General procedure for the synthesis of 6-chloropyridinecarbonyl chloride from 6-hydroxypyridine-2-carboxylic acid: to 20 g (144 mmol) of 6-hydroxypyridine-2-carboxylic acid was added 50.6 g (330 mmol) of phosphorus trichloride and 99.8 g (479 mmol) of phosphorus pentachloride, and the reaction was carried out by stirring the mixture for 8 hours at 90 °C. Upon completion of the reaction, the mixture was cooled, followed by the addition of 8.6 g of formic acid, and the reaction mixture was concentrated using a rotary evaporator to quantitatively obtain the target product 6-chloropyridinecarbonyl chloride.
