Isothiourea-Catalyzed Enantioselective Synthesis of Trans-3,4-dihydrothiopyranones: Harnessing Thiochalcones As Original Michael Acceptors.

A highly enantioselective formal (4 + 2)-cycloaddition between carboxylic acids and thiochalcones promoted by (+)-HBTM-2.1 isothiourea organocatalyst has been reported. The methodology relied on the generation of C1-ammonium enolate intermediates and proceeded through a nucleophilic 1,4-addition-thiolactonization cascade. It enabled the stereocontrolled preparation of sulfur-containing δ-thiolactones in good yields, moderate diastereoselectivity, and excellent enantiomeric excess (up to 99%). This annulation benefited from the peculiar reactivity of uncommon electron-rich thiochalcones used as Michael acceptors.