The general procedure for the synthesis of 5,6-dichloro-1H-indazole from 4,5-dichloro-2-methylaniline is as follows:
Step 2: Acetic anhydride (2.09 ml, 22.1 mmol) was slowly added to a solution of 4,5-dichloro-2-methylaniline (1.69 g, 9.6 mmol) in trichloromethane (25 ml) at 0 °C. The reaction mixture was gradually warmed up to room temperature and stirred for 1 hour during which a thick white precipitate gradually formed. Potassium acetate (283 mg, 2.88 mmol) was then added followed by slow addition of isoamyl nitrite (2.78 ml, 20.6 mmol). The reaction mixture was heated to reflux overnight. Upon completion of the reaction, the dark orange homogeneous reaction mixture was cooled to room temperature and concentrated. Water (10 mL) was added and concentrated again to obtain an orange solid. The solid was suspended in a conical flask, 15 mL of hydrochloric acid was added, heated at 60 °C for 2 h, then cooled to 0 °C and neutralized with 50% sodium hydroxide solution. It was extracted with ethyl acetate, dried over magnesium sulfate and concentrated to give an orange solid. The solid was dissolved in tetrahydrofuran/methanol (1:1, 25 mL) and 10% sodium hydroxide solution (3 mL) was added. The dark maroon colored reaction mixture was stirred at room temperature for 5 min, then neutralized with 1.0 M hydrochloric acid and diluted with water. It was extracted with ethyl acetate (2 times), dried over magnesium sulfate and concentrated. The residue was adsorbed on silica gel and purified by chromatography with 30% to 50% ethyl acetate/hexane solvent mixture to give 1.50 g (84% yield) of 5,6-dichloro-1H-indazole as a light orange solid.1H NMR (CDCl3, 300 MHz): δ (ppm) 8.04 (s, 1H), 7.89 (s, 1H), 7.68 (s, 1H). 1H).
