Brønsted Acid Promoted Reactions of γ-Hydroxy Acetylene Ketones. DFT Study of Cationic Intermediates and Concurrent Formation of 3-Furanones and/or Conjugated Enynones

Protonation of γ-hydroxy conjugated acetylene ketones Ph(O=)C–C≡C–C(OH)R1R2 in strong Brønsted acid H2SO4 or superacid TfOH (CF3SO3H, triflic acid) gives rise to O,O-diprotonated species Ph(+HO=)C–C≡C–C(OH2+)R1R2. Electrophilic, electronic and orbital properties of these dications have been studied by DFT calculations. Depending on the structure of the dications and acidity of Brønsted acid (nucleophilicity of the corresponding acid counter ion), two concurrent reaction products may be formed, 3(2H)-furanones in a way of cyclization and/or conjugated enynones in a way of dehydration. Plausible reaction mechanisms are discussed.