Catalytic Asymmetric [3 + 2] Cycloaddition of Exocyclic Enol Ethers for the Synthesis of Spiroketals

An efficient synthesis of chiral benzannulated spiroketals via catalytic asymmetric [3 + 2] cycloaddition of exocyclic enol ethers with p-quinones was achieved. The transformation was enabled by a chiral N,N'-dioxides/Tm^III complex as the Lewis acid catalyst and afforded a series of enantiomerically enriched benzannulated spiroketal derivatives in good yields (up to 99%) and enantioselectivities (up to 98% ee). Topographic steric maps and distribution of the buried volumes of the catalysts via Cavallo's SambVca 2 tool were used to elucidate the enantioinduction raised by the ligands and the metal ions.