[CH2:1]([O:3][C:4](=[O:14])[CH2:5][NH:6][CH2:7][C:8]1[CH:13]=[CH:12][CH:11]=[CH:10][CH:9]=1)[CH3:2].C([O:17][C:18](=[O:24])[CH2:19][CH2:20][CH2:21][CH2:22]Br)C.C(=O)([O-])[O-].[K+].[K+]>CN(C)C=O>[CH2:7]([N:6]([CH2:5][C:4]([O:3][CH2:1][CH3:2])=[O:14])[CH2:22][CH2:21][CH2:20][CH2:19][C:18]([OH:24])=[O:17])[C:8]1[CH:13]=[CH:12][CH:11]=[CH:10][CH:9]=1N-benzylglycine ethyl ester (1.87 ml) and 5-bromovaleric acid ethyl ester (1.92 ml) are dissolved in dimethylformamide (100 ml) and stirred in the presence of potassium carbonate (1.66 g) for 2 days at room temperature. The reaction is quenched with saturated aqueous ammonium chloride solution, and extraction is carried out with ethyl acetate. After drying over sodium sulphate the combined organic phases are concentrated by evaporation. From the obtained residue, 5-(benzyl-ethoxycarbonylmethyl-amino)-pentanoic acid is isolated in a yield of 30% by chromatography using silica gel (ethyl acetate/heptane 1:5).
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