Studies on the novel pyridine sulfide containing SDH based heterocyclic amide fungicide

Design, synthesis and antifungal activity of novel amide derivatives containing a pyrrolidine moiety as potential succinate dehydrogenase inhibitors

To discover new succinate dehydrogenase inhibitors (SDHI) fungicides, a series of amide derivatives containing a pyrrolidine moiety were designed and synthesized, and their antifungal activities were evaluated against Monilinia fructicola (M. fructicola), Rhizoctonia solani (R. solani), Fusarium graminearum schw (F. graminearum), Fusarium oxysporum (F. oxysporum), and Phytophthora infestans (P. infestans). Some compounds showed excellent antifungal activities against the five fungi. Among them, compound 6 showed broad-spectrum inhibitory activities. The EC50 of compound 6 against M. fructicola, R. solani, F. graminearum, F. oxysporum, and P. infestans were 2.13, 14.42, 1.69, 27.79, and 27.12 mg/L, respectively. In addition, compound 6 can effectively inhibit the spore germination of M. fructicola and has moderate damage to the cell membrane. Compound 6 can effectively inhibit succinate dehydrogenase (SDH) of M. fructicola, and can significantly increase the expression levels of SDHC and SDHD. Compound 6 can be used as a lead structure for developing new SDH inhibitors.

Enantioselective fate and risk assessment of chiral fungicide pydiflumetofen in rice-fish and wheat farming systems

Fungicides have been widely used for reducing the losses caused by plant diseases. Rice and wheat are the most basic food crops, and the potential risks after applying fungicides are worthy of attention. Especially rice-fish farming system is an ecological symbiosis system that is beneficial to both environmental and ecological protection. However, the application of pesticides will stress the ecosystem, and the pesticide residues in rice and fish would be transmitted along the food chain, which is harmful to human health. Here, the enantioselective behaviors of chiral pydiflumetofen in rice-fish and wheat farming systems were clarified. In the rice-fish farming system, pydiflumetofen enantiomers were preferentially attached to the plants, entering the paddy water and settling into the paddy soil, and then accumulating and dissipating in the fish. With the growth of rice, it was transported to rice fruits. The wheat farming system was similar. Enantioselective dissipation occurred in carp (Cyprinus carpio), brown rice and wheat soil, and S-(+)-pydiflumetofen was preferentially dissipated. In other words, R-(-)-pydiflumetofen showed higher concentrations, especially in carp, which meant R-(-)-pydiflumetofen was more easily accumulated in the environment, and posed a greater potential risk to the farming system. The pydiflumetofen residues in brown rice and wheat were lower than MRLs from the EFSA (0.02 mg/kg) and eCFR (0.3 mg/kg), respectively. What deserves attention is that the MRL of pydiflumetofen in fish is not clear. Meanwhile, pydiflumetofen in paddy soil and wheat soil had a persistent residual effect, and the risks could not be ignored. Combined with the previous research, developing S-(+)-pydiflumetofen products will help to reduce the dosage and reduce the risks to environment and people. This study evaluated the environmental fate and risk of chiral pydiflumetofen from the perspective of farming system, and would provide data support for its rational use and risk assessment.

Discovery of Novel Pyrazol-5-yl-benzamide Derivatives Containing a Thiocyanato Group as Broad-Spectrum Fungicidal Candidates

In an effort to promote the development of new fungicides, a series of 48 novel N-(1-methyl-4-thiocyanato-1H-pyrazol-5-yl)-benzamide derivatives A1-A36 and B1-B12 were designed and synthesized by incorporating a thiocyanato group into the pyrazole ring, and their fungicidal activities were evaluated against Sclerotinia sclerotiorum, Valsa mali, Botrytis cinerea, Rhizoctonia solani, and Phytophthora capsici. In the in vitro antifungal/antioomycete assay, many of the target compounds exhibited good broad-spectrum fungicidal activities. Among them, compound A36 displayed the best antifungal activity against V. mali with an EC50 value of 0.37 mg/L, which was significantly higher than that of the positive controls fluxapyroxad (13.3 mg/L) and dimethomorph (10.3 mg/L). Meanwhile, compound B6 exhibited the best antioomycete activity against P. capsici with an EC50 value of 0.41 mg/L, which was higher than that of azoxystrobin (29.2 mg/L) but lower than that of dimethomorph (0.13 mg/L). Notably, compound A27 displayed broad-spectrum inhibitory activities against V. mali, B. cinerea, R. solani, S. sclerotiorum, and P. capsici with respective EC50 values of 0.71, 1.44, 1.78, 0.87, and 1.61 mg/L. The in vivo experiments revealed that compounds A27 and B6 presented excellent protective and curative efficacies against P. capsici, similar to that of the positive control dimethomorph. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that compound B6 could change the mycelial morphology and severely damage the ultrastructure of P. capsici. The results of the in vitro SDH enzymatic inhibition experiments indicated that compounds A27 and B6 could effectively inhibit the activity of P. capsici SDH (PcSDH). Furthermore, molecular docking analysis demonstrated significant hydrogen bonds and Pi-S bonding between the target compounds and the key amino acid residues of PcSDH, which could explain the probable mechanism of action. Collectively, these studies provide a valuable approach to expanding the fungicidal spectrum of pyrazol-5-yl-benzamide derivatives.

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.

