General procedure for the synthesis of 3,4-dihydro-2H-benzo[b][1,4]oxazin-6-ol from 6-hydroxy-2H-benzo[b][1,4]oxazin-3(4H)-one: 6-Hydroxy-2H-benzo[b][1,4]oxazin-3(4H)-one (0.39 g, 2.35 mmol) was dissolved in tetrahydrofuran (THF) and at 0 °C 1 M borane-THF solution (7.05 mL, 7.05 mmol) was slowly added dropwise for a controlled time of 5 min. After the dropwise addition, the reaction mixture was gradually warmed to room temperature and stirred overnight. Upon completion of the reaction, the excess borane was quenched by careful addition of methanol. Subsequently, the solvent was removed by distillation under reduced pressure and the crude product was extracted with ethyl acetate (EtOAc) to afford pure 3,4-dihydro-2H-benzo[b][1,4]oxazin-6-ol as an oil in quantitative yield (0.34 g). The product was confirmed by nuclear magnetic resonance hydrogen spectroscopy (1H NMR, DMSO-d6): δ 8.48 (s, 1H), 6.37 (d, 1H, J = 8.47 Hz), 5.96 (d, 1H, J = 2.75 Hz), 5.82 (dd, 1H, J = 8.44, 2.76 Hz), 3.97 (t, 2H, J = 4.38 Hz), 3.18 ( t, 2H, J = 4.31 Hz). Nuclear magnetic resonance carbon spectrum (13C NMR, DMSO-d6): δ 152.2, 136.6, 135.8, 116.7, 103.8, 102.0, 65.0, 47.2. electrospray high-resolution mass spectrometry (ES-HRMS) measured [M + H]+ as 152.0711, which is in accordance with the theoretically calculated value (C8H10NO2).
