectivity of [4+2] cycloadditions. Simple olefins and β,γ-unsaturated α-ketoester were thus transformed to the corresponding exo or endo products, respectively, with excellent diastereoselectivity (up to 99:1 d.r.) and enantioselectivity (up to 99 % ee).