[Cl:1][C:2]1[CH:7]=[CH:6][N:5]=[C:4]([C:8]([CH:12]2[CH2:14][CH2:13]2)=[CH:9][O:10]C)[C:3]=1OC.S(=O)(=O)(O)O.[CH2:22]1COCC1>>[Cl:1][C:2]1[CH:7]=[CH:6][N:5]=[C:4]([CH:8]([CH:12]2[CH2:14][CH2:13]2)[CH:9]=[O:10])[C:3]=1[CH3:22]1.02 g of 2-(4-chloro-3-methoxy-2-pyridyl)-2-cyclopropyl-1-methoxyethene (XI) was dissolved in 10 ml of THF and 10 ml of dilute sulfuric acid was added to the resulting mixture and the resulting mixture was concentrated at 50° C. under reduced pressure. The resulting mixture was poured into water and neutralized with saturated sodium bicarbonate solution, and thereafter, extracted with chloroform. The resulting organic layer was washed with saturated salt water, and thereafter, dried over anhydrous sodium sulfate and the solvent was distilled off under reduced pressure. The resulting residue was purified by silica gel column chromatography (eluent; hexane:ethyl acetate=4:1) to obtain 0.81 g of 2-(4-chloro-3-methyl-2-pyridyl)-2-cyclopropylethanal (XII).
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