Synthetic intermediate of 1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosineA compound represented by the formula [1F]:wherein R1 represents a hydrogen atom or a hydroxyl protecting group； R2 represents a hydrogen atom or a hydroxyl-protecting group； R3C represents a hydrogen atom, a halogen atom, an optionally substituted alkyl group or -OR3a corresponding to a hydroxyl-protecting group； R3D represents a hydrogen atom, a halogen atom, an optionally substituted alkyl group or -OR3a corresponding to a hydroxyl-protecting group； R4 represents a halogen atom, a hydroxyl group, an optionally substituted alkylsulfonyloxy group or an optionally substituted arylsulfonyloxy group； R5 and R6 together represent =O or =NOR7 wherein R7 represents a hydrogen atom, an optionally substituted alkyl group, an optionally substituted aryl group, an optionally substituted heterocyclic group or an optionally substituted silyl group； or R4 and R5 together represent a sulfur atom； and R6 represents a hydroxyl group. The compound is useful as an intermediate for producing a thionucleoside.
