Chirality

Chirality of molecules is an important aspect of their structure in all fields, no less among insect substances. Many pheromones and hormones are chiral, and the behavioural response they produce may vary greatly with the chiral form. For example, the unnatural enantiomer of a pheromone can inhibit completely the response to the natural enantiomer, the natural pheromone may be a blend of unequal proportions of two enantiomers, or the response may be equal for both enantiomers. These and several other possibilities have been observed in practice. For any given pheromone compound, it is impossible to predict which enan-tiomer will be active or whether one or both or a blend of enantiomers will be the naturally occurring substance.

It is therefore important to determine chirality of the natural compound or mixture. There are several ways to do this. One is through recording the NMR spectrum with the aid of a chiral shift reagent. Another is by chromatography on a chiral stationary phase in gas or liquid chromatography. With some luck and skill, the enantiomers will be separated by chromatography on a chiral phase. Another is by chromatography of a derivative of the compound made with a chiral reagent. This gives two separable diastereomeric products. Usually it is also necessary to have at least one enantiomer of known chirality for comparison for either of these chromatographic methods. Another possibility is to treat the compound with an enzyme where the reaction proceeds with known stereochemistry. It is rare that an insect substance is available in sufficient quantity to determine its optical rotation, and in some cases, at least, the rotation is very small. Optical rotatory dispersion (ORD) at short UV wavelength, or, if the molecule has a UV chromophore, circular dichroism (CD) can be more useful. These techniques are little used with insects. Frequently all possible isomers and enantiomers have to be selectively synthesized for comparison with the natural compound by gas chromatography.

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