Discovery of Novel Thiazole Carboxamides as Antifungal Succinate Dehydrogenase Inhibitors

Design, Synthesis, and Evaluation of Novel Isothiazole-Purines as a Pyruvate Kinase-Based Fungicidal Lead Compound

Target identification is one of the most important bases for novel pesticide development; pyruvate kinase (PK) was discovered as a potent fungicide target in our previous studies. To continue the PK-based fungicide development, novel isothiazole-purine derivatives were rationally designed and synthesized. Bioassay results showed that compound 5ai displayed excellent in vitro activity against Rhizoctonia solani with an EC₅₀ of 1.5 μg/mL, which was superior to those of positive controls diflumetorim with its EC₅₀ of 19.8 μg/mL and PK-based lead YZK-C22 with its EC₅₀ of 4.2 μg/mL. Compounds 3b (5.2 μg/mL) and 3c (4.5 μg/mL) displayed better activities against Gibberella zeae with their EC₅₀s falling between 4.0 and 5.5 μg/mL, while YZK-C22 showed an EC₅₀ of 6.4 μg/mL. In addition, 5ah exhibited promising in vivo activity against Erysiphe graminis and Puccinia sorghi Schw. with 100% efficacy at 10 μg/mL and 90% efficacy at 2 μg/mL against P. sorghi Schw. Compound 5ai showed good PK inhibitory activity with an IC₅₀ of 38.8 μmol/L, and it was well docked into the active site of the target enzyme PK, which was slightly more active than YZK-C22 with its IC₅₀ of 42.4 μmol/L. Our studies discovered that isothiazole-purines were PK-based fungicidal leads deserving of further study.

Fabrication of Versatile Pyrazole Hydrazide Derivatives Bearing a 1,3,4-Oxadiazole Core as Multipurpose Agricultural Chemicals against Plant Fungal, Oomycete, and Bacterial Diseases

Developing multipurpose agricultural chemicals is appealing in crop protection, thus eventually realizing the reduction and efficient usage of pesticides. Herein, an array of versatile pyrazole hydrazide derivatives bearing a 1,3,4-oxadiazole core were initially synthesized and biologically evaluated the antifungal, antioomycetes, and antibacterial activities. In addition, the pyrazole ring was replaced by the correlative pyrrole, thiazole, and indole scaffolds to extend the molecular diversity. The results showed that most of these hybrid compounds were empowered with multifunctional bioactivities, which are exemplified by compounds a1-a6, b1-b3, b7, b10, b13, and b18. For the antifungal activity, the minimal EC₅₀ values could afford 0.47 (a2), 1.05 (a2), 0.65 (a1), and 1.32 μg/mL (b3) against the corresponding fungi Gibberella zeae (G. z.), Fusarium oxysporum, Botryosphaeria dothidea, and Rhizoctonia solani. In vivo pot experiments against corn scab (caused by G. z.) revealed that the compound a2 was effective with protective and curative activities of 90.2 and 86.3% at 200 μg/mL, which was comparable to those of fungicides boscalid and fluopyram. Further molecular docking study and enzymatic activity analysis (IC₅₀ = 3.21 μM, a2) indicated that target compounds were promising succinate dehydrogenase inhibitors. Additionally, compounds b2 and a4 yielded superior anti-oomycete and antibacterial activities toward Phytophora infestins and Xanthomonas oryzae pv. oryzae with EC₅₀ values of 2.92 and 8.43 μg/mL, respectively. In vivo trials against rice bacterial blight provided the control efficiency within 51.2-55.3% (a4) at 200 μg/mL, which were better than that of bismerthiazol. Given their multipurpose characteristics, these structures should be positively explored as agricultural chemicals.

