The general procedure for the synthesis of ethyl 3-oxocyclobutanecarboxylate from ethyl 3-hydroxycyclobutanecarboxylate was as follows: oxalyl chloride (1.79 mL, 21.2 mmol) was slowly added to dichloromethane (100 mL) at -78 °C, followed by dimethyl sulfoxide (2.51 mL, 35.3 mmol). After 30 minutes of reaction, a solution of cis- and trans-3-hydroxycyclobutanecarboxylic acid ethyl ester (2.68 g, 17.6 mmol) in dichloromethane (46 mL) was added dropwise. The reaction mixture was kept stirred at -78 °C for 30 min. After that, triethylamine (9.84 mL, 70.6 mmol) was added. The reaction mixture was slowly warmed up to room temperature over 2 hours. Upon completion of the reaction, the reaction was quenched by the addition of water and the organic and aqueous layers were separated. The organic layer was washed sequentially with 1N HCl, water, saturated sodium bicarbonate solution and brine, dried over anhydrous sodium sulfate, filtered and concentrated. The resulting crude product, ethyl 3-oxocyclobutanecarboxylate (2.36 g, 94% yield), could be used in the subsequent reaction without further purification. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3): δ 4.21 (q, 2H), 3.45-3.37 (m, 2H), 3.33-3.17 (m, 3H), 1.29 (t, 3H).
