Synthesis, Biological Evaluation And Structure-Activity Relationship Study Of Fluorine-Containing Stilbene Derivatives

Plant diseases have been recognized as a severe threat that limits crop production worldwide. Natural stilbenes are reported to exhibit considerable biological properties, especially in plant disease resistance, thus are regarded as a promising scaffold in the search for bioactive molecules. However, their biological evaluation as fungicidal agents are challenged by their oxidative degradation and limited availability. Recently, hybridization of natural stilbenes with bioactive moieties has been as an effective strategy in the search for novel hybrid molecules with improved bioactivity. Nevertheless, research regarding the biological activity of stilbene hybrids against plant pathogenic fungi remains sparse in the literature. Considering the biological profiles of 1,3,4-oxadiazole and fluorine-containing motifs, a new class of oxadiazole-stilbene hybrids was synthesized based on the concept of hybridization. Three-dimensional quantitative structure-activity relationship(3D-QSAR) derived from in vivo bioassay against phytopathogenic fungi was reasonably discussed. For further understanding the possible fungicidal mechnism of the stilbene, the effects on hyphal morphology, electrolyte leakage, and respiration of mycelial cell suspension were studied. In addition, homology modeling combined with molecular docking was also performed. The main conclusions were drawn as follows:(1) A total of 21 new series of oxadiazole-stilbene hybrids were prepared by the condensation of 4-methylbenzohydrazide and 4-fluorobenzaldehyde, followed by oxidative cyclization of acylhydrazones, bromination of N-bromosuccinimide(NBS), Arbuzov rearrangement, and Wittig-Horner olefination. Their chemical structures were characterized by 1H and 13 C nuclear magnetic resonance(NMR) and high-resolution mass spectrometry(HRMS).(2) The fungicidal activities of 3,4,5-trimethoxy-(series I) and fluorine-containing(series II) stilbene derivatives against Colletotrichum lagenarium and Pseudoperonospora cubensis were evaluated in vivo using the pot culture test. Derived from the bioassay results of series I, the 3D-QSAR analysis with good predictive ability(q2 = 0.516, r2 = 0.920) was reasonably discussed using the comparative molecular field(CoMFA) analysis. The results reavled that electron-withdrawing substituents at the meta or para position were favorable to increase the bioactivity. In this regard, the in vivo fungicidal activities of series II were further evaluated based on the 3D-QSAR analysis results. Bioassay results indicated that most of the compounds exhibited potent fungicidal activities. In particular, some of the newly synthesized compounds showed remarkably improved activities against the test fungi compared with series I, which further comfirmed the structure-activity trends. Notably, the activity of II-8 against C. lagenarium was comparable to that of Carbendazim.(3) To gain insights into the possible mode of action of the stilbene against C. lagenarium, the effects on hyphal morphology, electrolyte leakage, and respiration of mycelial cell suspension were studied. Microscopic observation showed considerably deformed mycelial morphology, which might result in hyphal lysis. The increased membrane permeability in the presence of II-8 was further confirmed its action on the cell membrane. Taken together with the actions of natural stilbenes, the fungicidal mechanism of stilbenes may result, at least in part, from its membrane-disruption effect that may be somewhat organism- and stilbene structure-dependent.(4) Some of the title compounds(II 13–21) were also evaluated for their in vitro fungicidal activity against Botrytis cinerea by mycelial growth inhibition assay. Bioassays indicated that II-19 showed potent inhibitory activitiy against B. cinerea with the EC50 value of 144.6 μg/m L, which was superior to that of resveratrol(EC50=315.6 μg/m L). In light of the catalytic potential of fungal cytochrome P450 s family CYP51 and human CYPs in bioconversion of stilbene derivatives, we postulated the possible interactions between the stilbenes and CYP51 enzyme. Homology modeling combined with molecular docking studies were performed to predict and explain the putative binding modes of both resveratrol and oxadiazole-stilbene hybrids with CYP51 from B. cinerea for the first time. Docking results suggested that the patterns of stilbene skeleton(introduction of oxadiazole ring, different substituents) are essential determinants of ligand affinity, which may account for their in vitro inhibitory activity. The structural information revealed from this study could provide important clues for mechanistic study and derivatization of stilbenes as alternative sources of fungicidal agents for plant disease control, and shed lights on precise information about the protein-ligand interactions from the enzyme inhibition assay.