Enantioselective [4+2] Cycloaddition of o-Quinone Methides and Vinyl Sulfides: Indirect Access to Generally Substituted Chiral Chromanes

A catalytic asymmetric [4 + 2] cycloaddition of ortho-quinone methides (o-QMs) is described. With the readily available vinyl sulfides as the key 2 pi partner and a properly chosen chiral phosphoric acid catalyst, the reaction proceeded under mild conditions to form the corresponding adduct with high enantio- and diastereoselectivity. Owning to the easy removal and conversion of the sulfenyl group in the product, the present process provides indirect access to generally substituted chromanes previously lacking easy access. Mechanistically, the reaction is also a new demonstration of the rarely achieved sole activation of o-QM for asymmetric control.