1173003-61-6

1173003-60-5

1173003-61-6

General procedure for the synthesis of 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-3-carboxylic acid from ethyl 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-3-carboxylate: Ethyl 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-3-carboxylate (9.67 g, 49.3 mmol) was was dissolved in a mixed solvent of tetrahydrofuran (150 mL) and ethanol (50 mL), and 1M aqueous lithium hydroxide (148 mL, 148 mmol) was slowly added at 0 °C. The reaction mixture was heated to 60 °C and the reaction was stirred for 2 days. After completion of the reaction, it was cooled to room temperature and the volatile solvent was removed by distillation under reduced pressure. The residue was transferred to a conical flask and an ice-water mixture was added. The reaction solution was acidified with 10 M hydrochloric acid to pH 2. Subsequently, the aqueous phase was extracted several times with a 3:1 chloroform/isopropanol solvent mixture. The organic layers were combined, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to afford the target product 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-3-carboxylic acid as a solid (8.14 g, 98% yield). The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 7.77 (s, 1H), 4.37 (t, 2H, J = 5.29 Hz), 4.07 (t, 2H, J = 6.11 Hz), 2.20-2.15 (m, 2H).