Novel Camphor Sulfonohydrazide and Sulfonamide Derivatives as Potential Succinate Dehydrogenase Inhibitors against Phytopathogenic Fungi/Oomycetes

Synthesis and Activity of Novel Pyrazole/Pyrrole Carboxamides Containing a Dinitrogen Six-Membered Heterocyclic as Succinate Dehydrogenase and Ergosterol Biosynthesis Inhibitors against Colletotrichum camelliae

Pyrazole carboxamide derivatives were initially extensively studied as succinate dehydrogenase inhibitors (SDHIs). In the present study, a series of pyrazole/pyrrole carboxamides containing a dinitrogen six-membered heterocyclic were designed based on our reported active skeletons with dual mode of action. Bioactivity results showed that the target compound Q18 demonstrated superior antifungal efficacy against Colletotrichum camelliae (C. camelliae) with an EC50 value of 6.0 mg/L. The in vivo protective activity of Q18 was 74.7% at 100 mg/L. Scanning electron microscopy and transmission electron microscopy showed that Q18 could disrupt the surface morphology of the mycelia and cause lipid peroxidation of cell membrane, which was further verified by the determination of relative conductivity and malondialdehyde contents. Combined with ergosterol content, docking results between Q18 with SDH and CYP51, and the IC50 value of Q18 for SDH (9.7 mg/L), it is concluded that Q18 is a potential SDHI and ergosterol biosynthesis inhibitor. Thus, the present study provides fresh insight into the study of derivatives of the amides.

Targeting tubulin protein to combat fungal disease: Design, synthesis, and its new mechanistic insights of benzimidazole hydrazone derivatives

As the vital the biomacromolecule in eukaryotic cells, tubulin protein is essential for preserving cell shape, facilitating cell division, and cell viability. Tubulin has been approved as promising target for anticancer, and antifungal therapy. However, there are still many gaps in tubulin-targeted fungicidal discovery. To expand the structural diversity of benzimidazoles and achieve the distinct interaction model, a series of novel benzimidazole hydrazone derivatives were therefore synthesized. Antifungal results showed that compound A9 was the most effective, achieving an EC50 value of 2.88 μg/mL in vitro against Colletotrichum sublineola. In vivo assay, compound A9 displayed encouraging efficacy, outperforming the reference agents ferimzone and tetramethylthiuram disulfide. Interestingly, mechanistic studies indicated that, compared with carbendazim, compound A9 might form stronger interactions with tubulin, exemplified by the presence of multiple hydrogen bonds and π-π interactions, leading to intracellular microtubule aggregation in compound A9-treated cells. The significantly different interactions models between A9-tubulin and carbendazim-tubulin complexes may endow to produce the low resistance risk. Additionally, compound A9 possessed low phytotoxicity and satisfactory ADME properties. This study not only provides a structural basis for the development of benzimidazole-based fungicides targeting tubulin but also offers new insights into the use of immunofluorescence assays in tubulin-targeting studies.

Design and synthesis of camphor-thiazole derivatives as potent antifungal agents: structure-activity relationship and preliminary mechanistic study

BACKGROUND

Plant pathogenic fungi pose a severe threat to crop yield and food security. This study aims to investigate the potential antifungal activity and mechanism of action of camphor-thiazole derivatives against six plant pathogenic fungi. A novel series of camphor-thiazole derivatives were designed, synthesized and evaluated for their antifungal effects against Rhizoctonia solani, Fusarium graminearum, Valsa mali, Alternaria solani, Colletotrichum orbiculare and Botryitis cinerea.

RESULTS

Most of the synthesized camphor-thiazole derivatives exhibited notable antifungal activity. Amongst them, compounds C5, C10 and C17 showed significant activity against R. solani with median effective concentrations (EC50) values in the range 3-4 μg mL-1, demonstrating superior antifungal efficacy to the control drug boscalid (EC50 = 1.23 μg mL-1). Structure-activity relationship and density functional theory analysis emphasized the critical role of substituent selection in optimizing the biological activity of these compounds. Moreover, preliminary mechanistic studies revealed that compound C5 induced abnormal mycelial and cellular morphology in R. solani as observed using scanning and transmission electron microscopy, and triggered the production and accumulation of reactive oxygen species. Additionally, the increased concentration of C5 resulted in enhanced cell membrane permeability.