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 and Biological Activity of Novel Pyrazol-5-yl-benzamide Derivatives as Potential Succinate Dehydrogenase Inhibitors

To promote the discovery and development of new fungicides, a series of novel pyrazol-5-yl-benzamide derivatives were designed, synthesized by hopping and inversion of amide groups of pyrazole-4-carboxamides, and evaluated for their antifungal activities. The bioassay data revealed that compound 5IIc exhibited an excellent in vitro activity against Sclerotinia sclerotiorum with an EC₅₀ value of 0.20 mg/L, close to that of commercial fungicide Fluxapyroxad (EC₅₀ = 0.12 mg/L) and Boscalid (EC₅₀ = 0.11 mg/L). For Valsa mali, compound 5IIc (EC₅₀ = 3.68 mg/L) showed a significantly higher activity than Fluxapyroxad (EC₅₀ = 12.67 mg/L) and Boscalid (EC₅₀ = 14.83 mg/L). In addition, in vivo experiments proved that compound 5IIc has an excellent protective fungicidal activity with an inhibitory rate of 97.1% against S. sclerotiorum at 50 mg/L, while the positive control Fluxapyroxad showed a 98.6% inhibitory effect. The molecular docking simulation revealed that compound 5IIc interact with TRP173, SER39, and ARG43 of succinate dehydrogenase (SDH) through a hydrogen bond and p-π interaction, which could explain the probable mechanism of the action between compound 5IIc and target protein. Also, the SDH enzymatic inhibition assay was carried out to further validate its mode of action. These results demonstrate that compound 5IIc could be a promising fungicide candidate and provide a valuable reference for further investigation.

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.

Synthesis, Biological Evaluation, and 3D-QSAR Studies of N-(Substituted pyridine-4-yl)-1-(substituted phenyl)-5-trifluoromethyl-1H-pyrazole-4-carboxamide Derivatives as Potential Succinate Dehydrogenase Inhibitors

A series of new fungicides that can inhibit the succinate dehydrogenase (SDH) was classified and named as SDH inhibitors by the Fungicide Resistance Action Committee in 2009. To develop more potential SDH inhibitors, we designed and synthesized a novel series of N-(substituted pyridine-4-yl)-1-(substituted phenyl)-5-trifluoromethyl-1H-pyrazole-4-carboxamide derivatives, 4a-4i, namely, 5a-5h, 6a-6h, and 7a-7j. The bioassay results demonstrated that some title compounds exhibited excellent antifungal activity against four tested phytopathogenic fungi (Gibberella zea, Fusarium oxysporum, Cytospora mandshurica, and Phytophthora infestans). The EC₅₀ values were 1.8 μg/mL for 7a against G. zeae, 1.5 and 3.6 μg/mL for 7c against F. oxysporum and C. mandshurica, respectively, and 6.8 μg/mL for 7f against P. infestans. The SDH enzymatic activity testing revealed that the IC₅₀ values of 4c, 5f, 7f, and penthiopyrad were 12.5, 135.3, 6.9, and 223.9 μg/mL, respectively. The molecular docking results of this series of title compounds with SDH model demonstrated that the compounds could completely locate inside of the pocket, the body fragment formed H bonds, and the phenyl ring showed a π-π interaction with Arg59, suggesting that these novel 5-trifluoromethyl-pyrazole-4-carboxamide derivatives might target SDH. These results could provide a benchmark for understanding the antifungal activity against the phytopathogenic fungus P. infestans and prompt us to discover more potent SDH inhibitors.

Design, synthesis and biological evaluation of pyrazole-aromatic containing carboxamides as potent SDH inhibitors

To continue our ongoing studies on discovery of new potent antifungal leads, 43 novel pyrazole-aromatic containing carboxamides were rationally designed and synthesized. Bioassays indicated that most target compounds displayed good in vitro antifungal activities against Botrytis cinerea, Rhizoctonia cerealis and Sclerotinia sclerotiorum and in vivo antifungal activity against R. solani. Compound 11ea exhibited the most significant in vitro activity against R. cerealis (EC₅₀ = 0.93 μg/mL) with about 2-fold more potent than a previously reported lead compound A1 (EC₅₀ = 2.01 μg/mL), and about 11-fold more potent than the positive control/commercial succinate dehydrogenase inhibitor thifluzamide (EC₅₀ = 23.09 μg/mL). Structure-activity relationship analysis and molecular docking simulations indicated that the presence of difluoromethyl pyrazole-(m-benzene) carboxamide scaffold obviously increased the antifungal activity. The further enzymatic bioassay showed that both thifluzamide and compound 11ea displayed excellent SDH inhibitory effects, and fluorescence quenching analysis suggested that they may share the same target SDH.

