To 1.26 g of Compound 3-1-1 obtained in Production Example 2, 0.255 ml of methanesulfonic acid and 88.3 mg of water were added, and the resulting mixture was stirred at 110° C. for 3.5 hours. After the reaction, water was added to the reaction solution thus obtained, and extraction with ethyl acetate was then carried out. The extract was washed with an aqueous solution of sodium hydrogen carbonate and saturated brine. After the organic layer was dried over anhydrous sodium sulfate, the solvent was distilled away.
