Synthesis
C. General procedure for the preparation of 4-hydroxyindazole from 4-aminoindazole: 4-aminoindazole (9.6 g, 0.072 mol) was dissolved in a glass reaction vessel containing 7.2 g of concentrated sulfuric acid in 75 mL of water. The mixture was transferred to a stainless steel autoclave, sealed and heated to 170°C for overnight reaction. Upon completion of the reaction, a large amount of black precipitate appeared in the reaction mixture. The reaction mixture was diluted with ethyl acetate and water and transferred to a dispensing funnel for distribution. The aqueous layer was extracted with ethyl acetate several times until 4-hydroxyindazole was completely transferred to the organic phase. The organic phases were combined, washed with saturated brine, dried over anhydrous magnesium sulfate, filtered and concentrated to give a dark brown or black oil. The crude product was purified by silica gel column chromatography with an eluent of 50% ethyl acetate/hexane mixture to give 4-hydroxyindazole as an off-white solid (3.3 g, 33% yield).
References
[1] Patent: WO2004/39796, 2004, A1. Location in patent: Page/Page column 71-72
[2] Patent: US6534504, 2003, B1
[3] Patent: US6841563, 2005, B1. Location in patent: Page column 20
[4] Journal of Medicinal Chemistry, 2012, vol. 55, # 8, p. 3975 - 3991
[5] Journal of the Chemical Society, 1955, p. 2412,2419
