Synthesis of Benzofuranones via Malonates Desymmetrization: Yield Increase by the Portion-wise Addition of Quinones.

The organocatalyzed addition of several malonates to 1,4-benzoquinones affords benzofuranones bearing a quaternary stereocenter with good enantioselectivity. This reaction is an intramolecular desymmetrization since it proceeds through the formation of an arylated achiral malonate that cyclizes to give the reaction product. The addition rate of the quinone dramatically affects the reaction yield which was originally low. The yield was considerably increased, in some cases, from less than 20 % to over 95 %, by adding the quinone in portions rather than at once, keeping similar enantioselectivity. A possible rationalization for the preferential formation of the indicated enantiomer has been investigated by DFT calculations.