Synthesis, Crystal Structures, and Biological Activity Evaluation of Novel Xanthine Derivatives Containing a Pyrethroid Moiety

On the basis of the structures of natural xanthines and pyrethroid insecticides, a series of novel xanthine derivatives Ia-Is containing pyrethroid motifs were synthesized and identified by means of melting points, 1H NMR, 13C NMR, and HRMS. The single crystals of compounds In and Iq were obtained, which further confirmed the structures and configurations of this type of compounds. The biological tests showed that some of them exhibited favorable insecticidal activities toward Mythimna separata Walker and Plutella xylostella L. and were superior to the natural methylxanthine compound caffeine and comparable with the insecticide tetramethrin (e.g., compound Im: LC50 = 0.6162 mg/L, against P. xylostella). Among others, Im, Ib, Ij, and Ik could serve as new insecticidal leading structures for further study. Moreover, some of the compounds showed favorable fungicidal activities against a broad spectrum of plant pathogenic fungi (e.g., compound Ie: EC50 = 6.0922 μg/mL, against Physalospora piricola; EC50 = 9.0637 μg/mL, against Rhizoctonia cerealis), which in turn would be an exciting new finding in xanthine chemistry; Ie, Ih, and Ii could be novel fungicidal leading compounds for further investigation. The structure-activity relationships of the compounds were also analyzed and discussed in detail. The research results presented in this paper provide a useful reference and guidance for the development of new natural product-based agrochemicals.