NHC-Catalyzed Chemoselective Reactions of Enals and Cyclopropylcarbaldehydes for Access to Chiral Dihydropyranone Derivatives

An N-heterocyclic carbene (NHC)-catalyzed chemoselective activation reaction of 1-cyclopropylcarbaldehydes and α-alkynyl enals is reported. NHC selectively catalyzes 1-cyclopropylaldehydes, followed by a [2 + 4] cycloaddition reaction with α-alkynyl enals. The target dihydropyranone derivatives bearing different substituents and substitution patterns can be obtained in good to excellent yields with excellent enantio- and diastereoselectivities under mild conditions.

Novel aromatic carboxamide potentially targeting fungal succinate dehydrogenase: Design, synthesis, biological activities and molecular dynamics simulation studies

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) emerging in fungicide markets are widely used in crop protection. Currently, the structural modification focusing on a structurally diverse 'core' moiety (aryl) of SDHIs is being gradually identified as one of the innovative strategies for developing novel, highly effective and low resistant fungicides.

RESULTS

By optimization of lead compound SCU2028, 30 novel aromatic carboxamides Ia-o and IIa-o without a pyrazol group were designed, synthesized and characterized by 1 H NMR, 13 C NMR and high resolution mass spectrum (HRMS). In vitro antifungal activities showed that most of the compounds Ia-o and IIa-o exhibited good antifungal activities against Rhizoctonia solani. Among them, compounds Ic and IIc (EC50  = 0.02 mg/L), with the 2-chloro-3-pyridyl moiety, and compounds Io (EC50  = 0.03 mg/L) and IIo (EC50  = 0.02 mg/L), with the 4-methyl-2-trifluoromethylthiazolyl moiety, all exhibited the equivalent antifungal activities against R. solani with compound SCU2028 (EC50  = 0.03 mg/L) and bixafen (EC50  = 0.04 mg/L). Additionally, in pot tests, compound IIc (EC50  = 3.63 mg/L) also had higher antifungal activity against R. solani than compound SCU2028 (EC50  = 7.63 mg/L). Furthermore, in vitro inhibitory activity against fungal SDH showed the inhibitory ability of compound IIc was equivalent with that of compound SCU2028, and molecular dynamics simulation of the SDH-compound IIc complex suggested that compound IIc could strongly bind to and interact with the binding site of SDH.

CONCLUSION

Novel aromatic carboxamides without a pyrazol group have potential as a class of SDHIs, and the strategy of replacing the pyrazol group with another aryl in the 'core' moiety might offer an alternative option in discovery of SDHI fungicides. © 2023 Society of Chemical Industry.

Synthesis, Antifungal, and Antibacterial Activities of Novel Benzoylurea Derivatives Containing a Pyrimidine Moiety

To explore more efficient and less toxic antibacterial and antifungal pesticides, we utilized 2,6-difluorobenzamide as a starting material and ultimately synthesized 23 novel benzoylurea derivatives containing a pyrimidine moiety. Their structures were characterized and confirmed by 1H NMR, 13C NMR, 19F NMR, and HRMS. The bioassay results demonstrated that some of the title compounds exhibited moderate to good in vitro antifungal activities against Botrytis cinerea in cucumber, Botrytis cinerea in tobacco, Botrytis cinerea in blueberry, Phomopsis sp., and Rhizoctonia solani. Notably, compounds 4j and 4l displayed EC50 values of 6.72 and 5.21 μg/mL against Rhizoctonia solani, respectively, which were comparable to that of hymexazol (6.11 μg/mL). Meanwhile, at 200 and 100 concentrations, the target compounds 4a-4w exhibited lower in vitro antibacterial activities against Xanthomonas oryzae pv. oryzicola and Xanthomonas citri subsp. citri, respectively, compared to those of thiodiazole copper. Furthermore, the molecular docking simulation demonstrated that compound 4l formed hydrogen bonds with SER-17 and SER-39 of succinate dehydrogenase (SDH), providing a possible explanation for the mechanism of action between the target compounds and SDH. This study represents the first report on the antifungal and antibacterial activities of novel benzoylurea derivatives containing a pyrimidine moiety.

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 antifungal activity of novel dithiin tetracarboximide derivatives as potential succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitor (SDHI) fungicides are an important class of agricultural fungicides with the advantages of high efficiency and a broad bactericidal spectrum. To pursue novel SDHIs, a series of N-substituted dithiin tetracarboximide derivatives were designed, synthesized, and characterized by ¹ H NMR, ¹³ C NMR, and high resolution mass spectrum (HRMS).

RESULTS

These engineered compounds displayed potent fungicidal activity against phytopathogens, including Sclerotinia sclerotiorum, Botrytis cinerea, and Rhizoctonia solani, comparable with that of the commercial SDHI fungicide boscalid. In particular, compound 18 stood out with prominent activity against S. sclerotiorum with a half-maximal effective concentration (EC₅₀ ) value of 1.37 μg ml^-1 . Compound 1 exhibited the most potent antifungal activity against B. cinerea with EC₅₀ values of 5.02 μg ml^-1 . As for R. solani, 12 and 13 exhibited remarkably inhibitory activity with EC₅₀ values of 4.26 and 5.76 μg ml^-1 , respectively. In the succinate dehydrogenase (SDH) inhibition assay, 13 presented significant inhibitory activity with a half-maximal inhibitory concentration (IC₅₀ ) value of 15.3 μm, which was approximately equivalent to that of boscalid (14.2 μm). Furthermore, molecular docking studies revealed that 13 could anchor in the binding site of SDH.

