Enhancing stereochemical diversity by means of microwave irradiation in the absence of solvent: synthesis of highly substituted nitroproline esters via 1,3-dipolar reactions.

Microwave irradiation induces the 1,3-dipolar cycloaddition of imines derived from alpha-aminoesters with beta-nitrostyrenes in the absence of solvent within 10-15 min. The reaction proceeds to give yields in the range 81-86% and three isomeric pyrrolidines are obtained in the cycloaddition. Consequently, the use of three imines and two beta-nitrostyrenes gives rise to a library of 18 nitroproline esters. The use of classical heating with longer reaction times (24 h) gives lower yields of products (below 50%) and only two stereoisomers can be detected in each reaction.