59.40 g out of 62.69 g of the crude 2,3,5-tri-O-benzoyl-1-O-methyl-L-ribofuranose synthesized in (2) above was added to a 1-L flask. Thereafter, acetic anhydride (31.2 ml; 2.65 equivalents), acetic acid (26.7 ml; 3.73 equivalents), and pyridine (7.1 ml; 0.8 equivalents) were added to the flask. The obtained mixture was cooled to an ice-cooled temperature, and thereafter, concentrated sulfuric acid (13.5 ml; 2.03 equivalents) was slowly added dropwise thereto. After completion of the reaction for 30 minutes, cold water (200 ml) was slowly added dropwise to the reaction product, and crystals were then filtrated. The obtained solid was dried. Thus, 69.83 g of crude 1-O-acetyl-2,3,5-tri-O-benzoyl-L-ribofuranose was obtained in the form of a light yellow solid. The obtained compound was analyzed by HPLC. As a result, the ratio of α-/β-anomers in the 1-O-acetyl-2,3,5-tri-O-benzoyl-L-ribofuranose was 4/96, and the reaction yield of the β-anomer from the total 3 steps was 84%. 69.83 g of the crude 1-O-acetyl-2,3,5-tri-O-benzoyl-L-ribofuranose was suspended in isopropyl alcohol (92 ml), and the suspension was then heated to reflux for 1 hour. Thereafter, the reaction solution, in which the aforementioned compound had been completely dissolved, was cooled to an ice-cooled temperature, and it was then stirred for 30 minutes. Thereafter, the resultant mixture was filtrated, and the obtained crystals were then dried under reduced pressure. Thus, 1-O-acetyl-2,3,5-tri-O-benzoyl-β-L-ribofuranose was obtained from L-ribofuranose in the form of white crystals with a purity of 99% or more at a total yield of 75%.
