Isobutyl chloroformate (8.43 g, 61.7 mmol) was slowly added to a solution of anhydrous THF (20 mL) containing N-Boc-azetidine-2-carboxylic acid (12.42 g, 61.7 mmol) and N-methylmorpholine (6.23 g, 61.7 mmol) under nitrogen protection, and the rate of addition was controlled to maintain a reaction temperature of less than -8 °C. After addition, the reaction mixture was stirred at -8 °C for 1 hour, then brought to room temperature and continued stirring for 3 hours. Methanol (200 mL) was added to the reaction solution and stirred at room temperature for 18 hours. Upon completion of the reaction, the volatile components were removed by distillation under reduced pressure and the residue was partitioned with water and diethyl ether. The organic layer was separated, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give the oily product methyl N-Boc-2-azetidinecarboxylate (11.15 g, 84.0% yield). The product was confirmed by NMR hydrogen spectrum (CDCl3): δ 1.43 (s, 9H), 2.20 (m, 1H), 2.50 (m, 1H), 3.78 (s, 3H), 3.90 (m, 1H), 4.00 (m, 1H), 4.60 (m, 1H).
