Sodium hydride (containing mineral oil at 40%, 0.68 g) was added to a solution of tert-butyl 5-chloro-2-(diethoxyphosphoryl)valerate (5.59 g) in THF (50 mL) in a nitrogen atmosphere at room temperature, and the reaction solution was stirred at room temperature for 30 minutes. A solution of 2-fluoro-5-methoxy-4-(4-methyl-1H-imidazol-1-yl)benzaldehyde (3.19 g) in THF (10 mL) was added to the reaction mixture, and the reaction solution was stirred at room temperature for 4 hours. Water and ethyl acetate were added to the reaction mixture and the organic layer was partitioned. The organic layer was washed with brine, and then dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The resulting residue was purified by silica gel chromatography (elution solvent: heptane-ethyl acetate system) to obtain tert-butyl (E)-5-chloro-2-(2-fluoro-5-methoxy-4-(4-methyl-1H-imidazol-1-yl)benzylidene)valerate (5.93 g). A solution of tert-butyl (E)-5-chloro-2-(2-fluoro-5-methoxy-4-(4-methyl-1H-imidazol-1-yl)benzylidene)valerate in trifluoroacetic acid (30 mL) was stirred while cooling with ice. After 2 hours, the reaction solution was concentrated under reduced pressure. Tert-butyl methyl ether was added to the resulting residue, and the resulting deposited solid was filtered to obtain 3 g of the title compound. Properties data of the compound are as follows.