CONCLUSION

Taken together, results suggested that the dithiin tetracarboximide scaffold possessed a huge potential to be developed as novel fungicides and SDHIs. © 2023 Society of Chemical Industry.

Design, Synthesis, and Fungicidal Evaluation of Novel N-Methoxy Pyrazole-4-Carboxamides as Potent Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH, EC 1.3.5.1, also known as complex II) has been recognized as one of the most promising targets of fungicides. Here, based on the binding mode of pydiflumetofen with SDH, the carbon-carbon double bond was introduced into the chemical of pydiflumetofen and then produced the target compounds 6a-6o. The enzymatic inhibitory activity and structure-activity relationship (SAR) study showed that the 2-position and 4-position in terminal benzene were positive to increasing activity. Furthermore, fungicidal activity results in greenhouses indicated that compound 6o showed good control effects against wheat powdery mildew (WPM), cucumber powdery mildew (CPM), and southern corn rust (SCR), showing its broad-spectrum property. Especially, compound 6o exhibited 95 and 75% control effects against CPM and SCR at 6.25 mg/L, respectively, which are better than pydiflumetofen (80% control effects against CPM and 15% against SCR), indicating its potency that is worthy of further development.

Potential Fungicide Candidates: A Dual Action Mode Study of Novel Pyrazole-4-carboxamides against Gibberella zeae

Pyrazole carboxamides are a class of traditional succinate dehydrogenase inhibitors (SDHIs) that have developed into a variety of commercialized fungicides. In the present work, a series of novel 1,5-disubstituted-1H-pyrazole-4-carboxamide derivatives were designed and synthesized based on the active backbone of 5-trifluoromethyl-1H-4-pyrazole carboxamide. Bioassay results indicated that some target compounds exhibited excellent in vitro antifungal activities against six phytopathogenic fungi. Notably, the EC₅₀ values of Y₄₇ against Gibberella zeae, Nigrospora oryzae, Thanatephorus cucumeris, and Verticillium dahliae were 5.2, 9.2, 12.8, and 17.6 mg/L, respectively. The in vivo protective and curative activities of Y₄₇ at 100 mg/L against G. zeae on maize were 50.7 and 44.2%, respectively. Three-dimensional quantitative structure-activity relationship (3D-QSAR) analysis revealed that the large steric hindrance and electronegative groups on the 5-position of the pyrazole ring were important for the activity. The IC₅₀ value of Y₄₇ against succinate dehydrogenase (SDH) was 7.7 mg/L, superior to fluopyram (24.7 mg/L), which was consistent with the docking results. Morphological studies with fluorescence microscopy (FM) and scanning electron microscopy (SEM) found that Y₄₇ could affect the membrane integrity of mycelium by inducing endogenous reactive oxygen species (ROS) production and causing peroxidation of cellular lipids, which was further verified by the malondialdehyde (MDA) content. Antifungal mechanism analysis demonstrated that the target compound Y₄₇ not only had significant SDH inhibition activity but could also affect the membrane integrity of mycelium, exhibiting obvious dual action modes. This research provides a novel approach to the development of traditional SDHIs and their derivatives.

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.

Design, Synthesis, Inhibitory Activity, and Molecular Modeling of Novel Pyrazole-Furan/Thiophene Carboxamide Hybrids as Potential Fungicides Targeting Succinate Dehydrogenase

To discover new fungicides targeting succinate dehydrogenase (SDH), 36 new furan/thiophene carboxamides containing 4,5-dihydropyrazole rings were designed, synthesized, and characterized. The crystal structure of compound 5l was determined with the X-ray diffraction (XRD) of single crystals. The antifungal activity of these compounds was studied against Botrytis cinerea, Pyricularia oryzae, Erysiphe graminis, Physalospora piricola, and Penicillium digitatum. Bioassay results were that most compounds had obvious inhibitory activity at 20 μg/mL. Compounds 5j, 5k, and 5l possessed outstanding inhibitory activity against B. cinerea. Their EC₅₀ values were 0.540, 0.676, and 0.392 μg/mL, respectively. They owned better effects than fluxapyroxad (EC₅₀ = 0.791 μg/mL). In the meantime, the inhibitory activity of 16 compounds was evaluated against SDH. It turned out that these compounds displayed excellent activity. The IC₅₀ values of compounds 5j, 5k, and 5l reached 0.738, 0.873, and 0.506 μg/mL, respectively, whereas the IC₅₀ value of fluxapyroxad was 1.031 μg/mL. The results of molecular dynamics (MD) simulation showed that compound 5l possessed a stronger affinity to SDH than fluxapyroxad.

Identification of novel nematode succinate dehydrogenase inhibitors: Virtual screening based on ligand-pocket interactions

To discover new nematicidal succinate dehydrogenase (SDH) inhibitors with novel structures, we conducted a virtual screening of the ChemBridge library with 1.7 million compounds based on ligand-pocket interactions. The homology model of Caenorhabditis elegans SDH was established, along with a pharmacophore model based on ligand-pocket interactions. After the pharmacophore-based and docking-based screening, 19 compounds were selected for the subsequent enzymatic assays. The results showed that compound 1 (ID: 7607321) exhibited inhibitory activity against SDH with a determined IC₅₀ value of 19.6 μM. Structural modifications and nematicidal activity studies were then carried out, which provided further evidence that compound 1 exhibited excellent nematicidal activity. Molecular dynamics simulations were then conducted to investigate the underlying molecular basis for the potency of these inhibitors against SDH. This work provides a reliable strategy and useful information for the future design of nematode SDH inhibitors.

