Enantioselective Synthesis of α,α-Disubstituted Cyclopentenes by an <i>N</i>-Heterocyclic Carbene-Catalyzed Desymmetrization of 1,3-Diketones

The enantioselective synthesis of α,α-disubstituted cyclopentenes using N-heterocyclic carbene catalysis is reported. This desymmetrization reaction proceeds via a chiral enol generated in situ from the combination of an enantiopure azolium salt and α,β-unsaturated aldehyde. This reactive enol undergoes addition to one of two enantiotopic ketones to afford an optically active β-lactone after the intermediate carbinol is involved in an internal acylation event to release the carbene catalyst. In the substrates where R = aryl, a decarboxylation occurs immediately after lactone formation to produce the quaternary center-containing cyclopentene products in up to 96% ee. When the R group is alkyl, the β-lactones are isolated in moderate yield and high diastereo- and enantioselectivity.