[C:1](O)(=O)[CH3:2].[CH:5](=O)[CH3:6].Cl.[CH2:9]([O:11][C:12](=[O:23])[CH:13]([CH2:15][C:16]1[CH:21]=[CH:20][C:19]([OH:22])=[CH:18][CH:17]=1)[NH2:14])[CH3:10].C([BH3-])#N.[Na+]>CO>[CH2:5]([N:14]([CH:13]([CH2:15][C:16]1[CH:17]=[CH:18][C:19]([OH:22])=[CH:20][CH:21]=1)[C:12]([O:11][CH2:9][CH3:10])=[O:23])[CH2:1][CH3:2])[CH3:6]Acetic acid (0.3 ml) and acetaldehyde (0.5 ml) were added to a solution of DL-tyrosine ethyl ester hydrochloride (491 mg) in methanol (5 ml). To the mixture sodium cyanoborohydride (126 mg) was added and the mixture was stirred at ambient temperature for 2 hours. At the end of this time the reaction mixture was concentrated. The residue was partitioned between ethyl acetate and water. The ethyl acetate layer was separated and washed with aqueous sodium hydrogencarbonate solution and dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was purified via chromatography on a silica gel column using methanol/dichloromethane=1/20 as the eluant to afford the desired compound (420 mg) as a syrup.
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