To a mixture of 0.20 g of 2-[(4-acetyl-3-hydroxy-2-propylbenzyl)thio]-5-mercapto-1,3,4-thiadiazole obtained in Example 8, 0.15 g of 7-bromoheptanoic acid, 0.18 g of anhydrous potassium carbonate and 5 ml of methyl ethyl ketone was added a catalytic amount of tetra-n-butylammonium bromide. The mixture was stirred at 60° C. for 1 hour. An aqueous sodium hydroxide solution and ethyl acetate were added to the reaction solution to perform fractionation. The aqueous phase was made acidic with 2N hydrochloric acid and extracted with ethyl acetate. After the extract was dried over anhydrous magnesium sulfate, the solvent was distilled off. The residue was applied to silica gel column chromatography. Elution with a mixture of hexane-ethyl acetate (2:1) gave 0.20 g of 7-[[5-[(4-acetyl-3-hydroxy-2-propylbenzyl)thio]-1,3,4-thiadiazol-2-yl]thio]heptanoic acid.
