Synthesis
Step 2: Ethyl (2S,4R)-4-hydroxypyrrolidine-2-carboxylate hydrochloride (146.2 g, 0.747 mol) was dissolved in a solvent mixture of dichloromethane (1,600 mL) and ethanol (100 mL) and cooled to 0°C. Triethylamine (113.4 g, 1.12 mol, 156.2 mL) was added to this solution with stirring. Subsequently, di-tert-butyl dicarbonate (195.6 g, 0.90 mol) was added in batches. The reaction mixture was stirred at 0 °C for 15 minutes, then brought to room temperature and continued stirring for 16 hours. Upon completion of the reaction, the mixture was concentrated to about 800 mL and washed with water. The organic layer was dried with magnesium sulfate, filtered and concentrated. Purification by silica gel column chromatography (eluent: 20% ethyl acetate-dichloromethane) afforded ethyl Boc-L-hydroxyproline (193.7 g, 100%) as a yellow oil. Mass spectrum (M + Na): m/e 282. 1H-NMR (CDCl3) δ 1.30 (t, 3H), 1.45 (s, 9H), 1.75 (m, 1H), 2.10 (m, 1H), 2.30 (m, 1H), 3.45 and 3.55 (d, 1H, two rotary isomers), 3.65 (dd, 1H), 4.25 (m, 2H ), 4.40 and 4.45 (t, 1H, two rotary isomers), 4.55 (broad s, 1H).
