[C:1](OC(=O)C)(=[O:3])[CH3:2].Cl.[CH2:9]([O:11][C:12](=[O:19])[C@@H:13]1[CH2:17][CH:16]([OH:18])[CH2:15][NH:14]1)[CH3:10].C(N(CC)CC)C.C(Cl)(Cl)Cl>O>[CH2:9]([O:11][C:12](=[O:19])[C@@H:13]1[CH2:17][CH:16]([OH:18])[CH2:15][N:14]1[C:1](=[O:3])[CH3:2])[CH3:10]345 g of acetic anhydride was added dropwise to a suspension of 600 g of 4-hydroxy-L-proline ethyl ester hydrochloride, 683 g of triethylamine and 2.4 L of chloroform at not higher than 10° C. while cooling. After stirring for 2 hours, water (0.6 L) was added, and the layers were separated. The organic layer was dried over magnesium sulfate, and concentrated under reduced pressure to obtain 1020 g of N-acetyl-4-hydroxy-L-proline ethyl ester as an oil.
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