Tandem double-Michael-addition/cyclization/acyl migration of 1,4-dien-3-ones and ethyl isocyanoacetate: stereoselective synthesis of pyrrolizidines.

Up to four adjacent stereocenters can be formed stereoselectively in the construction of a pyrrolizidine unit through a novel organocatalytic reaction that involves treatment of various dienones with ethyl isocyanoacetate (see scheme; DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). Mechanisms for this atom-economic, one-pot synthesis have been proposed.