To a cooled (10 °C) mixture of compound 4 and tetrahydrofuran (4.0 vol) were added triethylamine (1.5 equiv.), cyanoacetic acid (1.5 equiv.), and 1-ethyl-3-(3dimethylaminopropyl) carbodiimide hydrochloride (1.5 equiv.). The mixture was stirred at 25 °C for 1 h. After 5% citric acid aqueous solution and methyl tert-butyl ether were added to the mixture, the aqueous layer was separated. The organic solution was subsequently washed with 5% sodium bicarbonate aqueous solution and 10% sodium chloride aqueous solution. By repeatedly concentrating with methanol multiple times, the solvent is replaced with methanol. The volume of solution was adjusted to 4.5 vol with methanol to obtain a compound 5/methanol solution.