[CH:1]([NH:3][C@@H:4]1[C:9](=[O:10])[O:8][C:6](=[O:7])[CH2:5]1)=[O:2].C([O-])(=O)C.[Na+].Cl.[CH3:17][O:18][C:19](=[O:29])[C@H:20]([CH2:22][C:23]1[CH:28]=[CH:27][CH:26]=[CH:25][CH:24]=1)[NH2:21]>C(Cl)Cl>[CH3:17][O:18][C:19](=[O:29])[C@H:20]([CH2:22][C:23]1[CH:28]=[CH:27][CH:26]=[CH:25][CH:24]=1)[NH:21][C:6](=[O:7])[CH2:5][C@@H:4]([C:9]([OH:8])=[O:10])[NH:3][CH:1]=[O:2]In 179.4 g of methylenechloride, 14.3 g (0.1 mole) of N-formyl-L-aspartic anhydride was suspended, and 9.2 g (0.11 mole) of sodium acetate was added at 20°-25° C. with stirring, and 21.6 g (0.1 mole) of L-phenylalanine methyl ester hydrochloride was added at the same temperature during 30 minutes. The mixture was reacted for 5 hours with stirring at the same temperature. Precipitated crystals were filtered, washed and dried. Crystals of a mixture of N-formyl-α-L-aspartyl-L-phenylalanine methyl ester, and N-formyl-β-L-aspartyl-L-phenylalanine methyl ester were obtained in a yield of 30.0 g. The crystals obtained were analyzed by high performance liquid chromatography to give a ratio of α-isomer:β-isomer of 75.0:25.0. The crystals were purified by a conventional method to obtain pure crystals of N-formyl-α-L-aspartyl-L-phenylalanine methyl ester. The yield was 16.1 g (50% yield on L-phenylalanine methyl ester hydrochloride).
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