Benzoxazoles Derived from Eugenol as Potential Agents against Arboviruses: Synthesis, Antiviral Studies, and Mechanistic Insights

The synthesis of new eugenol-benzoxazole derivatives and their antiviral evaluation against Orthoflavivirus zikaense (ZIKV), Alphavirus chikungunya (CHIKV), and Alphavirus mayaro (MAYV) are reported. Derivative 16 shows the highest potency and selectivity indices: 25 for ZIKV (EC₅₀: 6.1 µM), 11 for CHIKV (EC₅₀: 14.2 µM), and 24 for MAYV (EC₅₀: 6.3 µM), 68 times more potent than ribavirin and amantadine drugs. RT-qPCR showed that benzoxazole 16 reduced the viral load of CHIKV after 24 and 48 h and of ZIKV after 24 h, demonstrating that it may be a promising antiviral agent. In cytopathic effect assays, benzoxazoles 16 and 23 showed cell monolayer protection against ZIKV and compounds 16 and 24 against CHIKV. None of these compounds showed virucidal activity, suggesting they act post-viral entry, targeting proteins related to viral replication rather than inactivating the virus before cellular infection. Molecular dynamics and MM/GBSA binding energy analysis of compound 16 on viral replication targets identified the capsid protein and nsP3 as the most promising targets, with binding free energies of -31.21 and -29.55 kcal/mol, respectively. The ligand remained deeply buried and well-confined in both binding sites, with minimal positional drift and a marked reduction in solvent-accessible surface area.