Enantioselective Synthesis of 4-Hydroxy-dihydrocoumarins via Catalytic Ring Opening/Cycloaddition of Cyclobutenones.

A highly diastereo- and enantioselective ring-opening/cycloaddition reaction of cyclobutenones with 2-hydroxyacetophenones or salicylaldehyde was achieved by employing a chiral N,N'-dioxide-scandium(III) complex as the catalyst. It provided various 3-phenylvinyl-4-hydroxy-dihydrocoumarins in good yields (up to 92%), high enantioselectivities (up to 93% ee), and excellent diastereoselectivities (>19:1 dr). Moreover, a possible catalytic cycle was proposed based on the control experiments and previous reports.