Synthesis, Fungicidal Activity, and Mechanism of Action of Pyrazole Amide and Ester Derivatives Based on Natural Products l-Serine and Waltherione Alkaloids

Evaluation of the antifungal activity of novel bis-pyrazole carboxamide derivatives and preliminary investigation of the mechanism

To facilitate the development of novel agricultural succinate dehydrogenase inhibitor (SDHI) fungicides, we synthesized three series of derivatives by introducing phenyl pyrazole fragments into the structure of pyrazol-4-yl amides. The results of the bioactivity assay showed that most of the target compounds possessed varying degrees of inhibitory activity against the tested fungi. At a concentration of 100 mg/L, the compound B8 exhibited effective protective activity against S. sclerotiorum in vivo. Molecular docking analysis and succinate dehydrogenase (SDH) inhibition assay indicated that B8 was not a potential SDHI. The preliminary antifungal mechanism of studies showed that B8 induced a large amount of reactive oxygen species (ROS) and severe lipid peroxidation damage in S. sclerotiorum mycelium, resulting in mycelial rupture and disruption of the integrity of the cell membrane and leakage of soluble proteins, soluble sugars and nucleic acids. Further transcriptome analysis showed that compound B8 blocked various metabolic pathways by downregulating the differentially expressed genes (DEGs) catalase, disrupting hydrogen peroxide hydrolysis, accelerating membrane oxidative damage, and upregulating neutral ceramidase, accelerating sphingolipid metabolism to disrupt cell membrane structure and cell proliferation and differentiation, potentially accelerating cell death. The above results indicated that the potential target of these dis-pyrazole carboxamide derivatives may be the cell membrane of pathogenic fungi.

Design, Synthesis, and Nematocidal Evaluation of Waltherione A Derivatives: Leveraging a Structural Simplification Strategy

Southern root-knot nematodes are among the most pernicious phytoparasites; they are responsible for substantial yield losses in agricultural crops worldwide. The limited availability of nematicides for the prevention and control of plant-parasitic nematodes necessitates the urgent development of novel nematicides. Natural products have always been a key source for the discovery of pesticides. Waltherione A, an alkaloid, exhibits potent nematocidal activity. In this study, we designed and synthesized a series of quinoline and quinolone derivatives from Waltherione A, leveraging a strategy of structural simplification. Bioassays have revealed that the quinoline derivatives exhibit better activity than quinolone derivatives in terms of both nematocidal and fungicidal activities. Notably, compound D1 demonstrated strong nematocidal activity, with a 72 h LC50 of 23.06 μg/mL, and it effectively controlled the infection of root-knot nematodes on cucumbers. The structure-activity relationship suggests that the quinoline moiety is essential for the nematocidal efficacy of Waltherione A. Additionally, compound D1 exhibited broad-spectrum fungicidal activity, with an EC50 of 2.98 μg/mL against Botrytis cinerea. At a concentration of 200 μg/mL, it significantly inhibited the occurrence of B. cinerea on tomato fruits, with an inhibitory effect of 96.65%, which is slightly better than the positive control (90.30%).

Recent Advances in Design and Development of Diazole and Diazine Based Fungicides (2014-2023)

With fungal diseases posing a major threat to agricultural production, the application of fungicides to control related diseases is often considered necessary to ensure the world's food supply. The search for new bioactive agents has long been a priority in crop protection due to the continuous development of resistance against currently used types of active compounds. Heterocyclic compounds are an inseparable part of the core structures of numerous lead compounds, these rings constitute pharmacophores of a significant number of fungicides developed over the past decade by agrochemists. Among heterocycles, nitrogen-based compounds play an essential role. To date, diazole (imidazole and pyrazole) and diazine (pyrimidine, pyridazine, and pyrazine) derivatives make up an important series of synthetic fungicides. In recent years, many reports have been published on the design, synthesis, and study of the fungicidal activity of these scaffolds, but there was a lack of a comprehensive classified review on nitrogen-containing scaffolds. Regarding this issue, here we have reviewed the published articles on the fungicidal activity of the diazole and diazine families. In current review, we have classified the molecules synthesized so far based on the size of the ring.

