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To a solution of compound 23 (200 mg, 0.67 mmol) in dry dichloromethane (10 mL), under argon and at 0° C., was added dropwise dry N,N-dimethylformamide (100 μL) and thionyl chloride (200 μL, 2.74 mmol). The reaction mixture was stirred at reflux for 3 h. After cooling to room temperature, the solvent was removed under reduced pressure to afford crude 6-iodoquinoxaline-2-carbonyl chloride. This was suspended in dry tetrahydrofuran (10 mL) and were successively added, under argon, p-nitrophenol (93 mg, 0.67 mmol) and dropwise a solution of triethylamine (95 μL, 0.68 mmol) in dry tetrahydrofuran (5 mL). The solution was stirred at 50° C. for 18 h. After cooling to room temperature, dichloromethane (30 mL) was added and the resulting solution was washed with a 5% aqueous sodium carbonate solution (20 mL). The aqueous layer was extracted with dichloromethane (5×15 mL) and the organic layers were combined, dried on magnesium sulfate and evaporated under reduced pressure. The resulting precipitate was triturated with ether (5 mL) and collected by filtration to afford ester 24 (225 mg, 0.53 mmol) as a beige solid. Yield 80%; mp 228-230° C.; 1H NMR (400 MHz, DMSO-d6) δ 7.71 (d, 2H, J=7 Hz), 8.09 (d, 1H, J=9 Hz), 8.30 (dd, 1H, J=2, 9 Hz), 8.41 (d, 2H, J=7 Hz), 8.72 (d, 1H, J=2 Hz), 9.60 (s, 1H).

1. temperatureat reflux for 3 h
2. customthe solvent was removed under reduced pressure