[N+:1]([C:4]1[O:8][C:7]([C:9]2[C:13]([C:14](N)=[O:15])=[CH:12][N:11]([C:17]3[CH:22]=[CH:21][CH:20]=[CH:19][CH:18]=3)[N:10]=2)=[CH:6][CH:5]=1)([O-:3])=[O:2].N([O-])=[O:24].[Na+]>O.S(=O)(=O)(O)O>[N+:1]([C:4]1[O:8][C:7]([C:9]2[C:13]([C:14]([OH:24])=[O:15])=[CH:12][N:11]([C:17]3[CH:22]=[CH:21][CH:20]=[CH:19][CH:18]=3)[N:10]=2)=[CH:6][CH:5]=1)([O-:3])=[O:2]Add 2 g of 3-(5-nitro-2-furyl)-1-phenylpyrazole-4-carboxamide to 4.7 ml of concentrated sulfuric acid while cooling on ice and then add thereto a solution of 0.78 g of sodium nitrite in 2 ml of water slowly and dropwise at a temperature not exceeding 7° C. Allow the resulting mixture to stand overnight; dilute with from 4 to 5 ml of concentrated sulfuric acid and then mix it (while cooling with ice) with 0.39 g of sodium nitrite in 1 ml of water. Allow the thus-prepared solution to stand at a temperature of 0° to 5° C for 2 days in a refrigerator and then pour it onto ice. Vacuum filter to separate formed precipitate and chromatograph with chloroform/ethanol (9:1) on a silica gel column to obtain 3-(5-nitro-2-furyl)-1-phenylpyrazole-4-carboxylic acid [m.p. 250° to 252° C].
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