Effect of Brønsted acid co-catalyst in asymmetric conjugate addition of 3-aryloxindoles to maleimide under base-free phase-transfer conditions

Highly diastereo- and enantioselective conjugate addition of 3-aryloxindoles to maleimide was accomplished under base-free phase-transfer conditions. The effect of Brønsted acid co-catalyst was examined, and we found that the use of a phenol derivative as co-catalyst was effective for obtaining the target conjugate adduct in good yield with high stereoselectivity.