The general procedure for the synthesis of 4-methoxy-1H-indazole-3-carboxylic acid from 4-methoxyindoline-2,3-dione was as follows: 4-methoxyindoline-2,3-dione (20.7 mmol) was mixed with 1M sodium hydroxide solution (23 mL) and the reaction mixture was heated to 30-40 °C and maintained at this temperature for 30 minutes. Subsequently, the reaction mixture was cooled to 0 °C and treated with an aqueous solution (5.1 mL) of sodium nitrite (20.7 mmol) and maintained for 20 minutes. This solution was slowly added dropwise to a mixture of concentrated sulfuric acid (2.24 mL) and water (43.3 mL) that had been pre-cooled to 0-5 °C, and the reaction mixture was kept at 0-5 °C for 0.5 hours. Next, a concentrated hydrochloric acid (19.6 mL) solution of tin(II) chloride (50.5 mmol) was added drop by drop and the reaction mixture was kept at 0-5 °C for 1 hour. Upon completion of the reaction, the precipitated solid was separated by filtration and dried to afford 4-methoxy-1H-indazole-3-carboxylic acid as a yellow solid (100% yield).
