Step 3: Manganese dioxide (0.991 g, 11.4 mmol) was added to a solution of (5-chloro-1H-indazol-3-yl)methanol (0.208 g, 1.14 mmol) in ethyl acetate (10 mL) at room temperature. The reaction mixture was stirred overnight. Subsequently, the reaction mixture was filtered through diatomaceous earth and the filtrate was concentrated. The resulting residue was purified by silica gel column chromatography (eluent: chloroform/methanol) to afford the target product 5-chloro-1H-indazole-3-carbaldehyde (0.106 g, 51% yield). The product was characterized by 1H NMR (300 MHz, DMSO-d6): δ 7.53 (dd, J = 2.2,8.8 Hz, 1H), 7.77 (d, J = 8.8 Hz, 1H), 8.11 (d, J = 2.2 Hz, 1H), 10.18 (s, 1H), 14.36 (br s, 1H).
