In 99 ml of an aqueous solution of 1N hydrochloric acid was dissolved 10.01 g of Nα -benzyloxycarbonyl-L-histidine hydrazide (3). After adding thereto 132 ml of ethyl acetate, 8.25 ml of an aqueous solution of 2.313 g of sodium nitrite was added to the mixture with stirring vigorously under ice-cooling. After performing the reaction at 0° C. for 5 minutes, 39.6 ml of an aqueous 50% potassium carbonate solution was added to the reaction mixture under ice-cooling to alkalify the solution. The reaction mixture was placed in a separation funnel and an organic layer thus formed was collected. Furthermore, the aqueous layer was extracted by 20 ml of ethyl acetate and the extract was combined with the foregoing organic layer. The mixture was dried over anhydrous sodium sulfte under ice-cooling for 10 minutes. By removing anhydrous sodium sulfate by filtration, 152 ml of an ethyl acetate solution of Nα -benzyloxycarbonyl-L-histidine azide (4) was obtained. The product was ice-cooled and 20 ml of an ethyl acetate solution of 5.703 g of L-proline benzyl ester (5) was added thereto. The mixture was reacted overnight at 0° C. and then the reaction mixture was concentrated to dryness. The residue was dissolved in 22 ml of chloroform-methanol (10:1) and subjected to silica gel column chromatography. The eluate by chloroform-methanol (95:5) was concentrated to dryness to provide 6.602 g of Nα -benzyloxycarbonyl-L-histidyl-L-proline benzyl ester (6).
