Phosphine-mediated enantioselective [4 + 1] annulations between ortho-quinone methides and Morita–Baylis–Hillman carbonates

Abstract

The first example of phosphine catalyzed enantioselective [4 + 1] annulations of ortho-quinone methides and Morita–Baylis–Hillman carbonates is described. In the presence of 1,2-bis[(2R,5R)-2,5-dimethylphospholano]benzene monoxide, Morita–Baylis–Hillman carbonates reacted with ortho-quinone methides smoothly to afford a series of optically active 2,3-dihydrobenzofurans in 32–67% yields with 88–94% ee and >20 : 1 dr. When (−)-1,2-bis[(2R,5R)-2,5-dimethylphospholano]benzene was employed as a catalyst, the [4 + 1] annulation between ortho-quinone methides and Morita–Baylis–Hillman carbonates furnished chiral 2,3-dihydrobenzofurans in 65–90% yields with 79–87% ee and >20 : 1 dr. This protocol represents a facile strategy for the synthesis of biologically important enantioenriched 2,3-dihydrobenzofurans.