Design, synthesis and acaricidal activities of Cyflumetofen analogues based on carbon-silicon isosteric replacement

The application of a carbon-silicon bioisosteric replacement strategy to find new acaricides with improved properties led to the discovery of Sila-Cyflumetofen 6B, a novel and highly potent acaricide. The essential t-butyl group in the beta-ketonitrile acaricide Cyflumetofen 6A could be swapped with the bioisosteric trimethyl-silyl group with retention of high level acaricidal activity and favourable pharmacological properties. Sila-Cyflumetofen 6B was found to possess similar preferred energy-minimized conformation and electrostatic potential surface compare to Cyflumetofen 6A. Herein we also report the development and application of the first homology model of the spider mite mitochondrial electron transport complex II (succinate ubiquinone oxidoreductase; SQR) and demonstrated that the active metabolite AB-1 of Cyflumetofen 6A and its sila-analogue Sila-AB-1 bind to the Qp site in same binding pose and that both compounds form two H-bonds and a cation-π interaction with Trp 165, Tyr 433 and Arg 279, respectively. Furthermore, we also developed a new mode of action test for spider mite Complex II using cytochrome c as electron acceptor and blocking its re-oxidation by addition of KCN resulting in a sensitive and convenient colorimetric assay. This new method avoids the use of non-specific artificial electron acceptors and allows to measure SQR inhibition in crude extracts of Tetranychus urtice. In this assay Sila-AB-1, the intrinsically active metabolite of Sila-Cyflumetofen, 6A exhibited even a somewhat lower IC₅₀ value than the metabolite of Cyflumetofen AB-1. Synthetic methodologies are described for the preparation of Sila-Cyflumetofen 6B and its active metabolite Sila-AB-1 which enable an efficient synthesis of these compounds in only 5 and 4 steps, respectively, from cheap commercial starting materials. Although the value of carbon-silicon bioisosteric replacements has already be demonstrated in the past it is to the best of our knowledge the first report of a successful application in crop protection research in the last two decades.