A series of 8-substituted 1-methyl-1,4-dihydropyrazolo[3′,4′:4,5]¬pyrrolo[2,3-d]pyrimidine (methylpyrazolo-fused 7-deazapurine) ribonucleosides has been designed and synthesized. Two synthetic approaches to the key heterocyclic aglycon 7, (i) a six-step classical heterocyclization starting from 5-chloro-1-methyl-4-nitropyrazole and (ii) a three-step cross-coupling and cyclization approach starting from the zincated 4,6-dichloropyrimidine, gave comparable total yield of 18% vs. 13%. The glycosylation of 7 was attempted by three different methods but only the Vorbrüggen silyl-base protocol was efficient and stereoselective to give desired β-anomeric nucleoside intermediate 17A. Its nucleophilic substitutions or cross-coupling reactions at position 8 and deprotection of the sugar moiety gave eight derivatives of pyrazolo-fused deazapurine ribonucleosides, some of which were weakly fluorescent. Methyl, amino and methylsulfanyl derivatives exerted submicromolar cytotoxic effects in vitro against a panel of cancer and leukemia cell lines, as well as antiviral effect against hepatitis C virus in the replicon assay.

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