[2-(Bromomethyl)-3-chlorophenyl]methanol → 2-(Bromomethyl)-3-chlorobenzaldehyde

Dichloromethane (15 ml) and manganese dioxide (1.69 g, 19.4 mmol) were added to [2-(bromomethyl)-3-chlorophenyl]methanol obtained in Example 10d (440 mg, 1.87 mmol), followed by stirring at mom temperature overnight. Manganese dioxide (1.2 g, 13.8 mmol) was further added, followed by heating with stirring at 40° C. for 2.5 hours. The reaction mixture was filtered through Celite and washed with ethyl acetate. The filtrate was concentrated under reduced pressure, and the resulting crude product was purified by column chromatography (silica gel, elution solvent: heptane/ethyl acetate=99/1->90/10) to give the title compound (289 mg, yield 66.2%).