Asymmetric Intramolecular Michael Reaction Catalyzed by N-Heterocyclic Carbenes

Significance

A highly diastereo- and enantioselective intramolecular Michael reaction catalyzed by N-heterocyclic carbenes has been described. The addition of the carbene catalyst to an α,β-unsaturated aldehyde, followed by β-protonation generates a reactive enol intermediate which undergoes conjugate addition to the enone. Triazolium salts have been identified as efficient catalysts for this reaction and catalyst 1 is the best catalyst. It is regenerated by the formation of a cyclic O-acylated enol which is intercepted by alcohols and amines to provide different esters and amides. With 10 mol% of catalyst 1, moderate to good yields (59-80%) and excellent enantioselectivities (er = 99.5:0.5) are obtained for different substituted enones.