Regiocontrolled Synthesis of γ‑Hydroxybutenolides via Singlet Oxygen-Mediated Oxidation of 2‑Thiophenyl Furans

Photooxygenation of 2-thiophenyl-substituted furans in ethanol leads to the rapid, regiocontrolled, and quantitative synthesis of γ-hydroxybutenolides. The carbonyl group in butenolide holds the position of thiophenyl moiety in reacting furans. Decomposition of the initially formed [4 + 2] endoperoxide into products through a radical chain mechanism is proposed, as the fate of thiophenyl moiety is its transformation into ethyl phenylsulfenate (PhS-OEt) and diphenyldisulfide. Under the reaction conditions, the sulfenate is fast oxidized into the corresponding sulfinate.