Quinoline Derivatives in Discovery and Development of Pesticides

Finding highly active molecular scaffold structures is always the key research content of new pesticide discovery. In the research and development of new pesticides, the discovery of new agricultural molecular scaffold structures and new targets still faces great challenges. In recent years, quinoline derivatives have developed rapidly in the discovery of new agriculturally active molecules, especially in the discovery of fungicides. The unique quinoline scaffold has many advantages in the discovery of new pesticides and can provide innovative and feasible solutions for the discovery of new pesticides. Therefore, we reviewed the use of quinoline derivatives and their analogues as molecular scaffolds in the discovery of new pesticides since 2000. We systematically summarized the agricultural biological activity of quinoline compounds and discussed the structure-activity relationship (SAR), physiological and biochemical properties, and mechanism of action of the active compounds, hoping to provide ideas and inspiration for the discovery of new pesticides.

Design, synthesis, antifungal activity and molecular docking of novel pyrazole-4-carboxamides containing tertiary alcohol and difluoromethyl moiety as potential succinate dehydrogenase inhibitors

BACKGROUND

Resistance problems with the long-term and frequent use of existing fungicides, and the lack of structure diversity of traditional pyrazole-4-carboxamide succinate dehydrogenase inhibitors, it is highly required to design and develop new fungicides to address the resistance issue.

RESULTS

Different from previous pyrazole-4-carboxamide succinate dehydrogenase inhibitors by breaking the norm of difluoromethyl at the C-3 position of pyrazole and introducing a tertiary alcohol group at the C-3 position, 27 novel pyrazole-4-carboxamide derivatives were designed, synthesized and characterized by proton (1 H) nuclear magnetic resonance (NMR), carbon-13 (13 C) NMR, fluorine-19 (19 F) NMR and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). The crystal structures of compounds A14 and C5 were analyzed by single crystal X-ray diffraction. Their in vitro antifungal activities were evaluated against phytopathogen Fusarium graminearum, Botrytis cinerea, Phytophthora capsica, Sclerotinia sclerotiorum, Thanatephorus cucumeris. The results displayed that most of them exhibited significant antifungal activities against S. sclerotiorum at 50 mg/L, the half maximal effective concentration (EC50 ) data of A8 and A14 were 3.96 and 2.52 mg/L, respectively. Their in vivo antifungal activities were evaluated against Pseudoperonospora cubensis, Puccinia sorghi Schw, Colletotrichum gloeosporioides, F. graminearum, Erysiphe graminis, Thanatephorus cucumeris, the control efficacies of A6, B3, C3, and C6 against E. graminis reached 100% at a concentration of 400 mg/L. The molecular docking results showed that the binding mode of the target compounds containing tertiary alcohols were similar to that of fluxapyroxad in succinate dehydrogenase. In addition, tertiary alcohols were involved in the formation of hydrogen bonds.

CONCLUSION

The excellent in vitro and in vivo inhibitory activities of novel pyrazole-4-carboxamide derivatives against succinate dehydrogenase were reported for the first time, and they could be used as the potential lead compounds. © 2023 Society of Chemical Industry.

Design, Synthesis, Antifungal Evaluation, Structure-Activity Relationship (SAR) Study, and Molecular Docking of Novel Spirotryprostatin A Derivatives

Phytopathogenic fungi cause plant diseases and economic losses in agriculture. To efficiently control plant pathogen infections, a total of 19 spirotryprostatin A derivatives and 26 spirooxindole derivatives were designed, synthesized, and tested for their antifungal activity against ten plant pathogens. Additionally, the intermediates of spirooxindole derivatives were investigated, including proposing a mechanism for diastereoselectivity and performing amplification experiments. The bioassay results demonstrated that spirotryprostatin A derivatives possess good and broad-spectrum antifungal activities. Compound 4d exhibited excellent antifungal activity in vitro, equal to or higher than the positive control ketoconazole, against Helminthosporium maydis, Trichothecium roseum, Botrytis cinerea, Colletotrichum gloeosporioides, Fusarium graminearum, Alternaria brassicae, Alternaria alternate, and Fusarium solan (MICs: 8-32 µg/mL). Compound 4k also displayed remarkable antifungal activity against eight other phytopathogenic fungi, including Fusarium oxysporium f. sp. niveum and Mycosphaerella melonis (MICs: 8-32 µg/mL). The preliminary structure-activity relationships (SARs) were further discussed. Moreover, molecular docking studies revealed that spirotryprostatin A derivatives anchored in the binding site of succinate dehydrogenase (SDH). Therefore, these compounds showed potential as natural compound-based chiral fungicides and hold promise as candidates for further enhancements in terms of structure and properties.

