title: Naturally Occurring Bio-Herbicides as Leads for Weed Control
Creators: name: Stewart, Craig
Publisher: name: The Australian National University
Year: 2023

description: Herbicidal compounds with novel modes of action are in great demand due to the rapidly increasing evolution of resistance in weed species to existing products. Allelochemical interactions provide the inspiration for the development of lead molecules due to their existing phytotoxicity. This Thesis will focus on the efforts to develop such lead molecules based on amino acid containing natural products. Chapter One discusses the important place natural products have in the history of the discovery and development of herbicides. It will discuss several landmark examples, some of the various factors contributing to the recent decline of herbicide development, and the lack of novel modes of action entering the industry pipeline. Chapter Two discusses the highly phytotoxic non-proteinogenic amino acid m-tyrosine, which is exuded from the roots of the Festuca spp. of grasses and grants them a competitive advantage. The mode of action of this allelochemical has only recently been elucidated. Proteomic experiments have confirmed m-tyrosine is misincorporated into proteins in place of p-tyrosine and phenylalanine leading to the loss of viability, and with a preference for mitochondrial proteins. This mechanism constitutes a novel mode of action and prompted our synthesis of a library of methoxyphenylalanine analogues with the aim to improve the phytotoxicity of m-tyrosine and alleviate the primary obstacle for its use as a herbicide, its short soil half-life due to microbial degradation. This synthesis was achieved through the classical Erlenmeyer-Ploechl oxazolone synthesis and resulted in the development of a concise pseudo-one pot hydrogenation-nucleophilic ring opening-deprotection reaction. In this manner a library of 32 racemic amino acids and synthetic intermediates was generated from readily available polymethoxylated-benzaldehydes. Chapter Three focusses on the synthesis and stereochemical examination of compounds containing the piperazine-2,5-dione scaffold. The piperazine-2,5-dione moiety is present in many phytotoxic bacterial and fungal secondary metabolites, and thus presents another opportunity for the development of lead molecules. A library of (methoxybenzylidene)piperazine-2,5-diones was prepared via the alkylation of 1,4-diacetyl-piperazine-2,5-dione with methoxybenzaldehydes, and the configuration of the alkenes determined using NOE spectroscopic experiments. A number of these (benzylidene)piperazine-2,5-diones were reduced through Parr hydrogenation to the corresponding (benzyl)piperazine-2,5-diones. The relative stereochemistry of these compounds was investigated through analysis of the magnetic anisotropic effect exerted by the two aromatic sidechains on 2-H and 3-H chemical shift values, and through analysis of the two dihedral angles chi and phi using the Karplus relationship. Chapter Four details the preliminary phytotoxicity bioassays performed on selected compounds from the libraries developed in Chapters Two and Three with the aim of identifying lead molecules for further investigation and development. The assay involved measuring the relative inhibition of the growth of germinating garden cress (Lepidium sativum L.) seedlings when treated with the test compounds. Four of the 32 compounds prepared in Chapter Two displayed superior activity to that of the m-tyrosine control, with the 2,5-dimethoxyphenyl and phenyloxazolone moieties appearing in two active compounds each. The (benzylidene)piperazine-2,5-dione compounds proved challenging to test in this assay due to limited solubility but displayed mild to moderate inhibitory activity across the test series. Several compounds were observed to stimulate radicle growth under these conditions, though this effect did not appear to impair seedling development. Chapter Five concludes the Thesis and proposes future directions for this research.