(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 (150 mg, 0.27 mmol), piperazin-2-one (60 mg, 0.6 mmol), bis(2-oxo-3-oxazolidinyl) phosphonic chloride (0.138 g, 0.54 mmol), triethylamine (0.25 mL, 1.62 mmol) and 8 mL of dichloromethane were added into the reaction flask, and 10 mL of N,N-dimethylformamide was then added to the flask. The reaction mixture was reacted at room temperature overnight and monitored by thin layer chromatography until the disappearance of the starting materials. The reaction mixture was concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography to obtain the title compound (R)-[3-oxo-3-[1-(3-oxo-piperazine-1-carbonyl)-3-trifluoromethyl-5,6-dihydro-8H-imidazo[1,5-a]pyrazin-7-yl]-1-(2,4,5-trifluoro-benzyl)-propyl]-carbamic acid tert-butyl ester 19a (140 mg, yield 82%) as a colourness oil.