Antifungal Mechanism of Metabolites from Newly Isolated Streptomyces sp. Y1-14 against Banana Fusarium Wilt Disease Using Metabolomics

Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) is one of the most destructive banana diseases in the world, which limits the development of the banana industry. Compared with traditional physical and chemical practices, biological control becomes a promising safe and efficient strategy. In this study, strain Y1-14 with strong antagonistic activity against Foc TR4 was isolated from the rhizosphere soil of a banana plantation, where no disease symptom was detected for more than ten years. The strain was identified as Streptomyces according to the morphological, physiological, and biochemical characteristics and the phylogenetic tree of 16S rRNA. Streptomyces sp. Y1-14 also showed a broad-spectrum antifungal activity against the selected 12 plant pathogenic fungi. Its extracts inhibited the growth and spore germination of Foc TR4 by destroying the integrity of the cell membrane and the ultrastructure of mycelia. Twenty-three compounds were identified by gas chromatography-mass spectrometry (GC-MS). The antifungal mechanism was investigated further by metabolomic analysis. Strain Y1-14 extracts significantly affect the carbohydrate metabolism pathway of Foc TR4 by disrupting energy metabolism.

Design, Synthesis, and Fungicidal Evaluation of Novel 1,3-Benzodioxole-Pyrimidine Derivatives as Potential Succinate Dehydrogenase Inhibitors

A series of novel 1,3-benzodioxole-pyrimidine derivatives were designed and synthesized. The in vitro bioassay indicated that compounds 4e, 4g, 4n, 5c, and 5e displayed excellent fungicidal activities against test fungal strains. Especially, in the in vitro experiments, 5c exhibited a broad spectrum of fungicidal activity against Botrytis cinerea, Rhizoctonia solani, Fusarium oxysporum, Alternaria solani, and Gibberella zeae with EC₅₀ values of 0.44, 6.96, 6.99, 0.07, and 0.57 mg/L, respectively, which were significantly more potent than those of positive control boscalid (EC₅₀: 5.02, >50, >50, 0.16, and 1.28 mg/L). In vivo testing on tomato fruits and leaves showed that 5c displayed considerable protective and curative efficacy against A. solani. Scanning electron microscopy analysis indicated that 5c possessed a strong ability to destroy the surface morphology of mycelia and seriously interfere with the growth of the fungal pathogen. In the in vitro enzyme inhibition assay, 5c exhibited pronounced succinate dehydrogenase (SDH) inhibitory activity with an IC₅₀ value of 3.41 μM, equivalent to that of boscalid (IC₅₀: 3.40 μM). In addition, fluorescence quenching experiment further confirmed the strong interaction of 5c with SDH. Through chiral resolution, 5c was separated into two enantiomers. Among them, (S)-5c exhibited stronger fungicidal activity (EC₅₀: 0.06 mg/L) and SDH inhibitory (2.92 μM) activity than the R-enantiomer (EC₅₀: 0.17 mg/L and SDH IC₅₀: 3.68 μM), which was in accordance with the molecular docking study (CDOCKER Interaction Energy for (R)-5c and (S)-5c: -28.23 and -29.98 kcal/mol, respectively). These results presented a promising lead for the discovery of novel SDHIs as antifungal pesticides.

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.

Syntheses, structures, in vitro cytostatic activity and antifungal activity evaluation of four diorganotin(iv) complexes based on norfloxacin and levofloxacin

Four organotin(iv) complexes have been designed and synthesized from the reactions of R2SnO (R = Me, Ph) with the corresponding ligands norfloxacin and levofloxacin. And the cytostatic and antifungal activity test have been done.

Synthesis and Biological Activity of Novel Antifungal Leads: 3,5-Dichlorobenzyl Ester Derivatives

Succinate dehydrogenase (SDH) is one of the most important molecular targets for the development of new fungicides. Carboxamide fungicides are a class of SDH inhibitors widely used to inhibit highly destructive plant pathogens. Although cases of resistance have been found in fungal pathogens due to the unrestricted use in recent years, there is still demand for new compounds with improved fungicidal activity. Therefore, a series of ester compounds were designed to investigate potential novel antifungal molecules. First, the antifungal activity of different benzyl alcohol compounds (A1-A21) was tested, and a highly active fragment (3,5-dichlorobenzyl alcohol) was found. Subsequently, various compounds were synthesized by esterification between different acids and 3,5-dichlorobenzyl alcohol, among which compound 5 exhibited remarkable antifungal activity against Botrytis cinerea and Rhizoctonia solani with EC₅₀ values of 6.60 and 1.61 mg/L, respectively, which were comparable to those of commercial fungicide boscalid (EC₅₀ = 1.24 and 1.01 mg/L). In vivo testing further demonstrated that compound 5 was effective in suppressing B. cinerea (200 mg/L, 50.9%). Moreover, SDH inhibition assays, fluorescence quenching analysis, and determination of mitochondrial membrane potential revealed that compound 5 has similar effects to boscalid. Furthermore, the fungicidal activity of target compounds can be maintained by modifying the amide bond to an ester bond. These results will provide basis for the development of novel fungicides.

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.

