Enantioselective Cycloadditions of Azomethine Ylides

Abstract

The asymmetric 1,3-DCR of azomethine ylides, which is generated from the corresponding imino ester and alkenes, is one of the most fascinating transformations because the configuration of the four new stereogenic centers of the finally obtained proline can be absolutely established in only one step with total atom economy. Since 2002, the catalyzed enantioselective 1,3-DCRs have been performed using chiral metal complexes. For example, chiral silver and copper complexes are the most employed catalysts. Silver complexes afforded selectively endo-cycloadducts, however, both exo- and endo-adducts were generated in the presence of several chiral Cu(I) complexes. Although chiral zinc complexes have also been studied in endo-selective processes, the published works are not so numerous. Bidentate ligands, such as bisphosphanes, nitrogenated phosphanes, and sulfur-containing phosphanes have shown very high enantioselectivity levels. Apart from the employment of chiral Lewis acids, the utilization of chiral bases or organocatalysts are also known, albeit with a large number of limitations and, in some cases, with lower enantioselections.