Influence of Spacer Chains Derived from Ring-Opened Crown Ethers on the Anti-Influenza A Virus Activity of closo-Decaborates Bearing Amino Acid Ester Pendant Moieties

A series of novel substituted derivatives of the closo-decaborate anion [B10H10]2− bearing alkoxy spacer chains of varying length derived from ring-opened crown ethers (12-crown-4, 15-crown-5, and 18-crown-6) with pendant L-amino acid methyl ester residues (L-tryptophan and L-histidine) have been synthesized. The synthetic approach involved the nucleophilic ring-opening of oxonium crown ether derivatives of the closo-decaborate anion followed by coupling with amino acid methyl esters via mixed anhydride activation. The obtained compounds were characterized by multinuclear NMR spectroscopy (1H, 11B, 13C), IR spectroscopy, elemental analysis, and electrospray ionization mass spectrometry (ESI-MS). The compounds were obtained as sodium salts and evaluated for their in vitro cytotoxic and antiviral properties against the influenza A virus strain A/Moscow/78/2020 (H1N1)pdm09, which contains the S31N mutation conferring resistance to adamantane-class drugs. Cytotoxicity was assessed on MDCK cells, and antiviral activity was determined by cell ELISA. The results revealed a clear structure–activity relationship: the length of the alkoxy spacer chain significantly influenced both antiviral activity and selectivity. The shortest spacer (derived from 12-crown-4) proved to be optimal, while elongation of the chain led to a decrease in antiviral potency. The compound containing a 12-crown-4-derived spacer and a tryptophan methyl ester residue exhibited the highest selectivity index (SI = 155) with low cytotoxicity (CC50 = 155 µg/mL) and high antiviral activity (IC50 = 1.0 µg/mL). Tryptophan-containing derivatives consistently outperformed their histidine analogues, confirming the key role of the indole side chain in antiviral activity. Overall, the closo-decaborate platform with crown ether-derived spacers and amino acid ester pendant groups represents a promising scaffold for the development of low-toxicity, highly selective inhibitors of influenza A virus replication. The compound with the 12-crown-4-derived spacer and a tryptophan methyl ester residue merits further investigation as the most promising among the synthesized samples.