Nucleosides and nucleotides. 232. Synthesis of 2'-C-methyl-4'-thiocytidine: unexpected anomerization of the 2'-keto-4'-thionucleoside precursor.

The synthesis of 2'-C-methyl-4'-thiocytidine (16) is described. Since the 2'-keto-4'-thiocytidine derivative 2beta unexpectedly isomerized to 2alpha and the methylation of 2beta proceeded predominantly from the less hindered alpha-face to give 7, the desired product 16 was synthesized via the Pummerer reaction of the sulfoxide 14 and N4-benzoylcytosine.