Chiral N,N′-Dioxide-Yttrium Triflate Complexes-Catalyzed Asymmetric Aldol Cyclization of α-Keto Esters with α-Isothiocyanato Imide

A highly effective aldol cyclization of -isothiocyanato imide to both ,-unsaturated -keto esters and aryl-substituted -keto esters has been developed. A chiral N,N-dioxide-yttrium triflate complex was used as the catalyst. A series of cyclic thiocarbamates bearing chiral quaternary stereocenters was synthesized in good to high yields, excellent diastereo- (up to 25:1 dr) and enantioselectivities (up to 99% ee). In addition, the reaction could be carried out on a gram-scale, and other functionalized derivatives are also conveniently transformed. Interestingly, a discrepancy of diastereoselection was observed between the reactions of ,-unsaturated -keto esters and aryl-substituted -keto esters. Moreover, a substrate dependency of non-linear effects was observed in this reaction. On the basis of the experimental results and the absolute configuration of the products, possible catalytic models have been proposed to explain the origin of the asymmetric process.