Solid-phase synthesis of fusaricidin/LI-F class of cyclic lipopeptides: Guanidinylation of resin-bound peptidyl amines.

Fusaricidins/LI-Fs and related cyclic lipopeptides represent an interesting new class of antibacterial peptides with the potential to meet the challenge of antibiotic resistance in bacteria. Our previous study (Bionda et al. ChemMedChem 2012, 7, 871-882) revealed the significance of the guanidinium group located at the termini of the lipidic tails of these cyclic lipopeptides for their antibacterial activities. Therefore, devising a synthetic strategy that will allow incorporation of guanidinium functionality into their structure is of particular practical importance. Since appropriately protected guanidino fatty acid building blocks are not commercially available, our strategy toward guanidinylated fusaricidin/LI-F analogs include solid-phase synthesis of a cyclic lipopeptide precursor possessing a lipidic tail with a terminal amino group followed by its conversion into corresponding guanidine. To find the optimal method for this conversion, we have examined commonly used guanidinylation reagents under the conditions compatible with standard solid-phase peptide synthesis. Described experimental results demonstrated superiority of N,N'-di-Boc-N″-triflylguanidine in solid-phase preparation of fusaricidin/LI-F class of cyclic lipopeptides. The triflylguanidine reagent gave a single monoguanidinylated product in excellent yield independently of the type of solid-support.