Discovery of imidacloprid and further developments from strategic molecular designs.

The invention of imidacloprid, the most important neonicotinoid insecticide, was initiated by replacement of the framework of nithiazine with an imidazolidine ring. Through the finding of 1-(6-chloro-3-pyridylmethyl)-2-nitromethyleneimidazolidine, imidacloprid was invented. At the same time cyanoiminothiazolidinyl neonicotinoid thiacloprid was discovered. These products possess pronounced systemic properties and improved photostability in addition to supreme insecticidal ability. Crystal structure analysis led to the drug-receptor interaction model consisting of the guanidine (amidine) part conjugated to a powerful electron-withdrawing group bearing an H-bond accepting tip such as NO(2) or CN, and the chloronicotinyl group enhances the binding to the receptor. The QSAR study not only supports the key pharmacophore but also clarifies the crucial involvement of the phamacokinetic factors in the insecticidal activity. A concept for strategic and rational design led to the discovery of alkylene-tethered bis-imidacloprid derivatives with unexpected systemic insecticidal property and the unique binding mechanism revealing the second cavity in the neonicotinoid receptor.