Enantioselective Formal [4 + 2] Annulation of ortho-Quinone Methides with ortho-Hydroxyphenyl α,β-Unsaturated Compounds.

The highly efficient enantioselective [4 + 2] cycloaddition of o-QMs with ortho-hydroxyphenyl-substituted α,β-unsaturated compounds was realized by using a chiral N, N'-dioxide-Sc(III) complex as catalyst. A variety of chiral chromane derivatives with three continuous stereocenters were obtained in excellent results (up to 99% yield, >19:1 dr, and 99% ee) under as low as 0.005-1 mol % catalyst loading. Besides, a catalytic cycle with a possible transition state model was proposed to elucidate the origin of the chirality.