Structure–Activity Relationship Studies of the Peptide Antibiotic Clovibactin

Our laboratory reported the chemical synthesis and stereochemical assignment of the recently discovered peptide antibiotic clovibactin. The current paper reports an improved, gram-scale synthesis of the amino acid building block Fmoc-(2<i>R</i>,3<i>R</i>)-3-hydroxyasparagine-OH that enables structure-activity relationship studies of clovibactin. An alanine scan reveals that residues Phe₁, d-Leu₂, Ser₄, Leu₇, and Leu₈ are important for antibiotic activity. The side-chain amide group of the rare d-Hyn₅ residue is not essential to activity and can be replaced with a methyl group with a moderate loss of activity. An acyclic clovibactin analogue reveals that the macrolactone ring is essential to antibiotic activity. The enantiomer of clovibactin is active, albeit somewhat less so than clovibactin. A conformationally constrained clovibactin analogue retains moderate antibiotic activity, while a backbone <i>N</i>-methylated analogue is almost completely inactive. X-ray crystallography of these two analogues reveals that the macrolactone ring adopts a crown-like conformation that binds anions.