Design, synthesis and biological evaluation of imidazo[4,5-<italic>c</italic>]pyrazole nucleosides as 5:5 bicyclic analogs of purine nucleosides.

Abstract

There has been an interest in the synthesis of 5:5 bicyclic heterocycles that could serve as substitutes for the aglycons of certain 5:6 bicyclic nucleosides such as purine nucleosides. This approach involving the removal of a carbon (C-2) from purine has led to the imidazo[4,5-<italic>c</italic>]pyrazole ring system as a potential purine bio-isostere. 3-Amino-6-(beta-D-ribofuranosyl)imidazo[4,5-<italic> c</italic>]pyrazole, the <italic>N-6</italic> ribosylated imidazo[4,5-<italic> c</italic>]pyrazole derivative, was synthesized <italic>via</italic> a <italic> N-N</italic> bond formation strategy by mononuclear heterocyclic rearrangement (MHR). 5-Amino-1-(5-<italic>O-tert</italic>-butyldimethylsilyl-2,3-<italic> O</italic>-isopropylidene-beta-D-ribofuranosyl)-4-(5-methyl-1,2,4-oxadiazol-3-yl)imidazole, prepared from 5-amino-1-(beta-D-ribofuranosyl)imidazole-4-carboxamide, underwent the MHR with sodium hydride in DMF or DMSO to afford the corresponding 3-acetamidoimidazo[4,5-<italic>c</italic>]pyrazole nucleosides in good yields. A direct removal of the acetyl group from 3-acetamidoimidazo[4,5-<italic> c</italic>]pyrazoles under numerous conditions was unsuccessful. Subsequent protecting group manipulations followed by deacetylation afforded the desired 3-amino-6-(beta-D-ribofuranosyl)imidazo[4,5-<italic>c</italic>]pyrazole as a 5:5 fused analog of adenosine. The synthesis of the <italic>N-1</italic> ribosylated imidazo[4,5-<italic> c</italic>]pyrazole derivatives was attempted from three different approaches. <italic> N-1</italic> Ribosylated 5-aminopyrazole-4-carbonylazides, prepared from the appropriate 5-aminopyrazole-4-carboxylates, underwent a Curtius rearrangement in the presence of the adjacent amino groups but did not afford the desired ring-cyclized imidazo[4,5-<italic>c</italic>]pyrazoles. Alternatively, a series of <italic>N1</italic>-substituted 4,5-diaminopyrazoles were obtained when the acyl azides underwent a Curtius rearrangement followed by quenching with alcohols. The cyclization of these 4,5-diaminopyrazole derivatives with several 1,1<super>'</super>-dielectrophiles was unsuccessful. However, the <italic> N-1</italic> ribosylated 4,5-diaminopyrazole was able to condense with glyoxal or 2,3-butanedione to afford pyrazolo[3,4-<italic>b</italic>]pyrazine nucleosides. The synthesis of the <italic>N-1</italic> ribosylated imidazo[4,5-<italic> c</italic>]pyrazole derivatives was accomplished by the intramolecular cyclodehydration of the corresponding 5-alkylamino-4-nitrosopyrazoles. 5-Amino-1-(2,3-<italic> O</italic>-isopropylidene-beta-D-ribofuranosyl)pyrazole, prepared from D-ribose, was acylated at both the 5<super>' </super>-hydroxy and 5-amino positions followed by a reduction with lithium aluminum hydride to afford the 5-ethylaminopyrazole and 5-benzylaminopyrazole nucleosides. Subsequent protecting group manipulations on the sugar moieties followed by the nitrosation and then the ring closure reaction in pyridine at reflux temperature afforded the desired <italic>N-1</italic> ribosylated 5-substituted imidazo[4,5-<italic>c</italic>]pyrazole. The newly synthesized imidazo[4,5-<italic>c</italic>]pyrazole and pyrazolo[3,4-<italic> b</italic>]pyrazine nucleosides and non-nucleoside analogs were screened for antiviral activities against human cytomegalovirus (HCMV) and herpes simplex virus type-1 (HSV-1) as well as for cytotoxicity. At the concentrations tested (100 muM or less), these compounds were inactive against both HCMV and HSV-1, and showed no cytotoxicity.