Step 3: Manganese dioxide (1.12 g, 12.9 mmol) was added to a solution of (5-fluoro-1H-indazol-3-yl)methanol (0.215 g, 1.29 mmol) in ethyl acetate (10 mL) at room temperature. The reaction mixture was stirred overnight and filtered through diatomaceous earth. The filtrate was concentrated and the resulting residue was purified by silica gel column chromatography (eluent: chloroform/methanol) to afford 5-fluoro-1H-indazole-3-carbaldehyde (0.150 g, 70% yield). The structure of the product was confirmed by 1H NMR (300 MHz, DMSO-d6): δ 7.41 (ddd, J = 2.2,8.8,9.5 Hz, 1H), 7.75-7.81 (m, 2H), 10.18 (s, 1H), 14.32 (br s, 1H).
