C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+].C(O)(=O)C.[C:19]([O:23][C:24]([N:26]1[CH2:32][CH2:31][CH2:30][C@H:27]1[CH:28]=O)=[O:25])([CH3:22])([CH3:21])[CH3:20].[F:33][C:34]1[CH:41]=[CH:40][C:37]([CH2:38][NH2:39])=[CH:36][CH:35]=1>ClCCCl.ClCCl>[C:19]([O:23][C:24]([N:26]1[CH2:32][CH2:31][CH2:30][C@@H:27]1[CH2:28][NH:39][CH2:38][C:37]1[CH:40]=[CH:41][C:34]([F:33])=[CH:35][CH:36]=1)=[O:25])([CH3:22])([CH3:21])[CH3:20]Sodium triacetoxyborohydride (15.9 g, 75.4 mmol) and enough acetic acid to bring the pH to 5 were added to a solution of N-tert-butoxycarbonyl-L-prolinal (5 g, 25 mmol) and 4-fluorobenzylamine (6.28 g, 50.3 mmol) in 200 mL of 1,2-dichloroethane. The mixture was stirred at ambient temperature overnight, and then it was diluted with dichloromethane, and washed with saturated sodium bicarbonate solution. The organic phase was dried over magnesium sulfate, filtered and concentrated to leave the crude product. Chromatography (elution with methanol/ethyl acetate) afforded 6.0 g of product.
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