Synthesis of pyrazole C-nucleosides via Tin(IV) chloride-promoted reactions of β-d-ribofuranosyl cyanide with β-dicarbonyl compounds

2,3,5-Tri-O-benzoyl-β-d-ribofuranosyl cyanide reacts with methyl acetoacetate and diethyl malonate in the presence of stoichiometric amounts of SnCl4 to give a β-d-ribofuranosyl-enaminoketoester and a β-d-ribofuranosyl-enaminodiester, respectively. The β-d-ribofuranosyl-enaminoketoester was debenzoylated, treated with 2,2-dimethoxypropane and tert-butyl-dimethylsilyl chloride to give the methyl 3-amino-3-(5′-O-tert-butyldimethylsilyl-2′,3′-O-isopropylidene-β-d-ribofuranosyl)-2-acetyl propanoate obtained in good yield together with small amounts of its α-anomer. The reactions of the β-anomer with hydrazine, methyl- and phenylhydrazine, carried out under controlled experimental conditions, afforded the pyrazole C-nucleosides in good yields as β-anomers.