5′-Hydroxy-5′-homoaristeromycin: Synthesis and antiviral properties

Graphical abstract

Abstract

Synthetically combining the C-4′ side-chain structural features of the antiviral candidates 5′-methylaristeromycin and 5′-homoaristeromycin into a diastereomeric pair of C-4′ side-chain dihydroxylated aristeromycins (6 and 7) is reported. Broad antiviral analyses of the both targets found promising effects towards HBV (6, 6.7 μM and 7, 7.74 μM) and HCMV (only 7, 0.72 μM). No other activity was found. Neither of the diastereomers was cytotoxic in the assays performed.

While the report of biologically inactive carbocyclic thymidine (1) in 1962¹ introduced a new class of nucleosides, it was the synthesis of racemic carbocyclic adenosine (aristeromycin, 2)² and subsequent isolation of the (−)-enantiomer from Streptomyces citricolor ³ that the era of carbocyclic nucleosides began and became a focal point for the pursuit of carbocyclic nucleosides as therapeutic candidates and as probes for biological processes.⁴ Our interest in aristeromycin and analogs therefrom began with the report of 5′-noraristeromycin (3) with activity towards human cytomegalovirus.⁵ Over the years⁶ since 1992 we have looked back to see what analogs lie in the wake of our work that suggested a further look into structural possibilities. Recently, in that regard, we were drawn to our reports that 5′-methylaristeromycin (4)⁷ and 5′-homoaristeromycin (5)⁸ have meaningful antiviral properties that had not been developed through analog design. This stimulated us to consider combining the two side chain features of 4 and 5 into diastereomers 6 and 7 (whose designation is derived from 5′-homoaristeromycin in blue possessing a 5′-hydroxyl). The outcome of that pursuit is presented here (see Fig. 1 ).

Fig. 1.

Relevant carbocyclic nucleosides.

Oxidation of alkenes to glycols is well established in the synthetic organic toolbox. Thus, for this investigation, the known N-6 protected carbocyclic adenine nucleoside with the unsaturated C-4′ side chain 8 (available from D-ribose)⁹ served as the starting point. To achieve the requisite diastereomers 9a and 9b ADmix-α (for 9a) and ADmix-β (for 9b) were employed, respectively. Deprotection of 9a and 9b with 2 N hydrochloric acid produced 6 and 7. The stereochemistry of 6 and 7 was determined by mesylation of 9a/9b to 10a/10b that were deprotected to 11a/11b. Reductive removal of the 6′-mesylate with lithium aluminum hydride yielded 12a/12b (a convenient, alternative synthesis of those diastereomers). The spectroscopic properties of 12a were identical to that previously reported for 4 (same as 12a).⁷ To address any possible structural ambiguity in this study, confirmation of 6 was achieved by an X-ray structural analysis.¹⁰

In an antiviral analysis,¹¹ both 6 and 7 showed moderate activity towards hepatitis B (EC₅₀ 7.1 μM and 7.4 μM, respectively; CC₅₀ >100 μM)) while only 7 was potent against human cytomegalovirus (EC₅₀ 0.72 μM; CC₅₀ >300 μM). Compound 6 was found to lack the significant yellow fever properties reported for 4 indicating addition of a hydroxyl to the methyl carbon of 4 (12a), resulted in an undesirable outcome for future development of 4 as a yellow fever antiviral candidate. A similar conclusion can be reached for the loss of the orthopox activity of 5 due to the presence of the extra hydroxyl group on the C-5′ position of both diastereomers 6 and 7 (see Scheme 1. ).

Scheme 1.

Synthetic steps to targets 6 and 7. Reagents and conditions: (a) ADmix-α for 9a; ADmix-β for 9b, t-butyl alcohol, H₂O, 67% for 9a; 79% for 9b; (b) 2 N HCl, MeOH, 93% for 6, 86% for 7; (c) MsCl, Et₃N, CH₂Cl₂, 80% for 10a, 78% for 10b; (d) 2 N HCl, MeOH, 79% for 11a, 78% for 11b; (e) LiAlH₄, THF, 89% for 12a, 90% for 12b.

In an antiviral analysis,¹¹ both 6 and 7 showed moderate activity towards hepatitis B (EC₅₀ 7.1 μM and 7.4 μM, respectively; CC₅₀ >100 μM)) while only 7 was potent against human cytomegalovirus (EC₅₀ 0.72 μM; CC₅₀ >300 μM). Compound 6 was found to lack the significant yellow fever properties reported for 4 indicating addition of a hydroxyl to the methyl carbon of 4 (12a), resulted in an undesirable outcome for future development of 4 as a yellow fever antiviral candidate. A similar conclusion can be reached for the loss of the orthopox activity of 5 due to the presence of the extra hydroxyl group on the C-5′ position of both diastereomers 6 and 7.

Compounds 6 and 7 were inactive towards polio virus, SARS coronavirus, respiratory syncytial virus, hepatitis C virus, herpes simplex 1 and 2 viruses, vaccinia virus, dengue, Rift Valley fever, Venezuelan equine encephalitis, H1N1 influenza A virus, and West Nile virus. No cytotoxicity was found for either 6 or 7 in the assays conducted.

In conclusion, a convenient synthesis of the diastereomeric hybridization of 5′-methylaristeromycin (4) and 5′-homoaristeromycin (5) to 5′-hydroxy-5′-homoaristeromycin (6 and 7) has provided a new C-4′ structural entity for the aristeromycin family of analogs that showed potent HBV (6 and 7) and moderate HCMV activities (7). It should be noted that the hydroxyl substituents offer the opportunity of making substituent changes at those centers for possible new aristeromycin structural variations.