Discovery, Identification, and Mode of Action of Phenolics from Marine-Derived Fungus Aspergillus ustus as Antibacterial Wilt Agents

The bacterial wilt caused by Ralstonia solanacearum seriously affects crop yield and safety and is difficult to control. Biological activity-guided screening led to the isolation of 11 phenolic compounds including three undescribed compounds (carnemycin H-I and stromemycin B) from the secondary metabolites of a marine-derived Aspergillus ustus. One new compound is an unusual phenolic dimer. Their structures were elucidated by comprehensive spectroscopic data and J-based configurational analysis. The antibacterial activities of the isolated compounds against R. solanacearum were evaluated. Compound 3 exhibited excellent inhibitory activity with an MIC value of 3 μg/mL, which was comparable to that of streptomycin sulfate. Additionally, 3 significantly changed the morphology and inhibited the activity of succinate dehydrogenase (SDH) to interfere with the growth of R. solanacearum. Molecular docking was conducted to clarify the potential mechanisms of compound 3 with SDH. Further in vivo experiments demonstrated that 3 could remarkably inhibit the occurrence of bacterial wilt on tomatoes.

Discovery of Fungicidal Hydrazide Lead Compounds Derived from Sinapic Acid and Mycophenolic Acid

Structure optimization based on natural products has become an effective way to develop new green fungicides. In this project, thirty-two novel NPs-derived hydrazide compounds were designed and synthesized by introducing the bioactive hydrazide substructure into sinapic acid and mycophenolic acid. The fungicidal bioassays indicated that the obtained hydrazide compounds showed excellent and selective fungicidal activity against specific pathogens, especially compounds C8, D7, and D8 with EC50 values of 0.63, 0.56, and 0.43 μg mL-1 against M. oryzae, respectively. SAR indicated that the introduction of 4-fluoro, 4-chloro, and 2,4-difluoro groups was conducive to improving the fungicidal activity, while the extension of the hydrazide bridge would affect the selectivity for inhibitory activity. Subsequently, the effects of hydrazide compounds on rice seedling and zebrafish growth were also investigated. The fungicidal mechanism implied that treatment with compound B4 would cause significant changes in metabolites of plasma membrane-related linolenic acid metabolism, arachidonic acid metabolism, and α-linolenic acid metabolism pathways, which further led to the wrinkled hyphae and the blurred plasma membrane and cytoplasm. Finally, the frontier molecular orbitals and charge distribution were calculated to analyze the differences in bioactivity from a structural perspective. These results provide important guidance for the development and practical application of novel fungicides.

Natural Alkaloid Waltherione F-Derived Hydrazide Compounds Evaluated in an Agricultural Fungicidal Field

In this project, quinoline and quinolone-containing hydrazide compounds were designed and synthesized by introducing a bioactive hydrazide group into the skeleton of waltherione F. The fungicidal activity revealed that some hydrazide compounds exhibited excellent and broad-spectrum fungicidal activity; especially, compounds E8, E12, and E16 showed more than 90% or even 100% inhibition rates against most pathogens at 50 μg·mL-1. The fungicidal mechanism indicated that compound E8 may affect the normal function of the plasma membrane, further generating changes in the morphology and subcellular structure of mycelia. Simultaneously, Fusarium graminearum may resist the E8-treated stress through the metabolic pathways related to l-glutamate, l-glutamine, and glutathione. Finally, the effect of compound E8 on wheat seedling's growth and the toxicity to zebrafish were accomplished. These results will provide important guidance to discover novel fungicidal lead compounds and explore new targets, which are effective ways to alleviate the increasingly severe drug resistance.

