In 20 ml of dimethylformamide was dissolved 1.25 g of carbobenzyloxy-L-proline. The solution was cooled to -10° C., then there was added 810 mg of phosgene iminium chloride, and the mixture was stirred for 2 hours. To the mixture were added dropwise 5 ml of dimethylformamide containing 1.8 g of (S)-2-oxopiperazine-1,3-diacetic acid di-t-butyl ester synthesized in Working Example 3, then 2 ml of triethylamine. The mixture was stirred for one hour. The reaction mixture was diluted with ethyl acetate and was poured into ice-water. The organic layer was washed with a 5% potassium hydrogensulfate and a saturated aqueous solution of sodium hydrogenphosphate. The extract solution was dried over anhydrous magnesium sulfate and was concentrated under reduced pressure to give a crude product. The crude product was purified by s silica gel chromatography (ethyl acetate) to afford 1.0 g of the title compound as amorphous powder.
