[CH:1]1[CH:6]=[CH:5][C:4]([CH2:7][O:8][C:9]([CH2:11][C@H:12]([NH2:16])[C:13]([OH:15])=O)=[O:10])=[CH:3][CH:2]=1.[CH2:17](N(CC)CC)C.[C:24](OC(=O)C)(=[O:26])[CH3:25].[OH-].[K+]>CN(C)C1C=CN=CC=1.O>[C:24]([NH:16][CH:12]([C:13](=[O:15])[CH3:17])[CH2:11][C:9]([O:8][CH2:7][C:4]1[CH:3]=[CH:2][CH:1]=[CH:6][CH:5]=1)=[O:10])(=[O:26])[CH3:25]
β-Benzyl L-aspartate (400 g, 1.79 mol) was suspended in triethylamine (748 ml, 5.37 mol) and acetic anhydride (676 ml, 7.16 mol) was dropwise added at 0° C. with stirring. After stirring for 30 minutes, 4-dimethylaminopyridine (20.0 g, 0.16 mol) was portionwise added under ice-cooling. The mixture was stirred overnight at room temperature, and ice was added under ice-cooling. At the end of the exothermic process, water (700 ml) was added. A 7.5N aqueous solution of potassium hydroxide was portionwise added to make its pH 9. The mixture was extracted three times with ethyl acetate (1,000 ml), and the organic layer was washed twice with 1N hydrochloric acid (1,000 ml), twice with a saturated aqueous solution of sodium hydrogencarbonate (500 ml) and with saturated brine (500 ml) in order. The layer was dried over magnesium sulfate and concentrated to dryness to give 390 g of the title compound.
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