Design, Synthesis, and Evaluation of Antifungal Bioactivity of Novel Pyrazole Carboxamide Thiazole Derivatives as SDH Inhibitors

Agricultural production is seriously threatened by plant pathogens. The development of new fungicides with high efficacy and low toxicity is urgently needed. In this study, a series of pyrazole carboxamide thiazole derivatives were designed, synthesized, and evaluated for their antifungal activities against nine plant pathogens in vitro. Bioassay results showed that most compounds (3i, 5i, 6i, 7i, 9i, 12i, 16i, 19i, and 23i) exhibited good antifungal activities against Valsa mali. In particular, compounds 6i and 19i exhibited better antifungal activities against Valsa mali with EC₅₀ values of 1.77 and 1.97 mg/L, respectively, than the control drug boscalid (EC₅₀ = 9.19 mg/L). Additionally, compound 23i exhibited excellent inhibitory activity against Rhizoctonia solani, with an EC₅₀ value of 3.79 mg/L. Compound 6i at 40 mg/L showed a satisfactory in vivo protective effect against Valsa mali. Scanning electron microscopy analyses revealed that compound 6i could significantly damage the surface morphology to interfere with the growth of Valsa mali. In molecular docking, the results showed that compound 6i interacts with TRP O: 173, SER P: 39, TYR Q: 58, and ARG P: 43 of succinate dehydrogenase (SDH) through hydrogen bonding and σ-π interaction, and its binding mode is similar to that of boscalid and SDH. The enzyme activity experiment also further verified its action mode. Our studies suggested that pyrazole carboxamide thiazole derivative 6i provided a valuable reference for the further development of succinate dehydrogenase inhibitors.

Rational Exploration of Novel SDHI Fungicide through an Amide-β-ketonitrile Bioisosteric Replacement Strategy

The identification of succinate dehydrogenase inhibitor (SDHI) fungicides bearing a novel scaffold is of great importance to control pathogenic fungi. Difluoromethyl-pyrazole β-ketonitrile derivatives were rationally designed through an innovative amide-β-ketonitrile bioisosteric replacement strategy and evaluated for their antifungal activities. In preliminary fungicidal screening, our new β-ketonitrile compounds showed outstanding in vitro activity. Compounds A7 and A14 exhibited EC₅₀ values of 0.116 and 0.165 μg/mL against Sclerotinia sclerotiorum, respectively, and A14 also displayed an EC₅₀ of 0.0774 μg/mL against Rhizoctonia solani. Furthermore, A14 exhibited moderate in vivo protective activity against rice sheath blight on rice plants. Results from SDH enzymatic assays demonstrated that A14 possesses significant inhibitory effect toward porcine heart SDH, with an IC₅₀ value of 0.183 μM, which was 20-fold more potent than that of fluxapyroxad (IC₅₀ = 3.76 μM). A docking study indicated that H-bonds, cation-π interactions, and edge-to-face π-π interactions play key roles in the binding of A14 with R. solani SDH. The CoMSIA model guided the approach to further structural optimizations and indicated that hydrophobic and steric substituents on the benzene ring have decisive effects on the fungicidal activity against R. solani. The present work describes for the first time the successful bioisosteric replacement of the common SDHI amide moiety by a β-ketonitrile group and highlights the potential of β-ketonitriles as an innovative novel SDHI subclass.

Discovery of Aryloxy-, Arylthio-, and Arylamino-Containing Acethydrazides as Fungicidal Agents

The development of new green fungicides is an effective way to solve the resistance of agricultural pathogens and plays an important role in promoting high-quality and sustainable development of modern agriculture. In this project, a series of aryloxy-, arylthio-, and arylamino-containing acethydrazide derivatives were designed, synthesized, and characterized by ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and high-resolution mass spectrometry (HRMS). The fungicidal bioassays indicated that some compounds showed excellent and broad-spectrum fungicidal activity, and the structure-activity relationship was discussed. The in vivo fungicidal activity demonstrated that compounds C4 and D8 exhibited good preventative effects against Fusarium graminearum infecting wheat leaves, of which the preventative activity of compound D8 was almost equal to that of the positive agents. Transmission electron microscopy (TEM) observation revealed that the plasma membrane in the C4-treated F. graminearum hyphal cells was severely contracted and separated with the cell wall, coupling with the visible lysosomes and the disappeared cytoplasm and organelles, which may be the reasons for the shriveled and even ruptured hyphae observed by scanning electron microscopy (SEM). Subsequently, transcriptomics and metabolomics were performed to further elucidate the fungicidal mechanism. The regulatory networks of differential genes and metabolites in plasma membrane-related sphingolipid metabolism, linoleic acid metabolism, α-linoleic acid metabolism, and arachidonic acid metabolism were constructed and elaborated. Additionally, preliminary investigation of seeding growth suggested that compounds C4 and D8 may have different degrees of influence on the growth indicators of wheat seedlings; however, this effect may be negligible as the plant grows.

