5.3 g (30 mmol) of t-butoxycarbonyl glycine was dissolved in 100 ml of purified THF and 4.6 ml (33 mmol) of triethylamine and 4.33 ml (33 mmol) of isobutyl chloroformate) were added thereto at -15° C. The mixture was stirred at this temperature for 10 minutes. To the resultant solution of a mixed acid anhydride was added a solution which had been prepared by dissolving 8.0 g (36 mmol) of β-benzyl-L-aspartic acid and 5.02 ml (36 mmol) of triethylamine in water and cooling to 0° C. The mixture was then stirred at 0° C. for 1 hour and further at room temperature for 15 hours. After removing the THF by concentration under reduced pressure, cold 10% citric acid was added thereto. The precipitate thus formed was extracted with ethyl acetate twice. The extracts were combined and washed 5 times with one fifth the volume of 5% citric acid and then 10 times with water (one tenth the volume thereof). The solution was dehydrated over anhydrous sodium sulfate and concentrated to dryness. Then the residue was dissolved in ether and precipitated with n-hexane. After repeating this precipitation procedure 3 times, the obtained product was dried, whereby 6.9 g of the titled compound was obtained.
