To a solution of inosine (0.998 g, 3.72 mmol) in dimethylformamide (DMF, 30 mL) was sequentially added 2,2-dimethoxypropane (1.8 mL, 14.59 mmol) and p-toluenesulfonic acid monohydrate (0.038 g, 0.372 mmol). The reaction mixture was stirred under argon (Ar) protection for 24 hours. Upon completion of the reaction, DMF was removed by evaporation under reduced pressure, followed by alkalinization of the reaction mixture with 3% ammonia solution and evaporation again to dryness. The resulting residue was purified by silica gel column chromatography (eluent: dichloromethane/methanol, 95/5, v/v) to afford the target compound 22 in white foamy form in 80% yield. The product was characterized by 1H-NMR (200 MHz, DMSO-d6): δ 1.34, 1.50 (6H, s, (CH3)2C), 3.6 (2H, t, CH2-5'), 4.37 (1H, m, H-4'), 4.94 (1H, m, H-3'), 5.18 (1H, t, OH-5'), 5.37 (1H, dd. J = 3.7 Hz, H-2'), 6.12 (1H, d, J = 2.4 Hz, H-1'), 8.05 (1H, s, H-2), 8.32 (1H, s, H-8), and 12.44 (1H, br s, NH-3).ESI-MS analysis showed: m/z 309.1 [M + H]+, molecular formula C13H16N4O5, molecular weight 308.29.
