[ClH:1].N[C:3]1[CH:4]=[CH:5][C:6]([Cl:10])=[N:7][C:8]=1[CH3:9].N([O-])=O.[Na+].[S:15](=[O:17])=[O:16]>O>[Cl:10][C:6]1[N:7]=[C:8]([CH3:9])[C:3]([S:15]([Cl:1])(=[O:17])=[O:16])=[CH:4][CH:5]=1Under ice-cooling, thionyl chloride (4 mL) was added dropwise to water (24 mL) over 20 min. The mixture was stirred at room temperature for 12 hr to give a sulfur dioxide-containing solution. Separately, to the concentrated hydrochloric acid (6 mL) was added 5-amino-2-chloro-6-methylpyridine (1.0 g) with stirring under ice-cooling, and a solution of sodium nitrite (0.5 g) in water (2 mL) was added dropwise over 10 min. The reaction mixture was gradually added at 5° C. to the above-mentioned sulfur dioxide-containing solution added with cuprous chloride (10 mg). Under ice-cooling, the mixture was further stirred for 30 min, and the precipitate was collected by filtration, and washed with water to give the title compound as a pale-yellow solid (yield 1.1 g, 67%).
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