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Isolation, Determination of Absolute Stereochemistry, and Asymmetric Synthesis of Insect Methyl-Branched Hydrocarbons


Methyl-branched hydrocarbons (MBCHs) are ubiquitous components of insect cuticular lipids. Several have been shown to function as contact pheromones, and it is likely that many more remain to be discovered. The majority of insect produced MBCHs are chiral, but there have been no studies to determine whether they are biosynthesized enantiospecifically. In fact, there have been only a handful of studies on the effects of chirality on the biological activities of MBCH contact pheromones. This is primarily a result of the small to vanishingly small specific rotations of MBCHs (~3º to a tiny fraction of a degree), which in the past made enantiomeric analysis through polarimetry impractical, particularly in light of the small amounts of hydrocarbons (ng to µg) that can be obtained from many insects. The problem was compounded by difficulties in isolation of individual MBCHs from the crude mixture, and the time-consuming synthesis of chiral MBCH standards, both of which have hindered research on MBCH chirality.

The first part of this dissertation describes a generic method for the isolation of insect MBCHs from crude cuticular hydrocarbon (CHC) extracts of insects. The isolation of pure MBCH compounds required the initial fractionation of crude CHC extracts with AgNO3-impregnated silica gel chromatography followed by 5 Å molecular sieves adsorption of saturated n-hydrocarbons. These simple fractionation techniques were then followed up by reverse phase HPLC, using non-aqueous solvent systems, using an evaporative light scattering detector to detect all components as they eluted. This combination enabled the separation of MBCHs by chain-length and branch point. Following the described protocol 36 MBCH compounds from 20 species of insects, spanning nine orders of the Insecta, were isolated. Stereochemical analysis of these isolated MBCHs with a digital polarimeter revealed that the absolute stereochemistry of these insect natural products is conserved through at least the nine orders of Insecta studied, and regardless of methyl branch position or chain length.

The second part of this work describes the development of an efficient asymmetric synthesis of enantiopure methyl-branched hydrocarbons. Evans' alkylation was utilized to induce the asymmetry of the methyl branch point. A library of 45+ chiral methyl-branched hydrocarbons were synthesized to be used in collaborative studies testing their bioactivity in various insect systems, and as standards to confirm the polarimetric analyses of the previously isolated insect MBCH compounds. To date, the enantiopure compounds have been used to test the structure-bioactivity relationships of the MBCH contact pheromone of a parasitic wasp, Lariophagus distinguendus, and other studies are in progress.

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