A facile synthesis of 8‐(β‐D‐ribofuranosyl)‐4‐thioxo‐3H‐pyrazolo[1,5‐a]‐1,3,5‐triazine and its α‐anomer

A versatile intermediate 2 for C‐nucleoside synthesis was treated with thiosemicarbazide to obtain thiosemicarbazone 6, which was then converted to 3‐aminopyrazol‐2N‐thiocarboxamide derivatives 7 and 8 by the reaction of 6 and sodium ethoxide. 4‐Thioxo‐4H‐pyrazolo[1,5‐a]‐1,3,5‐triazine C‐nucleosides 11 and 12 were obtained by the ring closure reaction of 7 and 8 with triethyl orthoformate. Brief treatment of 11 and 12 with 10% methanolic hydrogen chloride afforded C‐nucleosides 4 and 13, respectively, without anomerization. Identification of compounds 4 and 13 was made on the basis of 1H nmr and uv spectra, as well as chemical conversion to known compounds with established configurations. Model compounds were also synthesized in order to confirm the heterocyclic moieties.