Synthesis and Biological Profiling of Pyrazolo-Fused 7-Deazapurine Nucleosides.

A series of 8-substituted 1-methyl-1,4-dihydropyrazolo[3',4':4,5]pyrrolo[2,3-d]pyrimidine (methylpyrazolo-fused 7-deazapurine) ribonucleosides have 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 yields 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 effects against hepatitis C virus in the replicon assay.