Brønsted Acid Promoted Cyclization of Cross-Conjugated Enynones into Dihydropyran-4-ones

In triflic acid or sulfuric acid, diaryl-substituted cross-conjugated enynones undergo addition of the acid to the carbon–carbon triple bond to afford the corresponding vinyl triflates or sulfates. The vinyl triflates are stable under aqueous workup, whereas the vinyl sulfates are hydrolyzed to α,β-unsaturated 1,3-diketones (existing as conjugated enol forms). Extended reaction times lead to cyclization into dihydropyran-4-ones with yields of up to 95 %. The protonated forms of the vinyl triflates or sulfates in triflic and sulfuric acid, respectively, are studied as reactive intermediates by NMR spectroscopy. Plausible reaction mechanisms for the formation of dihydropyran-4-ones are discussed.