Stereochemistry of pesticides

Pasteur considered chirality to be the hallmark of life. Not surprisingly, bioactive products are often chiral and vice versa.

Natural chiral products are often generated in pure stereoisomeric form (L-series amino acids, D-series sugars). Those obtained by conventional chemical synthesis, except in the case of stereospecific catalysis, are generally mixtures of stereoisomers. Their separation is costly. For this reason, it has long been assumed - especially when the aim was to destroy (pesticides) rather than cure (drugs) the target species - that these separations could be dispensed with and the product used as a mixture, with the inactive or weakly active stereoisomers playing only a role as fillers. This overlooked the fact that a stereoisomer inactive as a pesticide on the target species could display antagonistic or other activity on the target, or act on other targets. If the chemist can clearly identify the stereoisomers of different products from their formula, the user may hesitate, long misled by a single name (brand and active substance) for the product mixture. It is therefore necessary to recognize in a chemical structure the motifs responsible for chirality or another type of stereoisomerism.