The general procedure for the synthesis of (2R,3S,4S,5S)-2-acetoxy-5-((benzoyloxy)methyl)tetrahydrofuran-3,4-diyl dibenzoate from ethanoic anhydride and azacitidine impurity 27 was as follows: with reference to the method of Recondo and Rinderknecht, the compound was prepared from L-ribose by the preparation of 1-O-acetyl-2,3,5-tri-O- benzoyl-β-D-ribofuranose (1, R1 = Ac), or the compound was prepared from D-ribose (R2 = Bz). This was done as follows: the L-ribose (150 g, 1.0 mol) was dissolved in methanol (2.5 L) containing 1% HCl and after stirring for 2 h, the reaction mixture was neutralized with pyridine (250 mL). Subsequently, the mixture was concentrated under vacuum and the residue was dissolved in pyridine (1 L). Benzoyl chloride (385 mL, 3.3 mol) was added dropwise to this solution under cooling at 0°C. The reaction mixture was allowed to stand at room temperature overnight and then concentrated under vacuum at 35-40 °C. The residue was dissolved in ethyl acetate (1.5 L), and the organic phase was washed sequentially with cold water (2 x 0.5 L), 1N H2SO4 (3 x 0.5 mL), water (0.5 L), and saturated sodium bicarbonate solution (2 x 0.5 mL), and dried with magnesium sulfate. The dried organic phase was concentrated under vacuum to a syrupy consistency, which was dissolved in a mixture of glacial acetic acid (200 mL) and acetic anhydride (0.5 L). Concentrated sulfuric acid was added dropwise to this solution at 0°C. After completion of the reaction, the solidified product was filtered and washed sequentially with cold water (2×0.5 L), saturated sodium bicarbonate solution (2×0.5 L), cold water (2×0.5 L), and finally recrystallized from methanol to afford compound 1 (225 g, 45% yield) with a melting point of 124-125 °C. The 1H-NMR spectrum of the compound was consistent with that of the D-isomer.
