Synthetic intermediate of 1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine, synthetic intermediate of thionucleoside, and method for producing the same

A compound represented by the following formula [1F]:

• wherein R1 represents a hydroxyl-protecting group;
• R2 represents a hydroxyl-protecting group;
• or R1 and R2 may together form an optionally substituted C1-3 alkylene group;
• and R3C represents a hydrogen atom, a halogen atom, a group represented by the following formula [16]:
• —OR3a [16]
• (wherein R3a represents a hydroxyl-protecting group), or an optionally substituted C1-6 alkyl group;
• R3D represents a hydrogen atom, a halogen atom, a group represented by the following formula [16]
• —OR3a [16]
• (wherein R3a has the same meanings as those described above), or an optionally substituted C1-6 alkyl group;
• R4 and R5 together represent a sulfur atom;
• and R6 represents a hydroxyl group, provided that,
• when R4 and R5 together represent a sulfur atom and one of R3C and R3D represents a hydrogen atom, the other represents a halogen atom, or an optionally substituted C1-6 alkyl group.