Cl[C:2]([O:4][CH2:5][C:6]1[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=1)=[O:3].[OH:12][C@H:13]1[CH2:17][NH:16][C@H:15]([C:18]([OH:20])=[O:19])[CH2:14]1.C([O-])(O)=O.[Na+].O>CC(C)=O>[OH:12][C@H:13]1[CH2:17][N:16]([C:2]([O:4][CH2:5][C:6]2[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=2)=[O:3])[C@H:15]([C:18]([OH:20])=[O:19])[CH2:14]1Benzyl chloroformate (8.6 mL, 0.06 mol) was dissolved in acetone (12 mL), and this solution was added dropwise to a stirred solution of trans-4-Hydroxy-L-proline (6.56 g, 0.05 mol) in satd. NaHCO3 (160 mL) and H2O (24 mL). The resulting solution was stirred for 6 h at room temperature. The solution was washed with diethyl ether, and the organic layer was discarded. The aqueous layer was acidified with to pH 3 with 1 M HCl and extracted with ethyl acetate. The organic layer was dried over MgSO4 and concentrated to afford 13.5 g (quantitative yield) of the desired product as an oil.
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