CONCLUSION

In this study, the designed and optimized compound C5 emerged as a promising candidate for potent antifungal agents. The results demonstrate that synthesized camphor-thiazole derivatives possess potent antifungal activity and can serve as lead compounds for further optimization in antifungal agent development. © 2024 Society of Chemical Industry.

Design, Synthesis, and Biological Evaluation of Novel Aryl Sulfonamide Derivatives as Potential Succinate Dehydrogenase Inhibitors Targeting Phytopathogenic Fungi

In our pursuit of novel succinate dehydrogenase inhibitor (SDHI) fungicides for agriculture, we replaced the traditional amide structure with a sulfonamide framework and introduced various heterocyclic and aromatic rings at the sulfonamide's termini. This strategy yielded 30 synthesized compounds, which were screened for antifungal activity against eight phytopathogenic fungi. The biological assay results demonstrated that several target compounds exhibited significant in vitro antifungal activity. Notably, compound 2f showed remarkable antifungal activity against Valsa mali with an EC50 value of 0.56 mg/L, outperforming Boscalid (EC50 = 1.79 mg/L). In vivo experiments revealed that compound 2f provided significant protection to apple fruits, comparable to Boscalid. SEM analysis of compounds 2f and 3e showed that compound 2f disrupted the structure and morphology of fungal hypha analysis, which suggested that the terminal polyhalogen-substituted pyridine moieties might be pivotal regions influencing their antifungal efficacy. Molecular docking analysis revealed that compound 2f and Boscalid exhibited a comparable binding mode to SDH. Furthermore, detailed SDH inhibition assays confirmed the potential of 2f (IC50 = 2.51 μM) as prospective SDH inhibitors. RNA transcriptomic analysis indicated that the application of compound 2f could influence gene expression in fungi, thereby exerting a defensive effect against plant pathogenic fungi. Consequently, compound 2f shows promise for developing a novel and efficient agrochemical fungicide.

Rational design and discovery of novel hydrazide derivatives as potent succinate dehydrogenase inhibitors inspired by natural d/l-camphor

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) have rapidly become one of the fastest-growing categories of fungicides used against plant pathogenic fungi. Recent research advancements have emphasized that structural modifications of SDHIs using naturally sourced scaffolds represent an innovative strategy for developing new, highly effective, broad-spectrum fungicides. A novel series of d/l-camphorhydrazide derivatives potentially targeting fungal succinate dehydrogenase (SDH) were designed, synthesized and evaluated for their antifungal effects against Rhizoctonia solani, Fusarium graminearum, Valsa mali and Botrytis cinerea.

RESULTS

Amongst them, compounds A1-7 (d-camphor) and A2-7 (l-camphor) displayed excellent in vitro activity against R. solani with median effective concentration (EC50) values of 0.38 and 0.48 μg mL-1, which were obviously superior to that of boscalid (0.87 μg mL-1). A2-5 (l-camphor, EC50 = 3.27 μg mL-1) exhibited good activity against V. mali. A2-7 (2.13 μg mL-1), A2-21 (5.2 μg mL-1) and A1-5 (5.15 μg mL-1) showed good antifungal activity against F. graminearum with EC50 values below that of boscalid (5.85 μg mL-1). Preliminary mechanistic studies, using scanning and transmission electron microscopy, indicated that compound A1-7 induced disordered entanglement of hyphae, shrinkage of hyphal surfaces, and vacuole swelling and rupture, which disrupted normal hyphal growth. Additionally, compound A1-7 induced the production and accumulation of reactive oxygen species, disrupted mitochondrial membrane potential, and effectively inhibited the germination and formation of sclerotia in R. solani. Moreover, the molecular docking results and SDH enzyme assays yielded promising outcomes.

