The general procedure for the synthesis of ethyl 3,3-difluorocyclobutanecarboxylate from ethyl 3-oxocyclobutanecarboxylate was as follows: to a solution of ethyl 3-oxocyclobutanecarboxylate (2.80 g, 15 mmol) in dichloromethane (100 mL) was added (diethylamino) sulfur trifluoride (4 mL, 29.5 mmol) at -4 °C. The reaction mixture was slowly warmed to room temperature and stirred continuously for 88 hours. Upon completion of the reaction, the mixture was poured into a mixture of sodium carbonate and ice for quenching and stirred for 10 minutes. Subsequently, the reaction mixture was extracted with dichloromethane (3 x 75 mL), the organic phases were combined and dried over sodium sulfate. After filtration, the solvent was evaporated under reduced pressure to give the crude product as an oil. Purification of the crude product by column chromatography (using dichloromethane as eluent) afforded ethyl 3,3-difluorocyclobutanecarboxylate as a yellow oil (2.30 g, 95% yield). The structure of the product was confirmed by 1H-NMR (400 MHz, CDCl3): δ1.25 (t, 3H), 2.60-3.00 (m, 4H), 4.15 (q, 2H).