Synthesis and fungicidal activity of novel benzimidazole derivatives bearing pyrimidine-thioether moiety against Botrytis cinerea

BACKGROUND

Botrytis cinerea is a serious plant fungus and strongly affects the yield and quality of crops. The main control strategy is the employment of fungicides. To research for efficient fungicide with novel structure, a series of novel benzimidazole derivatives bearing pyrimidine and thioether moieties were designed and synthesized.

RESULTS

Some target compounds such as 4h, 4i, 4k, 4l, 4m, 4s, 4t and 4u exhibited notable fungicidal activities, with half maximal effective concentration (EC₅₀ ) values in the range 0.13-0.24 μg mL^-1 , which means that their activities were comparable or higher than that of carbendazim (EC₅₀ = 0.21 μg mL^-1 ). Among them, N-(4-fluorophenyl)-2-((4-(1H-benzimidazol-2-yl)-6-(4-methoxyphenyl) pyrimidin-2-yl)thio)acetamide (4m) displayed the best activity (EC₅₀ = 0.13 μg mL^-1 ). Molecular electrostatic potential analysis of 4m elucidated that the NH moiety of benzimidazole ring was located in the positive potential region and may generate hydrogen bond with target amino acid residue. Molecular docking analysis revealed that there was one hydrogen bond and one 𝜋-𝜋 interaction between 4m and target protein.

CONCLUSIONS

This study demonstrated that the benzimidazole derivatives bearing pyrimidine and thioether moieties can be further optimized as a lead compound for the control of B. cinerea. The combination of molecular electrostatic potential and molecular docking analyses may provide a valuable reference for studying the interaction between the ligand and target protein. © 2021 Society of Chemical Industry.

Design, Synthesis, and Antifungal Activity of Novel Thiophene/Furan-1,3,4-Oxadiazole Carboxamides as Potent Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) is known as an ideal target for the investigations of fungicides. To develop novel SDH inhibitors, 30 novel thiophene/furan-1,3,4-oxadiazole carboxamide derivatives were designed and synthesized. In the in vitro antifungal assay, a majority of the target compounds demonstrated fair to potent antifungal activity against seven tested phytopathogenic fungi. Compounds 4b, 4g, 4h, 4i, and 5j showed remarkable antifungal activity against Sclerotinia sclerotiorum, affording EC50 values ranging from 0.1∼1.1 mg/L. In particular, compound 4i displayed the most potent activity against S. sclerotiorum (EC₅₀ = 0.140 ± 0.034 mg/L), which was superior to that of boscalid (EC₅₀ = 0.645 ± 0.023 mg/L). A further morphological investigation revealed the abnormal mycelia and damaged cell structures of compound 4i-treated S. sclerotiorum by scanning electron microscopy. Furthermore, the in vivo antifungal assay against S. sclerotiorum revealed that compounds 4g and 4i were effective for suppressing rape Sclerotinia rot at a dosage of 200 mg/L. In the SDH inhibition assay, compounds 4g and 4i also presented significant inhibitory activity with IC₅₀ values of 1.01 ± 0.21 and 4.53 ± 0.19 μM, respectively, which were superior or equivalent to that of boscalid (3.51 ± 2.02 μM). Molecular docking and molecular dynamics simulation of compound 4g with SDH revealed that compound 4g could form strong interactions with the key residues of the SDH. These results indicated that this class of derivatives could be a promising scaffold for the discovery and development of novel SDH inhibitors.

The research progress in and perspective of potential fungicides: Succinate dehydrogenase inhibitors

Succinate dehydrogenase inhibitors (SDHIs) have become one of the fastest growing classes of new fungicides since entering the market, and have attracted increasing attention as a result of their unique structure, high activity and broad fungicidal spectrum. The mechanism of SDHIs is to inhibit the activity of succinate dehydrogenase, thereby affecting mitochondrial respiration and ultimately killing pathogenic fungi. At present, they have become popular varieties researched and developed by major pesticide companies in the world. In the review, we focused on the mechanism, the history, the representative varieties, structure-activity relationship and resistance of SDHIs. Finally, the potential directions for the development of SDHIs were discussed. It is hoped that this review can strengthen the individuals' understanding of SDHIs and provide some inspiration for the development of new fungicides.

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

The development of new green fungicides based on the structural optimization of natural products can effectively solve the problems of low safety and high pathogen resistance of traditional fungicides. In this paper, based on pyrazole amide compound h-I-9 with excellent fungicidal activity discovered in the previous work, a series of l-serine-derived pyrazole amide and waltherione alkaloid-derived pyrazole ester derivatives were synthesized. The structures were successively identified by ¹H NMR, ¹³C NMR, high-resolution mass spectrometry, and X-ray single-crystal diffraction. The in vitro and in vivo fungicidal activity screening demonstrated that compound II-5 showed a good inhibition rate against Physalospora piricola. A transmission electron microscope and fluorescence microscope observation further revealed that compound II-5 may cause damage to the cell membranes and vacuoles, and the hyphae treated with II-5 could produce obvious and easily observed blue fluorescence. The succinate dehydrogenase (SDH) enzymatic activity and molecular docking simulation indicated that compounds I-3 and I-4 may be potential SDH inhibitors against Alternaria sp.