CONCLUSION

In this study, the designed and optimized compounds A1-7 and A2-7 emerged as promising candidates for SDH-targeting fungicides, demonstrating strong antifungal activity. These compounds hold potential as new antifungal agents for further research. © 2024 Society of Chemical Industry.

Integrating morphology, physiology, and computer simulation to reveal the toxicity mechanism of eco-friendly rosin-based pesticides

To mitigate the impact of traditional chemical pesticides on environment, and achieve sustainable crop protection, 24 eco-friendly rosin-based sulfonamide derivatives were synthesized and developed. The in vitro activity assessment showed that compound 4X (Co. 4X) exhibited excellent fungicidal activity against V. mali (EC50 = 1.106 μg/mL), marginally surpassing the positive control carbendazim (EC50 = 1.353 μg/mL). In vivo investigations suggested that Co. 4X exhibited moderate efficacy in mitigating V. mali infection in both apple trees and apples. Physiological assessments revealed that Co. 4X induced severe ultrastructural damage to the mycelium, heightened cell membrane permeability, and inhibited SDH protein activity. Subsequent biosafety evaluations affirmed the environment-friendly of Co. 4X on Zebrafish (LC50(96h) = 25.176 μg/mL). Toxicological research revealed that Co. 4X caused damage to the cells of Zebrafish gills, liver, and intestines, resulting in impaired respiratory, detoxification, digestion, and absorption functions of Zebrafish. In summary, the findings of this study contribute to a deeper understanding of the toxicity mechanisms of novel pesticides, decreasing environmental risks caused by traditional chemical pesticides, and improving the effective management of novel pesticide applications.

Comprehensive Overview of the Amide Linker Modification in the Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase inhibitors (SDHIs) have become one of the most important classes of agrochemical fungicides. According to the data from FRAC, the resistance risk for SDHIs had reached up to medium and even to high. In general, the chemical structure of SDHIs mainly contained three fragments: an acid core, a hydrophobic tail, and an amide linker, corresponding to three modification directions for each fragment. Among them, amide linker modification (ALM) has become a research hotspot for the design of novel SDHIs fungicides in recent years. We presented here a detailed review on the ALM strategy in the past decade, and some of them had entered the market. According to their chemical structures, ALM strategy were classified into four parts: (1) linked aliphatic chain between amide bond and hydrophobic tail, (2) introducing substituents to replacing hydrogen atom in the amide bond, (3) reverse extending the amide linker, and (4) changed with other bioisosteres. Moreover, the structure-activity relationship and the interaction mechanism of ALM-SDHI with SDH were discussed. This review aims to provide a global perspective on research and development of novel SDHIs, as well as suggestions for food safety management.

Design, synthesis and biological activity evaluation of eco-friendly rosin-based fungicides for sustainable crop protection

BACKGROUND

Utilizing fungicides to protect crops from diseases is an effective method, and novel eco-friendly plant-derived fungicides with high efficiency and low toxicity are urgent requirements for sustainable crop protection.

RESULT

Two series of rosin-based fungicides (totally 35) were designed and synthesized. In vitro fungicidal activity revealed that Compound 6a (Co. 6a) effectively inhibited the growth of Valsa mali [median effective concentration (EC50) = 0.627 μg mL-1], and in vivo fungicidal activity suggested a significant protective efficacy of Co. 6a in protecting both apple branches (35.12% to 75.20%) and apples (75.86% to 90.82%). Quantum chemical calculations (via density functional theory) results indicated that the primary active site of Co. 6a lies in its amide structure. Mycelial morphology and physiology were investigated to elucidate the mode-of-action of Co. 6a, and suggested that Co. 6a produced significant cell membrane damage, accelerated electrolyte leakage, decreased succinate dehydrogenase (SDH) protein activity, and impaired physiological and biochemical functions, culminating in mycelial mortality. Molecular docking analysis revealed a robust binding energy (ΔE = -7.29 kcal mol-1) between Co. 6a and SDH. Subsequently, biosafety evaluations confirmed the environmentally-friendly nature of Co. 6a via the zebrafish model, yet toxicological results indicated that Co. 6a at median lethal concentration [LC50(96)] damaged the gills, liver and intestines of zebrafish.

