Hydrochloric Acid → 6-Chloro-2-methylpyridine-3-sulfonyl chloride

Under 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%).