A Facile Synthesis of 6-Aryl-5-cyano-1-(β-d-pyranosyl or β-d-furanosyl)-2-thiocytosines

The treatment of a piperidinium salt of 6-aryl-5-cyano-2-thiouracil with an O-peracetyl-α-d-pyranosyl bromide produces a mixture of N1-(β-d-pyranosyl)-2-thiouracil and its N1,S2-disubstituted analog. By contrast, the reaction of a silyl derivative of the 2-thiouracil with an O-peracetyl-β-d-pyranose furnishes the mononucleoside selectively. Both the mononucleoside/dinucleoside mixture and pure mononucleoside undergo ammonolysis under mild conditions to give the β-d-nucleoside of 6-aryl-5-cyano-2-thiocytosine. The silyl method also provides an easy access to β-d-ribosyl nucleosides.