[CH2:1]=[O:2].[Cl-].[Mg+2].[Cl-].C(N(CC)CC)C.[F:13][C:14]1[CH:19]=[CH:18][C:17]([CH3:20])=[CH:16][C:15]=1[OH:21].Cl>C(#N)C>[F:13][C:14]1[C:15]([OH:21])=[C:16]([C:17]([CH3:20])=[CH:18][CH:19]=1)[CH:1]=[O:2]
Paraformaldehyde (3.86 g, 133 mmol), magnesium chloride (6.32 g, 66.5 mmol) and triethylamine (11.6 ml, 83.3 mmol) were added to a solution of 2-fluoro-5-methylphenol (4.19 g, 33.3 mmol) in acetonitrile (100 ml), and the mixture was vigorously stirred at 90° C. for 10 days. The reaction mixture was poured into 1N hydrochloric acid and extracted with ethyl acetate. After the organic layer was successively washed with 1N hydrochloric acid, water and a saturated aqueous NaCl solution, and dried with anhydrous sodium sulfate, it was concentrated under reduced pressure. The obtained residue was subjected to silica gel column chromatography (eluting solvent: hexane/ethyl acetate=4/1) to obtain crude 3-fluoro-2-hydroxy-6-methylbenzaldehyde. According to a method similar to Example (28-3) and Example (28-4), from the crude compound obtained in the above, tert-butyl 2-[(tert-butoxycarbonyl)oxy]-3-fluoro-6-methylbenzoate was obtained (624 mg, three-step total yield: 6%).
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