To a cooled (0 °C) solution of compound 3/methanol from Step 1B was added dropwise 4 N hydrogen chloride/ethyl acetate solution (1.8 equiv.) over 1.5 h, and then the mixture was stirred at 0 °C for 1 h. After n-heptane, water, and ethyl acetate were added to the reaction mixture, the organic layer was separated. The aqueous solution was washed with n-heptane, and then the aqueous solution was adjusted to weak basic condition with 1 N sodium hydroxide aqueous solution. After ethyl acetate was added to the mixture, the aqueous layer was separated. Ethyl acetate was added to the separated aqueous layer, and then the aqueous layer was separated. The combined organic layer was washed with 2.5% sodium chloride aqueous solution, and then the mixture was filtered through activated carbon. The activated carbon was washed with ethyl acetate. The mixed filtrate was concentrated under reduced pressure to 11.5 vol, and then n-heptane (10.0 vol) was added to the mixture. A solution of (-)-10-camphorsulfonic acid (0.17 equiv.) dissolved in tetrahydrofuran (1.6 vol) was added dropwise to the mixture over 2.5 h, and then the resulting slurry was stirred at 25 °C for 1 h. After a solution of (-)-10-camphorsulfonic acid (0.49 equiv.) dissolved in tetrahydrofuran (4.4 vol) was added dropwise to the slurry over 3.5 h, n-heptane (18.0 vol) was added dropwise over 5 h. The slurry was stirred at 25 °C for 1 h and filtrated. The filtrated solid was washed with a mixture of ethyl acetate (3.0 vol) and n-heptane (6.0 vol). The obtained solid was dried under reduced pressure at 25 °C to afford compound 4 (61% yield).