Thionyl Chloride → Quinoxaline-6-carbonyl chloride

Participants

Reaction Steps

A mixture of quinoxaline-6-carboxylic acid (2 g, 11.49 mmol) and thionyl chloride (30 mL) was stirred at reflux for 2 hours. The reaction mixture was concentrated to dryness using a rotary evaporator to afford quinoxaline-6-carboxylic acid chloride (crude quantitative). A solution of the above acid chloride (11.49 mmol) in DCM (50 mL) and pyridine (20 mL) was mixed with N,O-dimethyl hydroxylamine HCl salt (2.24 g, 23 mmol) and stirred at room temperature for 12 hours. The reaction was quenched by adding aqueous HCl (50 mL, 1 N), extracted with DCM (3×100 mL), concentrated using a rotary evaporator. The residue was further purified by column (SiO2, Hexanes/EtOAc=1:3) to yield quinoxaline-6-carboxylic acid methoxy-methyl-amide (2 g, 80%). To a solution of the above Weinreb amide (2.0 g, 9.2 mmol) in THF (30 mL) at 0° C. was added methyl magnesium bromide (3.9 mL, 11.6 mmol). The reaction mixture was stirred at 0° C. for 2 hours and then 1 hour at room temperature, quenched by adding aqueous HCl (20 mL, 1 N), extracted with DCM (3×100 mL), concentrated using a rotary evaporator. The residue was further purified by column (SiO2, Hexanes/EtOAc=1:3) to yield 6-acetylquinoxaline (1.17 g, 74%). A solution of 2-chloronicotinic acid ethyl ester (5.0 g, 27 mmol) in MeOH (25 mL) was mixed with sodium methoxide (25.6 mL, 112.5 mmol) and stirred at reflux for 12 hours. The reaction was quenched by adding water (100 mL), extracted with DCM (3×100 mL), concentrated using a rotary evaporator to afford 2-methoxynicotinic acid methyl ester (3.2 g, 71%). A solution of 6-acetylquinoxaline (0.62 g, 3.6 mmol), 2-methoxynicotinic acid methyl ester (0.64 g, 3.8 mmol), and sodium hydride (0.46 g, 11.4 mmol) in THF (100 mL) was stirred at room temperature for 16 hours. The reaction was quenched by adding water (100 mL) and AcOH (20 mL), extracted with dichloromethane (3×100 mL), and concentrated using a rotary evaporator. The residue was re-dissolved in DCM (5 mL) and MeOH (3 mL) and was diluted with Hexanes (50 mL). The solid was removed by filtration and the filtrate was concentrated to afford the diketo compound (0.7 g, 60%). A solution of the above diketone (0.4 g, 1.3 mmol) in AcOH (50 mL) and sulfuric acid (conc., 15 drops) was stirred at reflux for 1 hour. Most of the solvent was removed using a rotary evaporator. The residue was re-dissolved in MeOH and neutralized with potassium carbonate to pH=8. The solid residue was removed by filtration, washed with MeOH and DCM. The filtrate was extracted with CH2Cl2 (3×100 mL) and concentrated using a rotary evaporator. The solid residue was purified by column (SiO2, Hexanes/EtOAc/MeOH=2:2:1) to afford 2-(quinoxalin-6-yl)-4H-pyrano[2,3-b]pyridin-4-one (90 mg, 24%); MS (ES) m/z: 276 (M+1); MP 272.3-274.8° C.

Provenance & Evidence