(R)-7-[3-tert-Butoxycarbonylamino-4-(2,4,5-trifluoro-phenyl)-butyryl]-3-trifluoromethyl-5,6,7,8-tetrahydro-imidazo[1,5-a]pyrazine-1-carboxylic acid 2a (0.275 g, 0.5 mmol) and 4 mL of N,N-dimethylformamide were added into the reaction tube, and then 1-chloroethyl ethyl carbonate (0.092 g, 0.6 mmol), potassium iodide (0.0415 g, 0.25 mmol) and potassium carbonate (0.083 g, 0.6 mmol) were added successively under stirring. Then the reaction tube was sealed up. The reaction mixture was reacted at 65° C. in an oil bath for 2 hours and monitored by thin layer chromatography until the disappearance of the starting materials. After cooling to room temperature, 40 mL of water was added to the reaction tube. The reaction mixture was extracted with ethyl acetate (25 mL×3), and the combined organic phase was washed with water (20 mL×2), dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography to obtain the title compound, (R)-7-[3-tert-butoxycarbonylamino-4-(2,4,5-trifluoro-phenyl)-butyryl]-3-trifluoromethyl-5,6,7,8-tetrahydro-imidazo[1,5-a]pyrazine-1-carboxylic acid (1-ethoxycarbonyloxy)-ethyl ester 34a (0.26 g, yield 78.1%) as a white solid.
