Towards the Synthesis of Pyoverdines: Preparation and Reactivity of the N-Formylhydroxyornithine Residue

The Gram-negative bacterium Pseudomonas aeruginosa produces a family of peptide siderophores called pyoverdines that play a vital role in the mechanisms by which it acquires iron from the environment. A key component of various pyoverdines is the presence of one or more copies of L-δ-N-formyl-δ-N-hydroxyornithine (fOHOrn) as an ironbinding residue. In this study, we have developed an improved preparation of a derivative of fOHOrn that is suitable for use in solid-phase peptide synthesis, incorporating a novel N-oxidation protocol and a mild final deprotection with HCl/hexafluoroisopropanol (HFIP) that circumvents the unexpected deformylation of the fOHOrn side chain under acidic conditions. We have also devised a synthesis of the cyclic peptide component of pyoverdine D exploiting a selective side-chain deprotection strategy with HCl/HFIP that allows the application of readily available amino acids with standard tert-butyl side-chain protection and which facilitates the cyclisation step. These innovations open the way towards the convergent preparation of various pyoverdines and also other natural products that contain fOHOrn residues.