CONCLUSION

The above research offers a theoretical foundation for exploiting eco-friendly rosin-based fungicidal candidates in sustainable crop protection. © 2024 Society of Chemical Industry.

Design, Synthesis, and Biological Activity Evaluation of Eco-Friendly Rosin-Diamide-Based Fungicides as Potential Succinate Dehydrogenase Inhibitors for Sustainable Crop Protection

To develop novel succinate dehydrogenase (SDH) inhibitors for sustainable crop protection, a series of dehydroabietyl-diamide-based fungicides (a total of 21) were designed. In vitro fungicidal activity measurement showed that compound 3u exhibited excellent fungicidal activity against Valsa mali (half-maximal effective concentration, EC50 = 0.195 μg/mL), surpassing that of the positive control carbendazim (EC50 = 1.35 μg/mL). The in vivo fungicidal activity assessment suggested that 3u exhibited a protective effect on apple branches (69.7-48.1%) and apples (94.6-56.6%). Furthermore, biosafety evaluation indicated that 3u was significantly environmentally friendly toward zebrafish. Subsequently, morphology, physiology, and molecular docking were investigated to elucidate the mode of action of 3u against V. mali. Results demonstrated a strong binding between 3u and SDH, resulting in decreased SDH activity (half-maximal inhibitory concentration, IC50 = 11.7 μg/mL). Moreover, 3u disrupted the mycelial cell membrane and accelerated electrolyte leakage, ultimately resulting in the death of V. mali. These findings suggest that 3u could serve as a potent SDH inhibitor for sustainable crop protection.

Design, Synthesis, and Antifungal Evaluation of Novel Pyrazole-5-sulfonamide Derivatives for Plant Protection

To develop further novel environmentally friendly antifungal agents with high efficacy, a series of pyrazole-5-sulfonamide derivatives were designed and synthesized by using the active molecules synthesized in previous works as lead compounds. Their antifungal activities were evaluated in vitro against ten highly destructive plant pathogenic fungi. The bioassay results indicated that more than half of the target compounds displayed potent antifungal activities (inhibition rate ≥85%) against Valsa mali and Sclerotinia sclerotiorum at 20 mg/L. Among them, compound C22 exhibited significant broad-spectrum antifungal activities against V. mali, S. sclerotiorum, Rhizoctonia solani, Botrytis cinerea, and Trichoderma viride, with EC50 values of 0.45, 0.49, 3.06, 0.57, and 1.43 mg/L, respectively. Moreover, compounds C21 and C22 exhibited remarkable protective effects on apple Valsa canker similar to tebuconazole (89.5%) at 50 mg/L. Preliminary antifungal mechanism investigations demonstrated that compound C22 may have inhibited V. mali mycelial growth by inducing oxidative damage to the mycelium and compromising the integrity of the cell membrane. Meanwhile, compounds C21 and C22 exhibited no obvious toxicity to worker bees (Apis mellifera ligustica). Taken together, these pyrazole-5-sulfonamide derivatives, particularly compound C22, possess huge potential to be developed as novel environmentally friendly fungicides with high efficacy.

