A solution of 3.0 g of 1-β-D-arabinofuranosyl-1,5-dihydro-4H-imidazo[4,5-c]pyridin-4-one, eight ml of acetic anhydride, 150 ml of pyridine, and 50 mg of p-dimethylaminopyridine is stirred at room temperature for 24 hours, evaporated in vacuo, co-evaporated with xylenes, and distributed between water and ethyl acetate. The ethyl acetate layer is washed with saturated NaHCO3 and dried with MgSO4. Removal of the ethyl acetate provides 1-(2,3,5-tri-O-acetyl-β-D-arabinofuranosyl)-1,5-dihydro-4H-imidazo[4,5-c]pyridin-4-one. This is dissolved in 150 ml of dry pyridine, treated with 8.8 g phosphorus pentasulfide, and heated under reflux for 5 hours. The pyridine is removed in vacuo and the residue is treated with water. The precipitate is filtered, washed with water, and recrystallized from ethanol to afford 1-(2,3,5-tri-O-acetyl-β-D-arabinofuranosyl)-1,5-dihydro-4H-imidazo-[4,5-cl]pyridine-4-thione.
