To a solution of compound 6 and isopropyl acetate (10.0 vol) was added N,Ndimethylformamide (2.0 vol) and the mixture was cooled to 5 °C. Phosphorus oxychloride (0.92 equiv.) was added dropwise to the mixture, and the mixture was stirred at 5 °C for 9 h. After cooling to 0 °C, an aqueous solution of tripotassium citrate (1.5 equiv.) dissolved in water (10.0 vol) was added dropwise to the reaction mixture at 0 °C. After the aqueous solution separated at 5 °C, the obtained organic solution was washed with 2% sodium chloride aqueous solution. The obtained organic solution was concentrated under reduced pressure to 5.0 vol. After n-heptane (0.5 vol) was added to the mixture, the resulting slurry was stirred at 25 °C for 1.5 h. Then, n-heptane (13.0 vol) was added dropwise to the slurry, and the slurry was stirred at 25 °C for 3 h and filtrated. The filtrated solid was washed with a mixture of isopropyl acetate (1.0 vol) and n-heptane (4.0 vol), and then washed with n-heptane (4.0 vol). The obtained solid was dried under reduced pressure at 60 °C to afford compound 7 (83% yield). Step 4 (vol, wt, and equiv. were based on the amount of compound 7)