Copper(II) Bromide/Boron Trifluoride Etherate-Cocatalyzed Cyclization of Ketene Dithioacetals and p-Quinones: a Mild and General Approach to Polyfunctionalized Benzofurans

A new application of copper(II) bromide-activated ketene dithioacetals as nucleophiles in organic chemistry has been developed. Under the cocatalysis of copper(II) bromide (2.0 mol%) and boron trifluoride etherate (10 mol%), the conjugate addition and sequential cyclization of alpha-electron-withdrawing group-substituted ketene dithioacetals with p-quinones in acetonitrile at room temperature gave a variety of benzofurans. This formal [3+2] cycloaddition provides a general method for catalytic synthesis of polyfunctionalized benzofurans with the advantages of readily available starting materials, cheap catalysts, mild reaction conditions, good yields and wide range of synthetic potential for the benzofuran products. Further transformations of the resulting benzofurans to 2-aminobenzofurans and benzofuro[2,3-d]pyrimidine derivatives are also investigated.