General procedure for the synthesis of quinoxaline-5-sulfonyl chloride from quinoxalin-5-amine: 5.3 g of 5-aminoquinoxaline was dissolved in 16 ml of concentrated hydrochloric acid and subsequently cooled to -12 °C. A diazonium salt solution was obtained by slowly adding 2.5 g of sodium nitrite solution dissolved in 10 ml of water at a temperature not exceeding -5 °C. Another 65 g of water was cooled to 1 °C, then 22 g of thionyl chloride was carefully added dropwise and 0.1 g of copper chloride was added as a catalyst to prepare to obtain a sulfur oxide solution. The diazonium salt solution was slowly added to the sulfur oxide solution in batches while maintaining a temperature not exceeding -5°C. After completion of the reaction, three extractions were carried out using 60 ml of dichloromethane. The organic phases were combined and concentrated to dryness. The crude product was purified by recrystallization from ethyl acetate to give 6.1 g of light yellow solid product in 73% yield.