Pinonic Acid Derivatives Containing Thiourea Motif: Promising Antifungal Lead Compound Targeting Cellular Barrier of Colletotrichum fructicola

In order to explore novel antifungal lead compounds from plant essential oil, thirty-two pinonic acid derivatives containing thiourea groups were designed and synthesized using α-pinene as a raw material. One of these pinonic acid derivatives compound 3a exhibited noteworthy in vitro antifungal activity against Colletotrichum fructicola (EC50 = 9.22 mg/L), which was comparable to that of the positive control kresoxim-methyl (EC50 = 9.69 mg/L). Structure-activity relationship (SAR) studies demonstrated that the introduction of thiourea groups, F atoms, and Cl atoms into the structure of pinonic acid derivatives significantly improved their antifungal activity. The in vivo antifungal test revealed that compound 3a could effectively control pear anthracnose. It also proved that compound 3a showed low acute oral toxicity to honeybees (LD50 > 100 μg/bee) and low or no cytotoxicity to LO2 and HEK293 cell lines. The preliminary mechanism of action studies revealed that compound 3a caused mycelium deformity, increased cell membrane permeability, blocked the normal process of phospholipase C on the cell membrane, and reduced mycelium protein content. The results of molecular docking studies demonstrated the stable binding of compound 3a to phospholipase C and chitin synthetase. This study suggested that compound 3a could be used as a promising lead compound for the development of novel antifungal agents targeting the cellular barrier of C. fructicola.

Synthesis, antifungal activity and action mechanism of novel citral amide derivatives against Rhizoctonia solani

BACKGROUND

Rice sheath blight caused by Rhizoctonia solani is a severe threat to the yield and quality of rice. Due to the unscientific abuse of common fungicides causing resistance and environmental issues, the development of new fungicides is necessary. In this study, we used citral as the lead compound, designed and synthesized a series of novel citral amide derivatives, and evaluated their antifungal activity and mode of action against R. solani.

RESULT

Bioassay results indicated that the antifungal activities of most citral amide derivatives against R. solani were significantly improved compared to citral, with EC50 values ranging from 9.50-27.12 mg L-1. Among them, compound d21 containing the N-(pyridin-4-yl)carboxamide group exhibited in vitro and in vivo fungicidal activities, with curative effects at 500 mg L-1 as effectively as the commercial fungicide validamycin·bacillus. Furthermore, d21 prolonged the lag phase of the growth curve of R. solani, reduced the amount of growth, and inhibited sclerotium germination and formation. Mechanistically, d21 deformed the mycelia, increased cell membrane permeability, and inhibited the activities of antioxidant and tricarboxylic acid cycle (TCA)-related enzymes. Metabolome analysis showed the abundance of some energy-related metabolites within R. solani increased, and simultaneously the antifungal substances secreted by itself reduced. Transcriptome analysis showed that most genes encoding ATP-binding cassette (ABC) transporters and peroxisomes upregulated after the treatment of d21 and cell membrane destruction.

CONCLUSION

This study indicates that novel citral amide derivatives possess antifungal activity against R. solani and are expected to develop an alternative option for chemical control of rice sheath blight. © 2024 Society of Chemical Industry.

Discovery of Novel Acethydrazide-Containing Flavonol Derivatives as Potential Antifungal Agents

In this study, a series of novel hydrazide-containing flavonol derivatives was designed, synthesized, and evaluated for antifungal activity. In the in vitro antifungal assay, most of the target compounds exhibited potent antifungal activity against seven tested phytopathogenic fungi. In particular, compound C32 showed the best antifungal activity against Rhizoctonia solani (EC50 = 0.170 μg/mL), outperforming carbendazim (EC50 = 0.360 μg/mL) and boscalid (EC50 = 1.36 μg/mL). Compound C24 exhibited excellent antifungal activity against Valsa mali, Botrytis cinerea, and Alternaria alternata with EC50 values of 0.590, 0.870, and 1.71 μg/mL, respectively. The in vivo experiments revealed that compounds C32 and C24 were potential novel agricultural antifungals. 3D quantitative structure-activity relationship (3D-QSAR) models were used to analyze the structure-activity relationships of these compounds. The analysis results indicated that introducing appropriate electronegative groups at position 4 of a benzene ring could effectively improve the anti-R. solani activity. In the antifungal mechanism study, scanning electron microscopy and transmission electron microscopy analyses revealed that C32 disrupted the normal growth of hyphae by affecting the structural integrity of the cell membrane and cellular respiration. Furthermore, compound C32 exhibited potent succinate dehydrogenase (SDH) inhibitory activity (IC50 = 8.42 μM), surpassing that of the SDH fungicide boscalid (IC50 = 15.6 μM). The molecular dynamics simulations and docking experiments suggested that compound C32 can occupy the active site and form strong interactions with the key residues of SDH. Our findings have great potential for aiding future research on plant disease control in agriculture.