Discovery of N-Methoxy-(biphenyl-ethyl)-pyrazole-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) inhibitor is one of the research hotspots for the development of fungicides. Herein, we describe the design and synthesis of N-methoxy-(biphenyl-ethyl)-pyrazole-carboxamide derivatives with enhanced fungicidal activity by employing fragment combination strategy. The SDH enzymatic activity was evaluated for 24 title compounds, and compound 7s was identified as the highest activity against porcine SDH with an IC₅₀ value of 0.014 μM, 205-fold greater than that of fluxapyroxad. Furthermore, the greenhouse experiments showed that compound 7u exhibited potent fungicidal activity against wheat powdery mildew with an EC₅₀ value of 0.633 mg/L, higher activity than fluxapyroxad and benzovindiflupyr. The computational results showed that the fluorine atom substituted on the pyrazole ring formed an extra dipolar-dipolar interaction with C_S42 and then increased the van der Waals interaction between the compound and SDH. The structural and mechanistic insights obtained from the present work will provide a valuable clue to developing novel SDH inhibitors.

Design, Synthesis, and Biological Evaluation of Novel Pyrazol-5-yl-benzamide Derivatives Containing Oxazole Group as Potential Succinate Dehydrogenase Inhibitors

A series of pyrazol-5-yl-benzamide derivatives containing the oxazole group were designed and synthesized as potential SDH inhibitors. According to the results of the bioassays, most target compounds displayed moderate-to-excellent in vitro antifungal activities against Valsa mali, Sclerotinia scleotiorum, Alternaria alternata, and Botrytis cinerea. Among them, compounds C13, C14, and C16 exhibited more excellently inhibitory activities against S. sclerotiorum than boscalid (EC₅₀ = 0.96 mg/L), with EC₅₀ values of 0.69, 0.26, and 0.95 mg/L, respectively. In vivo experiments on rape leaves and cucumber leaves showed that compounds C13 and C14 exhibited considerable protective effects against S. sclerotiorum than boscalid. SEM analysis indicated that compounds C13 and C14 significantly destroyed the typical structure and morphology of S. scleotiorum hyphae. In the respiratory inhibition effect assays, compounds C13 (28.0%) and C14 (33.9%) exhibited a strong inhibitory effect on the respiration rate of S. sclerotiorum mycelia, which was close to boscalid (30.6%). The results of molecular docking indicated that compounds C13 and C14 could form strong interactions with the key residues TRP O:173, ARG P:43, TYR Q:58, and MET P:43 of the SDH. Furthermore, the antifungal mechanism of these derivatives was demonstrated by the SDH enzymatic inhibition assay. These results demonstrate that compounds C13 and C14 can be developed into novel SDH inhibitors for crop protection.

Synthesis, Structure, and Antifungal Activities of 3-(Difluoromethyl)-Pyrazole-4-Carboxylic Oxime Ester Derivatives

Fifteen new pyrazole-4-carboxylic oxime ester derivatives were conveniently synthesized, and their structures were confirmed by 1H NMR, 13C NMR, HRMS, and X-ray diffraction. Antifungal assays indicated that some of these compounds possessed good activity against S. sclerotiorum, B. cinerea, R. solani, P. oryae, and P. piricola at 50 ppm. Structure-activity relationships (SAR) were studied by molecular docking simulation.

