Bronsted Acid-Catalyzed, Highly Enantioselective Addition of Enamides to In Situ-Generated ortho-Quinone Methides: A Domino Approach to Complex Acetamidotetrahydroxanthenes

The highly enantioselective conjugate addition of enamides and enecarbamates to insitu-generated ortho-quinone methides, upon subsequent N,O-acetalization, gives rise to acetamido-substituted tetrahydroxanthenes with generally excellent enantio- and diastereoselectivities. A chiral BINOL-based phosphoric acid catalyst controls the enantioselectivity of the carbon-carbon bond-forming event. The products are readily converted into other xanthene-based heterocycles.