Synthesis and insecticidal activity of novel 1,2,4-triazole derivatives containing trifluoroacetyl moieties

A series of compounds containing trifluoroacetyl groups were synthesized, and their insecticidal activity against Nilaparvata lugens and Aphis craccivora was evaluated. The compound structure was identified by NMR, HRMS, and single-crystal diffraction. The bioassay results indicated that compound 4-1 (R¹ is chloropyridine, R² is H), 4-2 (R¹ is chlorothiazole, R² is H) and 4-19 (R¹ is benzyl, R² is isopropyl) had the best activity against Nilaparvata lugens (93.5%, 94.1% and 95.5%) at 100 mg/L concentration. The effect of different substituents of R¹ or R² on the activity was studied through the structure-activity relationship. Molecular docking of compounds 4-1 and 4-2 was discussed.

Design, Synthesis, Biological Evaluation, and Molecular Modeling of Novel 4H-Chromene Analogs as Potential Succinate Dehydrogenase Inhibitors

Thirty-one new 4H-chromene derivatives were designed and synthesized. Their structures were identified with IR, ¹H NMR, ¹³C NMR, and HRMS. The crystal structure of compound 2a was determined by single-crystal X-ray diffraction. Their antifungal activities were evaluated against Pyricularia oryzae, Erysiphe graminis, Coniella diplodiella, Pseudoperonospora cubensis, and Sclerotinia sclerotiorum. These results demonstrated that most compounds exhibited remarkable inhibitory activities at 20 μg/mL. Compounds 4b and 4c displayed excellent antifungal activity against S. sclerotiorum and possessed better efficacy than fluopyram. At the same time, the inhibitory activity of the bioactive compounds was evaluated against succinate dehydrogenase (SDH). The results showed that these compounds possessed outstanding activity. Compounds 4b and 4c displayed better inhibitory activity than fluopyram. The molecular modeling results revealed that compound 4c had stronger affinity to SDH than fluopyram. It is the first time that the inhibitory activity of 4H-chromene analogs against SDH has been reported.

Design, synthesis and antifungal/anti-oomycete activity of pyrazolyl oxime ethers as novel potential succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) play an increasingly important role in controlling plant diseases. However, the similar structures of SDHIs result in rapid development of cross-resistance development and a clear bottleneck of poor activity against oomycetes, therefore the need to seek new SDHI fungicides with novel structures is urgent.

RESULTS

Innovative pyrazolyl oxime ethers were designed by replacing amide with oxime ether based on the succinate dehydrogenase (SDH) structure, and 19 pairs of Z- and E-isomers were efficiently prepared for the discovery of SDHI compounds with a novel bridge. Their biological activities against four fungi and two oomycetes were evaluated, and substantial differences were observed between the Z- and E- isomers of the title compounds. Furthermore, most of these compounds exhibited remarkable activities against Rhizoctonia solani with EC₅₀ values of less than 10 mg L^-1 in vitro, and bioassay in vivo further confirmed that E-I-6 exhibited good protective efficacy (76.12%) at 200 mg L^-1 . In addition, Z-I-12 provided better activity against the oomycetes Pythium aphanidermatum and Phytophthora capsici (EC₅₀  = 1.56 and 0.93 mg L^-1 ) than those of boscalid. Moreover, E-I-12 exhibited excellent SDH inhibition (IC₅₀  = 0.21 mg L^-1 ) thanks to its good binding ability to the SDH by hydrogen-bonding interactions, π-cation interaction and hydrophobic interactions.

CONCLUSION

Novel pyrazolyl oxime ethers have the potential as SDHI compounds for future development, and the strategy of replacing an amide bond with oxime ether may offer an alternative option in SDHI fungicide discovery.

Novel Pyrazole-4-acetohydrazide Derivatives Potentially Targeting Fungal Succinate Dehydrogenase: Design, Synthesis, Three-Dimensional Quantitative Structure-Activity Relationship, and Molecular Docking

Succinate dehydrogenase inhibitors (SDHIs) have emerged in fungicide markets as one of the fastest-growing categories that are widely applied in agricultural production for crop protection. Currently, the structural modification focusing on the flexible amide link of SDHI molecules is being gradually identified as one of the innovative strategies for developing novel highly efficient and broad-spectrum fungicides. Based on the above structural features, a series of pyrazole-4-acetohydrazide derivatives potentially targeting fungal SDH were constructed and evaluated for their antifungal effects against Rhizoctonia solani, Fusarium graminearum, and Botrytis cinerea. Strikingly, the in vitro EC₅₀ values of constructed pyrazole-4-acetohydrazides 6w against R. solani, 6c against F. graminearum, and 6f against B. cinerea were, respectively, determined as 0.27, 1.94, and 1.93 μg/mL, which were obviously superior to that of boscalid against R. solani (0.94 μg/mL), fluopyram against F. graminearum (9.37 μg/mL), and B. cinerea (1.94 μg/mL). Concurrently, the effects of the substituent steric, electrostatic, hydrophobic, and hydrogen-bond fields on structure-activity relationships were elaborated by the reliable comparative molecular field analysis and comparative molecular similarity index analysis models. Subsequently, the practical value of pyrazole-4-acetohydrazide derivative 6w as a potential SDHI was ascertained by the relative surveys on the in vivo anti-R. solani preventative efficacy, inhibitory effects against fungal SDH, and molecular docking studies. The present results provide an indispensable complement for the structural optimization of antifungal leads potentially targeting SDH.

