Axially chiral amidinium ions as inducers of enantioselectivity in Diels-Alder reactions

Enantioselective catalysis of Diels-Alder reactions is mostly achieved by coordinating the dienophile to relatively strong chiral Lewis acids. Here we report on a novel approach employing the hydrogen bond mediated association of dienophiles to chiral host molecules, In a reaction forming the steroid skeleton of norgestrel, chiral amidinium ions induce 5:ent-5 ratios of up to 2,5:1. Improved and simplified amidinium catalysts may become interesting candidates to perform stereoselective transformations.