The aqueous layer was separated and then the obtained organic solution was subsequently washed with 2% citric acid aqueous solution and 5% sodium bicarbonate/5% sodium chloride aqueous solution. Then, the obtained organic solution was concentrated under reduced pressure to 5.5 vol. By repeatedly concentrating with tetrahydrofuran multiple times, the solvent is replaced with tetrahydrofuran. The volume of solution was adjusted to 5.0 vol with tetrahydrofuran to obtain a compound 2/tetrahydrofuran solution.