To a solution of methyl 2-(4-fluorophenyl)-3-(1,2,3,4-tetrahydroquinolin-1-yl)quinoxaline-6-carboxylate (300 mg, 0.73 mmol) in chloroform (50 mL) was added sulfuric acid (213.6 mg, 2.18 mmol) and thionyl chloride (514 mg, 4.36 mmol) dropwise with stirring at 85° C. for 3 h in an oil bath. The reaction was then quenched by the addition of ice-water, extracted with dichloromethane (3×80 mL). The organic layers combined and dried over magnesium sulfate, concentrated under vacuum to give the residue, which was applied onto a silica gel column with 2% ethyl acetate in petroleum to afford methyl 3-[6-(chlorosulfonyl)-1,2,3,4-tetrahydroquinolin-1-yl]-2-(4-fluorophenyl)quinoxaline-6-carboxylate as a yellow solid (260 mg, 70%).
