Step B: Preparation of 1-methyl-2-oxopyrrolidine-3-carboxylic acid: In a 250 mL round-bottomed flask, methyl 1-methyl-2-oxopyrrolidine-3-carboxylate (1.89 g, 12.0 mmol), potassium trimethylsilanolate (4.64 g, 36.1 mmol), and tetrahydrofuran (THF, 100 mL) were added in sequence. The reaction mixture was stirred at room temperature overnight. Upon completion of the reaction, 2.0 M ethyl ether hydrochloride solution (50 mL) was added to the mixture and stirring was continued for 5 min. The reaction solution was filtered to remove insoluble solids, and the filtrate was concentrated under reduced pressure to give 1.28 g of crude product in 74.2% yield. The crude product was directly used in the subsequent reaction without further purification. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3): δ 3.40-3.49 (m, 3H), 2.94 (s, 3H), 2.37-2.47 (m, 2H).
