Enantioselective sulfa-Michael addition of thioacids to α,β-unsaturated ketones with bifunctional organocatalyst

Organocatalytic conjugate addition of thioacids to α,β-unsaturated ketones has been studied in the presence of cinchona alkaloid derived urea catalyst. Both the enantiomers of products are accessible with the same level of enantioselectivity using pseudoenantiomeric quinine/quinidine derived catalysts. The catalytic process provides optically active thioesters with high chemical yields (up to 99%) and useful enantioselectivity (up to 83% ee). The reaction was performed with 1 mol % of catalyst in toluene at room temperature. A transition state model has been proposed to explain the stereochemical outcome of the reaction.