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Tautomerism and Rotamerism of Favipiravir and Halogenated Analogues in Solution and in the Solid State. | LitMetric

AI Article Synopsis

  • Favipiravir is a selective antiviral drug being repurposed for COVID-19 treatment, showing specific chemical states that influence its antiviral activity.
  • The study investigates different tautomeric forms of favipiravir and its analogues using various analytical techniques, both in solution and solid states, while also considering gas phase properties.
  • Tautomerization is affected by solvent polarity and the electronegativity of substituents, with specific rotameric states identified based on the type of tautomer present.

Article Abstract

Favipiravir is an important selective antiviral against RNA-based viruses, and currently, it is being repurposed as a potential drug for the treatment of COVID-19. This type of chemical system presents different carboxamide-rotameric and hydroxyl-tautomeric states, which could be essential for interpreting its selective antiviral activity. Herein, the tautomeric 3-hydroxypyrazine/3-pyrazinone pair of favipiravir and its 6-substituted analogues, 6-Cl, 6-Br, 6-I, and 6-H, were fully investigated in solution and in the solid state through ultraviolet-visible, H nuclear magnetic resonance, infrared spectroscopy, and X-ray diffraction techniques. Also, a study of the gas phase was performed using density functional theory calculations. In general, the keto-enol balance in these 3-hydroxy-2-pyrazinecarboxamides is finely modulated by external and internal electrical variations via changes in solvent polarity or by replacement of substituents at position 6. The enol tautomer was prevalent in an apolar environment, whereas an increase in the level of the keto tautomer was favored by an increase in solvent polarity and, even moreso, with a strong hydrogen-donor solvent. Keto tautomerization was favored either in solution or in the solid state with a decrease in 6-substituent electronegativity as follows: H ≫ I ≈ Br > Cl ≥ F. Specific rotameric states based on carboxamide, "cisoide" and "transoide", were identified for the enol and keto tautomer, respectively; their rotamerism is dependent on the tautomerism and not the aggregation state.

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Source
http://dx.doi.org/10.1021/acs.joc.3c00777DOI Listing

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