Discovery of Novel Pyrazole Carboxylate Derivatives Containing Thiazole as Potential Fungicides

Inspired by commercially established fluxapyroxad as the lead compound of novel efficient antifungal ingredients, novel pyrazole carboxylate derivatives containing a flexible thiazole backbone were successfully designed, synthesized, and detected for their in vitro and in vivo biological activities against eight agricultural fungi. The antifungal bioassay results showed that compound 24 revealed excellent bioactivities against Botrytis cinerea and Sclerotinia sclerotiorum, with median effective concentrations (EC₅₀) of 0.40 and 3.54 mg/L, respectively. Compound 15 revealed remarkable antifungal activity against Valsa mali, with an EC₅₀ value of 0.32 mg/L. For in vivo fungicide control against B. cinerea and V. mali, compounds 3 and 24 at 25 mg/L, respectively, displayed prominent efficacy on cherry tomatoes and apple branches. Molecular docking results demonstrated that compound 15 could form an interaction with several crucial residues of succinate dehydrogenase (SDH), and the in vitro enzyme assay indicated that the target compound 15 displayed an inhibitory effect toward SDH, with an IC₅₀ value of 82.26 μM. The experimental results indicated that phenyl pyrazole carboxylate derivatives displayed a weak antifungal property and low activity compared to the other title substituent pyrazole carboxylate derivatives. Compounds 3, 15, and 24 are promising antifungal candidates worthy of further fungicide development due to their prominent effectiveness.

Synthesis, Crystal Structure, Herbicidal Activity, and SAR Study of Novel N-(Arylmethoxy)-2-chloronicotinamides Derived from Nicotinic Acid

Nicotinic acid, also known as niacin, is a natural product, which is widely found in plants and animals. To discover novel natural-product-based herbicides, a series of N-(arylmethoxy)-2-chloronicotinamides were designed and synthesized. Some of the new N-(arylmethoxy)-2-chloronicotinamides exhibited excellent herbicidal activity against Agrostis stolonifera (bentgrass) at 100 μM. Compound 5f (2-chloro-N-((3,4-dichlorobenzyl)oxy)nicotinamide) possessed excellent herbicidal activity against Lemna paucicostata (duckweed), with an IC₅₀ value of 7.8 μM, whereas the commercial herbicides clomazone and propanil had values of 125 and 2 μM, respectively. The structure-activity relationships reported in this paper could be used for the development of new herbicides against monocotyledonous weeds.

Pine Rosin as a Valuable Natural Resource in the Synthesis of Fungicide Candidates for Controlling Fusarium oxysporum on Cucumber

To improve the effect of pine rosin in plant fungicides, four series of dehydroabietyl-1,3,4-thiadiazole derivatives from the natural product rosin were synthesized. Based on the evaluation of the in vitro antifungal activity against Sclerotinia sclerotiorum, Botrytis cinerea, Fusarium oxysporum, and Magnaporthe oryzae, rosin-based 1,3,4-thiadiazole compounds containing thiophene heterocycles were screened. Notably, compound 3e [dehydroabietyl-(1,3,4-thiadiazol-2-yl)-5-nitrothiophene-2-carboxamide] exhibited excellent antifungal property against F. oxysporum with an EC₅₀ of 0.618 mg/L, which was lower than that of the positive control carbendazim (0.649 mg/L). The in vivo antifungal activity results showed that 3e exerted a protective effect on cucumber plants. Physiological and biochemical studies showed that the primary mechanism of action of compound 3e on F. oxysporum was it changed the mycelial morphology, increased the cell membrane permeability, and inhibited the synthesis of ergosterol in the mycelia. Furthermore, the quantitative structure-activity relationship studies revealed that the frontier orbital energy in the molecule had a key role in the antifungal activity through the conjugation and electrostatic interaction between compound 3e and the receptors of the target. Thus, the present study highlighted the application of rosin-based fungicidal candidates and exploited efficient plant pesticides for sustainable crop production.

The crystal structure of (E)-3-chloro-2-(2-(2-fluorobenzylidene)hydrazinyl)pyridine, C12H9ClFN3

C12H9ClFN3, monoclinic, C2/c (no. 15), a = 24.713(13) Å, b = 15.459(13) Å, c = 17.507(12) Å, β = 17.515(11)°, V = 4742(6) Å3, Z = 16, R gt (F) = 0.0530, wR ref (F 2) = 0.1161, T = 113(2) K.

Design, synthesis and fungicidal activity of pyrazole-thiazole carboxamide derivatives

Twenty-one novel pyrazole-thiazole carboxamide derivatives were rationally designed and synthesized. Bioassay results indicated that 6d (EC₅₀ = 5.11 μg/mL) and 6j (EC₅₀ = 8.14 μg/mL) exhibited better in vitro activities than fluxapyroxad (EC₅₀ = 11.93 μg/mL) and thifluzamide (EC₅₀ = 22.12 μg/mL) against Rhizoctonia cerealis. Particularly, compound 6j showed promising in vivo protective activity against Rhizoctonia solani and Puccinia sorghi Schw. with 80% and 90% inhibition at 10 μg/mL, respectively. Our studies found that pyrazole-thiazole is a promising fungicide lead deserving for further derivation.

