In 25 ml of dimethylformamide was dissolved 2.65 g of N-benzyloxycarbonyl-L-leucine. After cooling to -15° C., 1.10 ml of N-methylmorpholine and then 1.37 ml of isobutyl chloroformate were added to the solution while stirring the solution at the same temperature. The mixture was sitrred at -15° C. for 5 minutes. On the other hand, 3.25 g of L-argininal dibutyl acetal hydrochloride (Example 2) was dissolved in 30 ml of dimethylformamide and 1.40 ml of triethylamine was added to the solution. The resulting solution was dropwise added at -10° C. to the mixed acid anhydride suspension which had been previously prepared. Stirring was continued at the same temperature for an hour and then at 0° C. for an hour. The resulting reaction solution was concentrated under reduced pressure and the residue was dissolved in 100 ml of chloroform. The solution was washed successively with 10% citric acid aqueous solution (30 ml×2), saturated sodium chloride aqueous solution (30 ml×1), saturated sodium hydrogencarbonate aqueous solution (30 ml×2) and saturated sodium chloride aqueous solution (30 ml×2). After drying over anhydrous magnesium sulfate, the solvent was distilled off under reduced pressure. The obtained residue was subjected to column chromatography using silica gel as a carrier and developed with chloroform: methanol=20:1 (v/v). The fraction showing Rf: 0.48 (under the same conditions as in Example 1 (A) 4) which was positive in the Sakaguchi reaction, was concentrated under reduced pressure to give 3.73 g of N-benzyloxycarbonyl-L-leucyl-L-argininal dibutyl acetal.
