Step (i)[3] : (2S,4R)-1-(tert-butoxycarbonyl)-4-methylpyrrolidine-2-carboxylic acid (57 g, 262.52 mmol) was dissolved in ethyl ether (150 mL), and an ethyl ether solution of HCl was added slowly (50 mL) with stirring. The reaction mixture was stirred at room temperature overnight. After the reaction was completed, the white solid was collected by filtration and washed with ether (50 mL). The filtrate was cooled to -0 °C and kept for 6 h. The precipitated solid was filtered again and washed with cold water (25 mL). The two batches of solid products were combined and dried under reduced pressure in the presence of phosphorus pentoxide (P2O5) to constant weight to give a white solid (31 g). The structure of the product was confirmed by 1H NMR (D2O): δ 1.94-2.01 (m, 1H), 2.14-2.19 (m, 1H), 3.30-3.48 (m, 2H), 3.71-3.76 (m, 1H), 3.93-4.08 (m, 2H), 4.67-4.75 (m, 1H).
