Biorational pesticides: Pest control inspired by natural compounds

Klimavicz, James
Major Professor
Joel R Coats
Committee Member
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Over the last several hundred million years, plants have evolved to produce an incredible variety of secondary metabolites. Many of these compounds are repellent or insecticidal and are generated by the plant to reduce herbivory. However, these compounds frequently lack the ideal chemical and physical properties that would make them suitable for larger-scale use as insect repellents or agricultural insecticides. Using a biorational approach, it is possible to chemically modify these natural metabolites to produce new compounds that are more ideal for commercialization.

Plant essential oils often contain a variety of monoterpenoids in high abundance, and these components of the oil are usually very fragrant. Although some of these compounds may be highly repellent or toxic to insects, the high volatility of these chemicals typically makes them unsuitable for use as long-lasting spatial insect repellents. The first part of this dissertation explores the chemical modification of monoterpenoid alcohols by esterification, which leads to compounds with higher molecular weight, and a concomitant drop in volatility, while maintaining satisfactory repellency characteristics against Culex pipiens, the northern house mosquito. Slight changes in the structure of the parent monoterpenoid or the esterifying group can often lead to substantial differences in potency of the repellency, and several of the synthesized compounds were repellent for more than seven hours against C. pipiens.

Plant-parasitic nematodes are a tremendous economic burden throughout the world, and although several terpenoids and phenylpropanoids have been shown to be somewhat toxic to some of these nematodes, treatment of infected soil is still often cost-prohibitive. A biorational approach was used to develop analogs of cinnamaldehyde, a compound previously shown to be effective against multiple species of nematodes in in vitro assays. Many of these compounds were significantly more potent against the soybean cyst nematode (Heterodera glycines) than the parent compound.

The synthesis of analogs of a natural compound is an integral part of the biorational design of pesticides. The last portion of this dissertation focuses on two new synthetic methods, one for producing ketene dithioacetals, and one on the synthesis of 1,2,4-thiadiazoles. Both of these classes of compounds have been underexplored in agricultural chemistry, and the existing methods of creating these chemicals are limited. The new methods make these compounds in acceptable-to-good yields from readily-available starting materials.