To 100 ml of desiccated methylene chloride were added 10 g of Nα -t-butyloxycarbonyl-Nim -tosyl-L-histidine (47) and 6.50 g of L-proline benzyl ester hydrochloride and the mixture was cooled in an ice bath. After adding thereto 2.72 g of triethylamine, 6.05 g of DCC was further added to the mixture and the resultant mixture was stirred for 30 minutes in an ice-bath and stirred overnight at room temperature. Insoluble matters were filtered off and the filtrate was concentrated. The residue thus formed was purified by silica gel column chromatography. By eluting the product with ethyl acetate-benzene (1:1), 13.5 g of Nα -t-butyloxycarbonyl-Nim -tosyl-L-histidyl-L-proline benzyl ester (49) was obtained.
