Biosynthetic studies on pseudomonic acid

Abstract

Chapter one reviews the literature published on the pseudomonic acids. The chemistry and structural characteristics of these compounds is covered together with the semi-synthetic modifications which have been investigated to enhance their anti-bacterial properties. Some of the synthetic routes to pseudomonic acid C are reviewed along with its subsequent conversion to pseudomonic acid A. The previous biosynthetic studies are described in detail.

Chapter two briefly reviews the use and methods of detection of radioactive and stable isotopes in biosynthetic studies. The maintance of the P. fluorescens culture and the conditions for the production and isolation of the metabolites are described. Intial incorporation experiments using [l-¹³C, ¹⁸O₂]acetate, [1-¹³C, ²H₃]acetate and ¹⁸O₂ gas are discussed and the results from these experiments allow for mechanisms for the formation of the pyran ring and unsaturated ester linkage to be proposed. Synthesis and incorporation of ¹⁴C and ¹³C₂ labelled forms of ß-hydroxy-ß-methyl glutarate and its non-intact incorporation into pseudomonic acid is discussed. The synthesis of ¹³C labelled malonate and the results of its incorporation into pseudomonic acid are described and indicate that malonate is an important precursor. Differential levels of enrichment were observed with malonate going preferentially into the 9-hydroxynonanoic acid side chain.

In chapter three, a stepwise mechanism for the formation of pseudomonic acid is proposed and discussed. The synthesis of three of the proposed chain assembly intermediates, ie. acetoacetate; (2S, 3S)-3-hydroxy-2-methylbutanoic acid and 5-hydroxy-4-methylhex-2-enoic acid are described. Results of intial incorporation studies using 5% ²H₂O in the culture medium and [2-²H₃]acetate are detailed together with the attempted incorporation of (2S, 3S)-[2'-²H₃]-3-hydroxy-2-methylbutanoic acid chain assembly intermediate.