To a mixture of 3.0 g of tert-butyl (1S,4S)-2,5-diazabicyclo[2.2.1]heptane-2-carboxylate, 3.2 g of 1-(3-dimethyl-aminopropyl) -3-ethylcarbodiimide hydrochloride, 2.6 g of 1-hydroxybenzotriazole, 2.9 g of 2,3-dichlorobenzoic acid and 60 ml of chloroform was stirred at room temperature for 17 hours and then diluted with chloroform. An insoluble matter was filtered off using Celite and the resulting mixture was washed with water with saturated brine. The resulting organic layer was dried over magnesium sulfate and filtered, the filtrate was concentrated in vacuo and the resulting residue was purified by a silica gel column chromatography (eluent: hexane/ethyl acetate=9/1 to 1/1) to give the title compound.
