“One-Pot” Access to 4<i>H</i>-Chromenes with Formation of a Chiral Quaternary Stereogenic Center by a Highly Enantioselective Iminium-allenamine Involved Oxa-Michael−Aldol Cascade

An unprecedented organocatalytic highly enantioselective cascade Michael−aldol reaction has been developed in high yields under mild reaction conditions. The “one-pot” process affords an efficient approach to the synthetically and biologically important chiral 4H-chromenes bearing a quaternary stereogenic center. The study significantly expands the scope of less explored organocatalytic iminium-allenamine chemistry.