Synthesis of Trifluoromethylated Analogues of the Cyclic Lipopeptide Iturin A and Evaluation of their Antifungal Activity

The rise of antifungal resistance threatens public health and agriculture. Iturin A, a cyclic lipopeptide produced by Bacillus species, is known for its antifungal activity against various pathogens. In this study, three novel trifluoromethylated analogues of iturin A were synthesised as potential ¹⁹F NMR probes and to compare their bioactivity with the natural compound. Trifluoromethylation targeted the D-tyrosine and iturinic acid residues, which are critical for antifungal activity. Fluorinated building blocks were prepared via oxidative radical trifluoromethylation for D-tyrosine and, notably, via electrophilic trifluoromethylation combined with a chiral auxiliary-based approach for the iturinic acid, marking the first synthesis of a terminally trifluoromethylated long-chain β-amino fatty acid. Peptide assembly was achieved through solid-phase synthesis followed by on-resin cyclisation, alongside a high-yielding late-stage aromatic trifluoromethylation method. Bioactivity assays revealed that the mono-trifluoromethylated tyrosine analogue exhibited slight activity loss against Candida albicans and greater loss against Fusarium graminearum. The bis-trifluoromethylated tyrosine analogue lost activity against both fungi, while the alkyl-trifluoromethylated analogue retained full activity against C. albicans and showed a minor activity loss against F. graminearum. These analogues provide insights into site-specific trifluoromethylation effects, can serve as valuable ¹⁹F NMR probes, and can be a platform for further iturin A analogue development.