Merging Chiral Brønsted Acid/Base Catalysis: An Enantioselective [4 + 2] Cycloaddition of o-Hydroxystyrenes with Azlactones.

An enantioselective [4 + 2] cycloaddition of o-hydroxylstyrenes with azlactones has been established by merging chiral Brønsted acid (chiral phosphoric acid) and base (chiral guanidine) catalysis, which constructed a biologically important dihydrocoumarin scaffold in an efficient and enantioselective style (up to 99% yield, 96:4 er). This approach has not only realized the successful application of o-hydroxylstyrenes as oxa-diene precursors in catalytic asymmetric cycloadditions but also established a new cooperative catalytic system of chiral phosphoric acid and chiral guanidine.