1,2,4-Oxadiazole-Based Bio-Isosteres of Benzamides: Synthesis, Biological Activity and Toxicity to Zebrafish Embryo

To discover new compounds with broad spectrum and high activity, we designed a series of novel benzamides containing 1,2,4-oxadiazole moiety by bioisosterism, and 28 benzamides derivatives with antifungal activity were synthesized. These compounds were evaluated against four fungi: Botrytis cinereal, FusaHum graminearum, Marssonina mali, and Thanatephorus cucumeris. The results indicated that most of the compounds displayed good fungicidal activities, especially against Botrytis cinereal. For example, 10a (84.4%), 10d (83.6%), 10e (83.3%), 10f (83.1%), 10i (83.3%), and 10l (83.6%) were better than pyraclostrobin (81.4%) at 100 mg/L. In addition, the acute toxicity of 10f to zebrafish embryo was 20.58 mg/L, which was classified as a low-toxicity compound.

Expedient Discovery for Novel Antifungal Leads Targeting Succinate Dehydrogenase: Pyrazole-4-formylhydrazide Derivatives Bearing a Diphenyl Ether Fragment

The pyrazole-4-carboxamide scaffold containing a flexible amide chain has emerged as the molecular skeleton of highly efficient agricultural fungicides targeting succinate dehydrogenase (SDH). Based on the above vital structural features of succinate dehydrogenase inhibitors (SDHI), three types of novel pyrazole-4-formylhydrazine derivatives bearing a diphenyl ether moiety were rationally conceived under the guidance of a virtual docking comparison between bioactive molecules and SDH. Consistent with the virtual verification results of a molecular docking comparison, the in vitro antifungal bioassays indicated that the skeleton structure of title compounds should be optimized as an N'-(4-phenoxyphenyl)-1H-pyrazole-4-carbohydrazide scaffold. Strikingly, N'-(4-phenoxyphenyl)-1H-pyrazole-4-carbohydrazide derivatives 11o against Rhizoctonia solani, 11m against Fusarium graminearum, and 11g against Botrytis cinerea exhibited excellent antifungal effects, with corresponding EC₅₀ values of 0.14, 0.27, and 0.52 μg/mL, which were obviously better than carbendazim against R. solani (0.34 μg/mL) and F. graminearum (0.57 μg/mL) as well as penthiopyrad against B. cinerea (0.83 μg/mL). The relative studies on an in vivo bioassay against R. solani, bioactive evaluation against SDH, and molecular docking were further explored to ascertain the practical value of compound 11o as a potential fungicide targeting SDH. The present work provided a non-negligible complement for the structural optimization of antifungal leads targeting SDH.

Design, synthesis biological activity, and docking of novel fluopyram derivatives containing guanidine group

Succinate dehydrogenase (SDH), a crucial bridge enzyme between the respiratory electron transfer chain and tricarboxylic acid (or Krebs) cycle, has been identified as an ideal target for the development of effective fungicide. In this study, a series of 24 novel SDH inhibitors (SDHIs) were designed, synthesized, and characterized by ¹H NMR, ¹³C NMR, and HRMS. In vitro fungicidal activity experiments, most of the compounds exhibited broad-spectrum antifungal activities against five plant pathogenic fungi. Compounds 9j and 9k showed excellent activities against Pythium aphanidermatum with EC₅₀ values of 9.93 mg/L and 10.50 mg/L, respectively, which were superior to the lead compound Fluopyram with an EC₅₀ value of 19.10 mg/L. Furthermore, the toxicity of these compounds was also tested against Meloidogyne incognita J2 nematodes. The results indicated that compound 9x exhibited moderate nematicidal activity (LC₅₀/48 h = 71.02 mg/L). Molecular docking showed that novel guanidine amide of 9j formed hydrogen bonds with crucial residues, which was crucial to the binding of an inhibitor and SDH. This present work indicates that these derivatives may serve as novel potential fungicides targeting SDH.

Synthesis and Biological Activity of Benzamides Substituted with Pyridine-Linked 1,2,4-Oxadiazole

To find pesticidal lead compounds with high activity, a series of novel benzamides substituted with pyridine-linked 1,2,4-oxadiazole were designed by bioisosterism, and synthesized easily via esterification, cyanation, cyclization and aminolysis reactions. The structures of the target compounds were confirmed by ¹H-NMR, ¹³C-NMR and HRMS. The preliminary bioassay showed that most compounds had good larvicidal activities against mosquito larvae at 10 mg/L, especially compound 7a, with a larvicidal activity as high as 100%, and even at 1 mg/L was still 40%; at 50 mg/L, all the target compounds showed good fungicidal activities against the eight tested fungi. Moreover, compound 7h exhibited better inhibitory activity (90.5%) than fluxapyroxad (63.6%) against Botrytis cinereal. Therefore, this type of compound can be further studied.

Synthesis and Pesticidal Activities of New Quinoxalines

Natural products are a source of many novel compounds with biological activity for the discovery of new pesticides and pharmaceuticals. Quinoxaline is a fused N-heterocycle in many natural products and synthetic compounds, and seven novel quinoxaline derivatives were designed and synthesized via three steps. Pesticidal activities of title quinoxaline derivatives were bioassayed. Most of these compounds had herbicidal, fungicidal, and insecticidal activities. The compounds 2-(6-methoxy-2-oxo-3-phenylquinoxalin-1(2H)-yl)acetonitrile (3f) and 1-allyl-6-methoxy-3-phenylquinoxalin-2(1H)-one (3g) were the most active herbicides and fungicides. Mode-of-action studies indicated that 3f is a protoprophyrinogen oxidase-inhibiting herbicide. Compound 3f also possessed broad-spectrum fungicidal activity against the plant pathogen Colletotrichum species. Some of these compounds also had insecticidal activity. Molecular docking and DFT analysis can potentially be used to design more active compounds.