Direct Asymmetric Michael Reaction of alpha,beta-Unsaturated Aldehydes and Ketones Catalyzed by Two Secondary Amine Catalysts

A direct asymmetric Michael reaction of alpha,beta-unsaturated aldehydes and ketones proceeded in the presence of two pyrrolidine-type catalysts, a diphenylprolinol silyl ether and hydroxyproline, to afford synthetically useful delta-keto aldehydes with excellent diastereo- and enantioselectivity. Although there are several iminium ions and enamines in the reaction mixture, the iminium ion generated by the former catalyst reacts preferentially with the enamine generated by the latter catalyst.