Isolating Antipathogenic Fungal Coumarins from Coriaria nepalensis and Determining Their Primary Mechanism In Vitro

Through bioassay-guided isolation, eight undescribed coumarins (1-8), along with six reported coumarins (9-14), were obtained from Coriaria nepalensis. The new structures were determined by using IR, UV, NMR, HRESIMS, and ECD calculations. The results of the biological activity assays showed that compound 9 exhibited broad spectrum antifungal activities against all tested fungi in vitro and a significant inhibitory effect on Phytophthora nicotianae with an EC50 value of 3.00 μg/mL. Notably, compound 9 demonstrated greater curative and protective effects against tobacco balack shank than those of osthol in vivo. Thus, 9 was structurally modified to obtain new promising antifungal agents, and the novel derivatives (17b, 17j, and 17k) exhibited better effects on Sclerotinia sclerotiorum than did lead compound 9. Preliminary mechanistic exploration illustrated that 9 could enhance cell membrane permeability, destroy the morphology and ultrastructure of cells, and reduce the exopolysaccharide content of P. nicotianae mycelia. Furthermore, the cytotoxicity results revealed that compound 9 exhibited relatively low cytotoxicity against HEK293 cell lines with an inhibition rate of 33.54% at 30 μg/mL. This research is promising for the discovery of new fungicides from natural coumarins with satisfactory ecological compatibility.

Computation-Directed Molecular Design, Synthesis, and Fungicidal Activity of Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase inhibitors (SDHIs) are a class of fungicides targeting the pathogenic fungi mitochondrial SDH. Here, molecular docking, three-dimensional quantitative structure-activity relationship (3D-QSAR), and molecular dynamics (MD) simulations were used to guide SDHI innovation. Molecular docking was performed to explore the binding modes of SDH and its inhibitors. 3D-QSAR models were carried out on 33 compounds with activity against Rhizoctonia cerealis (R. cerealis); their structure-activity relationships were analyzed using comparative molecular field analysis and comparative molecular similarity indices analysis. MD simulations were used to assess the stability of the complexes under physiological conditions, and the results were consistent with molecular docking. Binding free energy was calculated through the molecular mechanics generalized born surface area method, and the binding free energy was decomposed. The results are consistent with the activity of bioassay and indicate that van der Waals and lipophilic interactions contribute the most in the molecular binding process. Afterward, we designed and synthesized 12 compounds under the guidance of the above-mentioned analyses, bioassay found that F9 was active against R. cerealis with the EC50 value of 9.43 μg/mL, and F4, F5, and F9 were active against Botrytis cinerea with an EC50 values of 5.80, 3.17, and 1.63 μg/mL, respectively. They all showed good activity between positive controls of pydiflumetofen and thifluzamide. Our study provides new considerations for effective SDHIs discovery.

Synthesis and Antifungal Activity of 2-(2-Benzoxazolyl)-1-arylethanone and 2-(2-Benzoxazolyl)-1-alkylethanone Derivatives

To discover more effective antifungal candidates, 33 benzoxazole derivatives, were designed, synthesized, and evaluated for their antifungal activity against seven phytopathogenic fungi by the mycelium growth rate method. Among 33 benzoxazole derivatives had thirteen derivatives no reported, and new derivatives C17 exhibited good inhibitory activity against Phomopsis sp. with EC50 values of 3.26 μM. Structure-activity relationship (SAR) of these derivatives analysis indicated that the substituent played a key role in antifungal activity in ortho-, meta- and para- substituted acetophenones. The preliminary mechanistic exploration demonstrated that C17 might exert its antifungal activity by targeting the mycelia cell membrane, which was verified by the observed changes in mycelial morphology, the formation of extracellular polysaccharides, cellular contents, cell membrane permeability and integrity, among other effects. Furthermore, C17 had potent curative effect against Phomopsis sp. in vivo, which indicated that C17 may be as a novelty potent antifungal agent.

