The general procedure for the synthesis of 4-methyl-3,4-dihydroquinoxalin-2(1H)-one from formaldehyde and 3,4-dihydroquinoxalin-2(1H)-one was as follows: sodium cyanoborohydride (425 mg, 6.76 mmol) was sequentially added to a methanol (3 mL) solution of 3,4-dihydroquinoxalin-2(1H)-one (500 mg, 3.378 mmol), paraformaldehyde (102 mg) and acetic acid (30 μL). The reaction mixture was stirred at room temperature for 4 hours. Subsequently, another portion of sodium cyanoborohydride (212 mg, 3.37 mmol), paraformaldehyde (51 mg) and concentrated hydrochloric acid (10 μL) was added to the reaction mixture. The reaction mixture was stirred at 50 °C for 5 h and then cooled to room temperature. The pH was adjusted to 8 with base and the solvent was evaporated under reduced pressure. The target product was extracted with ethyl acetate and washed with brine and dried over anhydrous sodium sulfate. After evaporation of the solvent under reduced pressure, 4-methyl-3,4-dihydroquinoxalin-2(1H)-one (537 mg, 98% yield) was obtained as a beige powder.LC-MS ESI m/z 163 [M + H]+.
