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

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.