Methanol → 5-[5-(N-acetyl-anilino)-2, 3-pentadienylidene]-2,7-diphenyl-4-methylpyrazolo[3,4-b]pyridine-3,6-dione

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In a solution prepared by adding methanol (50 ml) and triethylamine (2.8 ml) to 2,7-diphenyl-4-methylpyrazolo(3,4-b) pyridine-3,6-dione (3.17 g), 1,7-diphenyl-1,7-diaza-1,3,5-heptatriene hydrochloride (2.85 g) was dissolved and then acetic anhydride (1.88 ml) was added, and the mixture was stirred at room temperature for one hour. The resulting precipitate was collected by filtration, washed with methanol and dried to obtain 5-[5-(N-acetylanilino)-2,3-pentadienylidene]-2,7-diphenyl-4-methylpyrazolo(3,4-b)pyridin-3, 6-dione (1.9). To these crystals (1.5 g), N,N-dimethylformamide (60 ml), 2,7-diphenyl-4-methylpyrazolo(3,4-b)pyridin-3,6-dione (0.95 g) and triethylamine (0.42 g) were added, and stirred at 50° C. for 2 hours. After a trace amount of insoluble matters were removed by filtration, ethyl acetate of 10-fold the volume of the filtrate was added thereto while stirring for crystallization. The resulting crystals were redissolved in a small amount of N,N-dimethylformamide, and a 10-fold volume of ethyl acetate was added thereto for crystallization. The resulting crystals were collected by filtration and dried to obtain compound II-2 (1.3 g). Gold crystals, melting point 300° C. or more, λmaxDMF 792 nm

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