ETHYL 6,7-DIHYDRO-5H-PYRAZOLO[5,1-B][1,3]OXAZINE-2-CARBOXYLATE

Synthesis

Step 1: Preparation of ethyl 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-2-carboxylate: To a stirred suspension of ethyl 5-keto-4,5-dihydro-1H-pyrazole-3-carboxylate (10.34 g, 0.66 mol) and potassium carbonate (36.62 g) in acetonitrile (500 mL) was slowly added 1,3-dibromopropane (14.7 g). The reaction mixture was heated to reflux for 16 hours. After completion of the reaction, the mixture was cooled to room temperature, filtered and the solid was washed with acetonitrile. The filtrate and washings were combined and concentrated to dryness under reduced pressure to give an oily residue. The residue was dissolved in ethyl acetate and extracted with water. The organic phase was separated, dried with anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give the crude product. By further purification, ethyl 6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-2-carboxylate (8.80 g, 68% yield) was obtained with a melting point of 44-46 °C and a mass spectrum showing the (M + H)+ peak at 197.

References

[1] Journal of Medicinal Chemistry, 2006, vol. 49, # 15, p. 4623 - 4637
[2] Patent: WO2003/93279, 2003, A1. Location in patent: Page/Page column 137
[3] Patent: US2006/276445, 2006, A1. Location in patent: Page/Page column 42
[4] Patent: WO2017/145013, 2017, A1. Location in patent: Page/Page column 54-55
[5] Journal of Heterocyclic Chemistry, 1993, vol. 30, # 4, p. 1097 - 1100