An n-butyl lithium-n-hexane solution (1.6 M) was added to a mixture of 3-(2-thienyl)-5-trifluoromethyl-1H-pyrazole and THF at −60° C. or below, followed by stirring at 0° C. for 50 minutes. Ethyl chloroformate was added to the reaction solution at −60° C. or below. After stirring at −78° C. for 1 hour, it was subjected to purification in the usual way to give a mixture of ethyl 5-(1-ethoxycarbonyl-5-trifluoromethyl-1H-pyrazol-3-yl)thiophene-2-carboxylate and ethyl 5-(1-ethoxycarbonyl-3-trifluoromethyl-1H-pyrazol-5-yl)thiophene-2-carboxylate as a light yellow solid. Then, a mixture of this mixture with sodium bicarbonate, ethanol, 1,4-dioxane and water was stirred at room temperature for 3 days, and it was subjected to purification in the usual way to give ethyl 5-(5-trifluoromethyl-1H-pyrazol-3-yl)thiophene-2-carboxylate as colorless powder crystals. This was hydrolyzed with a base in the usual way to give 5-(5-trifluoromethyl-1H-pyrazol-3-yl)thiophene-2-carboxylic acid.
