General procedure for the synthesis of diisopropyl 3-oxocyclobutane-1,1-dicarboxylate from diisopropyl 3,3-dimethoxycyclobutane-1,1-dicarboxylate: in a round-bottomed flask containing a solution of 3,3-dimethoxycyclobutane-1,1-dicarboxylic acid diisopropyl 3,3-dimethoxycyclobutane-1,1-dicarboxylate (1.5 g, 5.20 mmol) in dichloromethane (20 mL), trifluoroacetic acid (2 mL, 26.0 mmol). The reaction mixture was stirred at room temperature for 2.5 hours. After the reaction was completed, the reaction mixture was diluted with ethyl acetate. The organic phase was washed sequentially with water, saturated sodium bicarbonate solution and brine. The organic phase was dried with anhydrous sodium sulfate, filtered and concentrated to give diisopropyl 3-oxocyclobutane-1,1-dicarboxylate (1.2 g). The product was characterized by 1H NMR (400 MHz, CDCl3): δ 5.13 (dt, J = 12.5, 6.3 Hz, 2H), 3.64-3.53 (m, 4H), 1.32-1.24 (m, 12H).
