ClC(OCC)=O.[C:7]([O:11][C:12]([N:14]1[CH2:18][C@@H:17]([OH:19])[CH2:16][C@H:15]1[C:20]([OH:22])=O)=[O:13])([CH3:10])([CH3:9])[CH3:8].[CH2:23]([NH2:25])[CH3:24].[Cl-].[Na+]>O1CCCC1.C(N(CC)CC)C>[C:7]([O:11][C:12]([N:14]1[CH2:18][C@@H:17]([OH:19])[CH2:16][C@H:15]1[C:20](=[O:22])[NH:25][CH2:23][CH3:24])=[O:13])([CH3:8])([CH3:9])[CH3:10]15.25 ml of triethylamine and subsequently a solution of 10.48 ml of ethyl chloroformate in 50 ml of dry tetrahydrofuran were added at -25° C. to a solution of 23.13 g of (2S, 4S)-1-t-butoxycarbonyl-4-hydroxy-2-pyrrolidinecarboxylic acid dissolved in 350 ml of dry tetrahydrofuran, and the mixture was stirred at the same temperature for 30 minutes. At the end of this time, a 70% by volume aqueous ethylamine solution was added at -22° C., the reaction temperature was allowed to rise gradually and the reaction finished when the temperature reached 10° C. The reaction mixture was then mixed with a small amount of an aqueous solution of sodium chloride, after which it was extracted thrice with ethyl acetate. The combined extracts were washed with an aqueous solution of sodium chloride and dried over anhydrous magnesium sulfate. The solvent was then removed by distillation under reduced pressure, to afford 23.60 g of the title compound as a colorless oil.
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