Cl[CH:2](Cl)[C:3]1[CH:4]=[N:5][N:6]([CH3:11])[C:7]=1[N+:8]([O-:10])=[O:9].C([OH:15])C>O.[N+]([O-])([O-])=O.[Ag+]>[CH3:11][N:6]1[C:7]([N+:8]([O-:10])=[O:9])=[C:3]([CH:2]=[O:15])[CH:4]=[N:5]1A hot solution of silver nitrate (4.7 g, 27.4 mmol) in ethanol (15 mL) and water (4 mL) was added to a stirring solution of 4-dichloromethyl-1-methyl-5-nitro-1H-pyrazole (1.44 g, 6.85 mmol) in ethanol (50 mL) at 50° C. The mixture was stirred vigorously under reflux for 1 h, cooled to room temperature, and diluted with water (14 mL). The aqueous layer was extracted with chloroform and filtered through a layer of celite. Concentration and chromatography on a silica gel column with hexane and ethyl acetate (6:1) gave 1-methyl-5-nitro-1H-pyrazole-4-carbaldehyde (0.42 g, 41%): 1H NMR (300 MHz, CDCl3) δ 9.93 (s, 1H), 8.08 (s, 1H), 4.36 (s, 3H).
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