Synthesis, biological activity and toxicity to zebrafish of benzamides substituted with pyrazole-linked 1,2,4-oxadiazole

To find pesticidal lead compounds with high activity, a series of novel benzamides substituted with pyrazole-linked 1,2,4-oxadiazole was designed and synthesized by using the splicing principle of active substructures. The chemical structures of the target compounds were confirmed by 1H NMR, 13C NMR and HRMS. The preliminary bioassay showed that most compounds displayed good larvicidal activities against mosquito larvae at 10 mg L-1. In particular, compound 12g exhibited obvious activity; its lethal rate reached up to 100% (at 5 mg L-1) and 55% (at only 2 mg L-1). Furthermore, compound 12f (70.6%) and 12h (100%) showed good fungicidal activities against Pyricularia oryzae, with EC50 values of 8.28 and 5.49 μg mL-1, respectively, which were superior to that of the control drug bixafen (9.15 μg mL-1). Finally, the LC50 of compound 12h to zebrafish embryo was 0.39 mg L-1, so it was classified as a high-toxic compound. Thus, this compound may be used as a potential lead compound for further structural optimisation to develop new compounds with high activity and low toxicity.

Design, synthesis and mechanism study of novel natural-derived isoquinoline derivatives as antifungal agents

In screening for natural fungicidal leads, two series of novel 3-aryl-isoquinoline derivatives 8 and 9 were designed and synthesized based on sanguinarine, chelerythrine and berberine. Their structures were confirmed by 1D, 2D NMR and HRMS. Most of the title compounds showed medium to excellent antifungal activity in vitro at 50 mg/L, which were much more active than the lead of sanguinarine. Especially, 9f possessed the best effective against Alternaria solani (80.4%), Alternaria alternata (88.2%) and Physalospora piricola (93.8%). Furthermore, the EC₅₀ of 9f (3.651 mg/L) against P. piricola was marginally better than chlorothalonil (3.869 mg/L). In vivo antifungal activity of 9f against P. piricola was studied on apples. The curative and protection results at the dosage of 50 and 100 mg/L showed as 70.45 ~ 81.67% and 64.96 ~ 80.34%, respectively, which were equal to that of chlorothalonil (80.30 ~ 86.67%, 73.08 ~ 76.92%). Molecular electrostatic potential and molecular docking analysis revealed that 9f was fully covered by positive potential contour, which was easier to interact with the negative amino acid resides of succinate dehydrogenase than 8f. 9f could be used as a novel antifungal lead compound for further study. Two series of novel isoquinoline derivatives 8, 9 containing 3-aryl were rational designed and synthesized based on quaternary isoquinoline alkaloids. The bioassay and interaction mechanism studies indicated that 9f should be considered as potential antifungal lead.

Comprehensive Overview of Carboxamide Derivatives as Succinate Dehydrogenase Inhibitors

Up to now, a total of 24 succinate dehydrogenase inhibitors (SDHIs) fungicides have been commercialized, and SDHIs fungicides were also one of the most active fungicides developed in recent years. Carboxamide derivatives represented an important class of SDHIs with broad spectrum of antifungal activities. In this review, the development of carboxamide derivatives as SDHIs with great significances were summarized. In addition, the structure-activity relationships (SARs) of antifungal activities of carboxamide derivatives as SDHIs was also summarized based on the analysis of the structures of the commercial SDHIs and lead compounds. Moreover, the cause of resistance of SDHIs and some solutions were also introduced. Finally, the development trend of SDHIs fungicides was prospected. We hope this review will give a guide for the development of novel SDHIs fungicides in the future.

Research on Crystal Structure and Fungicidal Activity of the Amide Derivatives Based on the Natural Products Sinapic Acid and Mycophenolic Acid

Structural optimization based on natural products is an important and effective way to discover new green pesticides. Here, two series of amide derivatives based on sinapic acid and mycophenolic acid were designed in combination with the fungicidal natural product piperlongumine and synthesized by preparing the carboxylic acid into acyl chloride and then reacting with the corresponding aromatic amines, respectively. The resulting structures were successively characterized by 1H NMR, 13 C NMR, and HRMS. The crystal structures of molecules I-4 and II-5 were analyzed for structure validation. The in vitro inhibitory activity indicated that most of the target products exhibited fungicidal activity equivalent to or even better than fluopyram against Physalospora piricola. The in vivo fungicidal activity demonstrated that the compounds I-5 and II-4 displayed almost the same preventative activity as carbendazim and fluopyram at 200 μg mL−1. The TEM observation revealed that the fungicidal activity of the target molecules against Physalospora piricola may be due to the influence on the mitochondria in the cell structure. These results will provide valuable theoretical guidance for developing the new green fungicides.