37784-17-1

24424-99-5

344-25-2

37784-17-1

Step-1: Synthesis of N-tert-butoxycarbonyl-(D)-proline: To a stirred solution of D-proline (100 g, 868.58 mmol) in dioxane (400 mL, 8 vol.) was added NaHCO3 (182.4 g, 2.5 equiv.) and water (800 mL, 8 vol.) and the reaction mixture was stirred for about 30 min at room temperature. The reaction mixture was cooled to 0-5 °C, di-tert-butyl dicarbonate (Boc2O) (224.26 g, 1.2 eq.) was slowly added and stirring was continued at 0-5 °C for 1 hour. Subsequently, the reaction mixture was gradually warmed to room temperature and stirred overnight (12-16 h). The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the dioxane solvent was removed by evaporation under reduced pressure. The aqueous layer was acidified to pH 2-3 with 4N HCl solution at 0-5 °C. The aqueous layer was extracted with ethyl acetate (4 x 200 mL), the organic layers were combined, washed with water and dried with anhydrous Na2SO4. The organic layer was concentrated under reduced pressure to give a white solid product. The white solid was added to heptane (200 mL) and stirred at room temperature for 2 hours. The solid was collected by filtration and dried under vacuum at 45-50 °C to give the target compound (wt: 125 g, yield: 90-95%).1H NMR (300 MHz, CDCl3): δ 9.20 (br, 1H), 4.34 (t, 1H), 3.54-3.13 (m, 2H), 2.31-2.25 (m, 1H), 2.09- 1.88 (m, 3H), 1.48-1.42 (b, 9H); MS: [M + Na]+ 238 (100%).