A mixture of (2S,4R)-4-hydroxypyrrolidine-2-carboxylicacid(50 mmol, 6550 mg) and Ac2O (250 mmol, 23.5 mL) in AcOH (100 mL) wasstirred at reflux for 6 h. The resulting mixture was concentrated to give thecrude intermediate. The solution of this crude intermediate in 2 N HCl (50 mL)was stirred at reflux for 3 h. Then activated charcoal (1 g) was added, the hotmixture was ltered immediately through a Celite layer. The filtrate was concentrated in vacuo to give the crude intermediate.The crude intermediate was dissolved in acetone and filtered. The residue was washed twice with ether anddried in vacuo to give cis-4-Hydroxy-D-proline hydrochloride as a white solid (7.54 g, 90percent).