Organocatalytic Asymmetric Formal [4 + 2] Cycloaddition of in Situ Oxidation-Generated ortho-Quinone Methides and Aldehydes

An unprecedented chiral secondary amine-catalyzed formal [4 + 2] annulation of aldehydes and oxidation-generated β-unsubstituted o-QMs is reported. This asymmetric protocol allows direct functionalization of the benzylic C–H bonds and furnishes [4 + 2] cycloadducts, chromanols, with excellent enantioselectivity and in up to 92% yield. The usability of this approach was further demonstrated by the enantioselective synthesis of anticancer Rhinacanthins derivative NKPLS8.