5-Chloro-2,4-dihydroxybenzaldehyde (493 mg, 2.86 mmol), dodecanal (641 mg, 3.48 mmol) and calcium chloride dehydrate (3.02 mg, 2.03 mmol) were dissolved in methanol (6 ml), added with a potassium hydroxide methanol solution (1.0 M, 4.0 ml) at 0° C. and stirred for 24 hours. The reaction mixture was made acidic with 1 M hydrochloric acid and extracted with ethyl acetate, and after post-treatment, the crude product (1.18 g) was purified by silica gel column chromatography (hexane:ethyl acetate=7:1) to obtain 5-chloro-2,4-dihydroxy-3-(1-hydroxydodecyl)benzaldehyde (colorless solid 475 mg, yield 46%) and 5-chloro-2,4-dihydroxybenzaldehyde (209 mg, recovery 42%).
