Bifunctional Catalysts for Asymmetric Nazarov Cyclizations

Significance

An organocatalytic asymmetric Nazarov cyclization is presented. The catalyst, a bifunctional structure with a thiourea moiety, and a primary amine function is suggested to activate the substrate by a double hydrogen-bonding ­interaction (eq. a). Although the reaction proved to be slow, the provided products, containing all-­carbon quaternary stereocenters, were synthesized in high yields and enantioselectivities. It would be interesting to speculate a mechanism involving truly covalent enamine and iminium intermediates, as such providing an explanation for the low turnover due to product inhibition (eq. b).