Concentrated sulfuric acid (0.2 mL) was slowly added dropwise to a solution of 3-oxocyclobutanecarboxylic acid (1 g, 8.77 mmol) in methanol (appropriate amount), followed by heating and refluxing the reaction mixture at 75 °C. The reaction progress was monitored by thin layer chromatography (TLC) and after complete conversion of the feedstock, saturated sodium bicarbonate solution was added to the reaction solution to quench the reaction. The solvent methanol was removed by distillation under reduced pressure and the resulting residue was separated by extraction with ethyl acetate and water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give methyl 3-oxocyclobutanecarboxylate (1.1 g, 99% yield) as a colorless oil.1H NMR (400 MHz, CDCl3) δ 3.22-3.34 (m, 3H), 3.40-3.47 (m, 2H), 3.78 (s, 3H).
