Potassium Permanganate → 5-fluoro-1-methyl-1H-indole-3-carboxylic acid

In acetone (50 ml) was dissolved 5-fluoro-1-methylindole-3-carbaldehyde (1.33 g, 7.52 mmol). To the resulting solution was added dropwise an 1N aqueous solution of potassium permanganate (7.15 ml, 7.15 mmol) under stirring at 0° C. After stirring at room temperature for 18 hours, another 1N aqueous solution of potassium permanganate (4.0 ml, 4.0 mmol) was added, and the mixture was stirred at room temperature for 18 hours. The mixture was filtered through Celite. After washing with water, the filtrate was added with 1N HCl. The crystals thus precipitated were collected by filtration under reduced pressure, washed with water and dried under reduced pressure to give 5-fluoro-1-methylindole-3-carboxylic acid (885 mg, 61%) as a white crystalline powder.