(a) Synthesis of 3,4-dihydro-2H-pyrido[3,2-b][1,4]oxazine: To a solution of 2H-pyrido[3,2-b][1,4]oxazin-3(4H)-one (5.00 g, 33.3 mmol) in tetrahydrofuran (THF, 40 mL) was slowly added lithium aluminium hydroxide (66.6 mL, 1.0 M THF solution. 66.6 mmol). After addition, the reaction mixture was heated to reflux. After 18 hours of reaction, the reaction was cooled to 0 °C and water (4 mL), sodium hydroxide solution (4 mL, 15%) and water (10 mL) were added sequentially to quench the reaction. Subsequently, the reaction mixture was filtered through diatomaceous earth and the filtrate was concentrated to afford the target product 3,4-dihydro-2H-pyrido[3,2-b][1,4]oxazine (3.87 g, 85% yield) as a blue-gray powder. The product was characterized by 1H NMR (500 MHz, DMSO-d6): δ 7.53 (dd, J=4.5,1.0 Hz, 1H), 6.90-6.89 (m, 1H), 6.61 (br s, 1H), 6.44 (dd, J=8.0,3.0 Hz, 1H), 4.08 (t, J=4.5 Hz, 2H), 3.39- 3.36 (m, 2H); mass spectrum (ESI) m/e 131 (M+H)+.
![2H-Pyrido[3,2-b][1,4]oxazin-3(4H)-one](/CAS/GIF/20348-09-8.gif)
![3,4-Dihydro-2H-pyrido[3,2-b][1,4]oxazine](/CAS/GIF/20348-23-6.gif)