To a mixture of 3-t-butyl-5-(4-ethynylphenyl)-4-(2-methoxyphenyl)-4,5-dihydro-1H-pyrrolo[3,4-c]pyrazol-6-one (1.51 g, 3.9 mmol), ethyl 2-chloro-2-hydroxyiminoacetate (1.19 g, 7.8 mmol) and THF (4 mL), triethylamine (1.09 mL, 7.8 mmol) in THF (3 mL) was added dropwise under ice-cooling conditions, and the mixture was stirred at 0° C. for 4 hr. Ethyl 2-chloro-2-hydroxyiminoacetate (1.19 g, 7.8 mmol) was added, and further triethylamine (0.81 mL, 5.9 mmol) in THF (3 mL) was added dropwise. The mixture was stirred at 0° C. for 2 hr and left stand overnight. The precipitate was removed by filtration, the filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (ethyl acetate/n-hexane) to give 3-t-butyl-5-[4-(3-ethoxycarbonyl-5-isoxazolyl)phenyl]-4-(2-methoxyphenyl)-4,5-dihydro-1H-pyrrolo[3,4-c]-pyrazol-6-one (1.26 g, 64%) as a colorless solid.
