To a mixed solution of phosphorus oxychloride (257 μL, 2.76 mmol) and THF (10.0 mL) was dropwisely added a mixed solution of (6S,9aS)-N-benzyl-8-((6-(3-(4-ethylpiperazin-1-yl)azetidin-1-yl)pyridin-2-yl)methyl)-6-((2-fluoro-4-hydroxyphenyl)methyl)-4,7-dioxo-2-(prop-2-en-1-yl)-octahydro-1H-pyrazino[2,1-c][1,2,4]triazine-1-carboxamide (1.00 g, 1.38 mmol) described in Example 1, triethylamine (768 μL, 5.52 mmol) and THF (10.0 mL) over 10 minutes under a nitrogen atmosphere at −15° C. or lower. The reaction mixture was stirred at −15° C. or lower for 1 hour and 10 minutes. To the reaction mixture was dropwisely added a mixed solution of a 5 N aqueous sodium hydroxide solution (2.03 mL, 10.1 mmol) and water (21.0 mL) over 5 minutes at 10° C. or lower. The reaction mixture was stirred at room temperature for 35 minutes, then ethyl acetate was added to the reaction mixture, and then the resultant solution was partitioned. An aqueous layer was further washed with ethyl acetate, and then 5 N hydrochloric acid was added to the solution to adjust the pH value of the solution at a neutral value. The solvent was distilled away from the reaction mixture under a reduced pressure. Dichloromethane, ethanol and methanol were added to the resultant residue and then stirred, and then the resultant suspension was filtrated using Celite. The solvent was distilled away from the filtrate under a reduced pressure, and the resultant residue was purified by ODS silica gel column chromatography (elution solvent: water/methanol) to give the title compound (881 mg, yield: 79%).
