New synthetic approaches to pyoverdine D derivatives

Abstract

Pseudomonas aeruginosa is a clinically opportunistic pathogen and is a major threat to patients with compromised immune systems and cystic fibrosis. Iron is an essential nutrient for virtually all microbes, including P. aeruginosa, which acquires iron by the secretion of certain iron-chelating siderophores. The most important of these siderophores is the cyclic peptide derivative, pyoverdine D. As an endogenous siderophore, pyoverdine D is of considerable interest as it could provide a starting point for new ways to deliver toxic materials to P. aeruginosa, for instance by linking with antibiotics. This requires an efficient and flexible synthetic route to be developed for this natural product.
The work herein describes a new modular synthesis of pyoverdine D, with a focus on three important areas – the preparation of the novel amino acid, N-formyl-N-hydroxyornithine; the solid phase synthesis of the cyclic peptide component; and the synthesis of the tricyclic “chromophore” unit. In particular, a new oxidation procedure was identified for the preparation of N-hydroxyornithine derivatives using Cs2CO3 and dibenzoyl peroxide, which should be applicable to the side chain N-oxidation of similar -amino acids. An efficient approach to the chromophore unit was also developed starting from D-Glu. This improves on several steps in Mashiach and Meijler’s published approach and also provides a potential route to some pyoverdine derivatives that have not been previously synthesised. In addition, a new protecting group strategy for the synthesis of the peptide component of the siderophore was devised featuring selective removal of t-butyl protection from Ser and Thr residues from a fully protected linear precursor using hexafluoroisopropanol/HCl. Removal of t-butyl protection from the C-terminal Thr was found to be essential for efficient cyclisation. It was also observed that some reported deprotection reactions with hexafluoroisopropanol/HCl (e.g. for Arg(Pbf)) could not be reproduced. Finally, the feasibility of a modular synthesis of pyoverdine D by a combined solid phase and solution approach was demonstrated. The successful preparation of the key building blocks from this work will enable future synthesis of the natural product and siderophore-antibiotic conjugates.

Date of Award	24 Apr 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Ian Eggleston (Supervisor) & Albert Bolhuis (Supervisor)