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.