[Br:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][CH:11]=1)[NH:8][C:7](=[O:12])[CH2:6][NH:5]2.C=O.O.[C:16](O)(=O)C.[BH3-]C#N.[Na+]>CO>[Br:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][CH:11]=1)[NH:8][C:7](=[O:12])[CH2:6][N:5]2[CH3:16]To a solution of 6-bromo-3,4-dihydroquinoxalin-2(1H)-one (370 mg, 1.63 mmol) in 10 ml methanol was added a solution of 37% formaldehyde in water (59 mg, 1.95 mmol) and acetic acid (196 mg, 3.26 mmol). The mixture was stirred at room temperature for 2 h, then refluxed for 1 h. After addition of NaBH3CN (307 mg, 4.89 mmol) stirring was continued for 2.5 h at room temperature. Then methanol was removed under vacuum. To the remaining mixture water was added. Then the solution was basified to pH 8-9 with an aqueous solution of sodium hydrogen carbonate and extracted several times with ethyl acetate. The combined organic layers were washed with brine, dried over MgSO4 and evaporated, affording 6-bromo-4-methyl-3,4-dihydroquinoxalin-2(1H)-one (369 mg, 1.53 mmol, 94%) as off-white solid.
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