Studies on the Synthesis, Crystal Structures, Biological Activities and Molecular Docking of Novel Natural Methylxanthine Derivatives Containing Piperazine Moiety

A series of novel methylxanthine Mannich base derivatives containing substituted piperazine groups were synthesized through Mannich reaction. The structures of these new compounds were confirmed by NMR, HRMS or elemental analyses, and X-ray single crystal diffraction. Bioassay results showed that some of the compounds exhibit favorable fungicidal and insecticidal potentials. Particularly, compounds IIk, IIq, IIs and compounds If, IIk against Physalospora piricola and Rhizoctonia cerealis, respectively, were comparable with Azoxystrobin and Chlorothalonil; compound Ik exhibited higher potency than Triflumuron against Plutella xylostella L., suggesting its potential as a lead compound for further development in insecticidal applications. Despite possessing weak herbicidal activities, the target compounds, especially the methylxanthine S-Mannich base derivatives I displayed remarkable inhibitory activities toward ketol-acid reductoisomerase (KARI); compounds Ib, If, and Ik which had K values of 2.41-8.08 µmol/L can be novel potent KARI inhibitors for deeper exploration. The SARs were analyzed in detail. The molecular docking studies on the highly active inhibitors with KARI provided possible binding modes between inhibitor and the target enzyme. The physicochemical parameter predictions indicated that compounds Ik, IIk, IIq and IIs have "druglike structure" features. The research results in this article may bring a new inspiration to the extensive explorations on new methylxanthine derivatives in pesticide area.