Diethyl Oxalate → 1,4-Dihydrobenzo[g]quinoxaline-2,3-dione

According to Step 1 of Reference Example 10-3, diethyl oxalate (3.4 mL, 25 mmol) and ytterbium trifluoromethanesulfonate (196 mg, 0.32 mmol) were added to naphthalene-2,3-diamine (1.0 g, 6.3 mmol) and the mixture was stirred at 80° C. for 2 hours. After water was added to the reaction mixture, the precipitate was washed with ethanol. Thus, benzo[g]quinoxaline-2,3(1H,4H)-dione (1.4 g, yield: 99%) was obtained.