Synthesis of 2-(5H)-furanones by cyclization of alkyl allene carboxylates in triflic acid

Alkyl allene carboxylates [R2(R3)C=C=C(R1)–C(=O)OAlk] in Brønsted superacid CF3SO3H (triflic acid, TfOH) at room temperature or 70 °C for 30 min are cyclized to form 2-(5H)-furanones in yields up to 83%. The reaction cationic intermediates, O-monoprotonated [R2(R3)C=C=C(R1)–C(=O+H)OAlk] and O,C-diprotonated [R2(R3)C+–CH=C(R1)–C(=O+H)OAlk] forms of starting allenes, have been studied experimentally by NMR and theoretically by DFT calculations. It has been found that the cyclization proceeds via O,C-diprotonated forms of starting allenes.