Nucleophilic Asymmetric Epoxidation Catalyzed by Cyclic Guanidines

Asymmetric nucleophilic epoxidation of chalcone and its derivatives was studied using chiral cyclic guanidine compounds.Pentacyclic guanidine 1a, which has a characteristic closed-type cavity, catalyzed the reaction to give epoxides with 39-60% ee.The newly developed tricyclic guanidine 4 drastically accelerated the reaction, and induced the asymmetric induction of chalcones with almost the same level of 1a.Development of novel efficient catalysts for asymmetric reactions is one of the important and challenging topics in synthetic organic chemistry.Phase-transfer catalysts (PTCs) are particularly attractive since they have some advantages over homogeneous catalysts, and many efficient asymmetric PTCs have been explored.1 In the course of our studies aimed at the development of PTCs, we have reported the pentacyclic guanidine catalyst 1 (Figure 1). 2 The structures of 1 were rationally designed to have a C 2 -symmetrical chiral reaction cavity around the guanidine group; i.e., a substrate recognition/activation site.One of these catalysts, 1a, which has a closed-type cavity and methyl substituents on its ether rings, was found to induce the high enentioselectivity in the alkylation reaction of the Schiff base imine.2b In this communication, we describe the asymmetric nucleophilic epoxidation to α,β-unsaturated ketones catalyzed by 1 and newly synthesized tricyclic guanidine 4.Among asymmetric epoxidation, catalytic asymmetric epoxidation of electron-deficient olefins is one of