Discovery of Pyrazole-5-yl-amide Derivatives Containing Cinnamamide Structural Fragments as Potential Succinate Dehydrogenase Inhibitors

To promote the development of novel agricultural succinate dehydrogenase inhibitor (SDHI) fungicides, we introduced cinnamamide and nicotinamide structural fragments into the structure of pyrazol-5-yl-amide by carbon chain extension and scaffold hopping, respectively, and synthesized a series of derivatives. The results of the biological activity assays indicated that most of the target compounds exhibited varying degrees of inhibitory activity against the tested fungi. Notably, compounds G22, G28, G34, G38, and G39 exhibited excellent in vitro antifungal activities against Valsa mali with EC50 values of 0.48, 0.86, 0.57, 0.73, and 0.87 mg/L, respectively, and this result was significantly more potent than boscalid (EC50 = 2.80 mg/L) and closer to the specialty control drug tebuconazole (EC50 = 0.30 mg/L). Compounds G22 and G34 also exhibited excellent in vivo protective and curative effects against V. mali at 40 mg/L. The SEM and TEM observations indicated that compounds G22 and G34 may affect normal V. mali mycelial morphology as well as the cellular ultrastructure. Molecular docking analysis results indicated that G22 and boscalid possessed a similar binding mode to that of SDH, and detailed SDH inhibition assays validated the feasibility of the designed compounds as potential SDH inhibitors. Compounds G22 and G3 were selected for theoretical calculations, and the terminal carboxylic acid group of this series of compounds may be a key region influencing the antifungal activity. Furthermore, toxicity tests on Apis mellifera l. revealed that compounds G22 and G34 exhibited low toxicity to A. mellifera l. populations. The above results demonstrated that these series of pyrazole-5-yl-amide derivatives are promising for development as potential low-risk drug-resistance agricultural SDHI fungicides.

Design, Synthesis, and Biological Evaluation of Novel Camphor-Based Hydrazide and Sulfonamide Derivatives as Laccase Inhibitors against Plant Pathogenic Fungi/Oomycetes

To discover novel natural product-based fungicidal agrochemicals, 41 novel camphanic acid hydrazide and camphor sulfonamide derivatives were designed, synthesized, and tested for their antifungal profile against four plant pathogenic fungi and three oomycetes. As a result, some derivatives presented pronounced inhibitory activities toward Botryosphaeria dothidea, Fusarium graminearum, Phytophthora capsici, and Phytophthora nicotianae. Especially, compound 4b demonstrated the most potent anti-B. dothidea activity (EC50 = 1.28 mg/L), much stronger than positive control chlorthalonil. The in vivo assay showed that 4b displayed significant protective and curative effects on apple fruits infected by B. dothidea. The primary antifungal mechanism study revealed that 4b could obviously enhance the cell membrane permeability, destroy the mycelial surface morphology and the cell ultrastructure, and reduce the ergosterol and exopolysaccharide contents of B. dothidea. Further, 4b showed potent laccase inhibitory activity in vitro with an IC50 value of 11.3 μM, superior to positive control cysteine. The molecular docking study revealed that 4b could dock well into the active site of laccase by forming multiple interactions with the key residues in the pocket. The acute oral toxicity test in rats presented that 4b had slight toxicity with an LD50 value of 849.1 mg/kg bw (95% confidence limit: 403.9-1785.3 mg/kg bw). This research identified that the camphanic acid hydrazide derivatives could be promising leads for the development of novel laccase-targeting fungicides.