N4-Acetyl-1-(2, 3-di-O-acetyl-4-thio-β-D-arabinofuranosyl) cytosine (2) was synthesized in three steps from 1-(4-thio-β-D- arabinofuranosyl)cytosine (1). The reaction of this partially blocked 4′-thio-ara-C derivative 2 with 2-chloro-4H-1,3,2-benzodioxaphosphorin-4- one gave the 5′-phosphitylate derivative 3, which on reaction with pyrophosphate gave the 5′-nucleosidylcyclotriphosphite 4. Product 4 was then oxidized with iodine/pyridine/water and deblocked with concentrated ammonium hydroxide to provide the desired 4′-thio-ara-C-5′- triphosphate 5. This triphosphate 5 was converted to 4-thio-ara-C-5′- monophosphate 6 by treatment with snake venom phosphodiesterase I. The details of the synthesis, purification, and characterization of both nucleotides are described. Copyright © Taylor & Francis, Inc.