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, and Evaluation of the Antifungal Activity of Novel Pyrazole-Thiazole Carboxamides as Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) is regarded as a promising target for fungicide discovery. To continue our ongoing studies on the discovery of novel SDH inhibitors as fungicides, novel pyrazole-thiazole carboxamides were designed, synthesized, and evaluated for their antifungal activity. The results indicated that compounds 9ac, 9bf, and 9cb showed excellent in vitro activities against Rhizoctonia cerealis with EC₅₀ values from 1.1 to 4.9 mg/L, superior to that of the commercial fungicide thifluzamide (EC₅₀ = 23.1 mg/L). Compound 9cd (EC₅₀ = 0.8 mg/L) was far more active than thifluzamide (EC₅₀ = 4.9 mg/L) against Sclerotinia sclerotiorum. Compound 9ac exhibited promising in vivo activity against Rhizoctonia solani (90% at 10 mg/L), which was better than that of thifluzamide (80% at 10 mg/L). The field experiment showed that compound 9ac had 74.4% efficacy against Rhizoctonia solani on the 15th day after two consecutive sprayings at an application rate of 4.80 g a.i./667 m², which was close to that of thifluzamide (83.3%). Furthermore, molecular docking explained the possible binding mode of compound 9ac in the RcSDH active site. Our studies indicated that the pyrazole-thiazole carboxamide hybrid is a new scaffold of SDH inhibitors.

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

BACKGROUND

Succinate dehydrogenase (SDH) has been identified as one of the most significant targets for fungicide discovery. To date, 23 commercial SDH inhibitor (SDHI) fungicides have been approved for plant protection since the first launch of carboxin in 1966, and extensively applied to combat destructive plant fungi.

RESULTS

In this project, 20 novel pyridine sulfide derivatives containing SDH-based heterocyclic amide fungicide were designed, synthesized, and characterized by proton nuclear magnetic resonance (¹ H-NMR), carbon-13 (¹³ C)-NMR and high-resolution mass spectrometry (HRMS). In vitro fungicidal activity experiment, the target compound I-1 displayed excellent inhibitory rates against the common agricultural pathogens with half maximal effective concentration (EC₅₀ ) values of 5.2 to 39.8 μg mL^-1 . The in vivo fungicidal activities demonstrated that the compound I-1 could effectively prevent Botrytis cinerea from infecting tomato and cucumber leaves with the preventative rates of 67% and 50%. The mitochondrial membrane potential detection, SDH enzyme assay and the molecular docking simulation revealed that the mechanism of action of the compound I-1 and the relevant interactions with the target enzyme may be similar to those of the control fluopyram.

CONCLUSION

The biological activity screening and validation of mechanism of action indicated that the compound I-1 could be identified as a potential SDH inhibitor for further study. © 2020 Society of Chemical Industry.

Sulfur-containing natural hinduchelins derivatives as potential antifungal agents against Rhizoctonia solani

Aryl-oxazole alkaloids are an important class of heterocyclic natural products, and which has been demonstrated to exhibit broad biological functions. During the course of our research for highly active compounds from natural products, the natural hinduchelins A-D with typical aryl-oxazole unit have been synthesized and investigated. So, in order to develop highly potential functional molecules, a series of novel sulfur-containing aryl-oxazole compounds derived from natural hinduchelins was designed and synthesized, and their in vitro fungicidal activities against four common plant pathogenic fungi (oomycetes Phytophthora capsici, ascomycetes Sclerotinia sclerotiorum, deuteromycetes Botrytis cinerea and basidiomycetes Rhizoctonia solani) were evaluated, the results demonstrated that compounds 7b and 7c displayed good selectivity and specificity in vitro against basidiomycetes R. solani. In addition, the in vivo antifungal activities also indicated compounds 7b and 7c can protect the horsebean against infection by R. solani, and the possible mechanism of antifungal action for these compounds has also been investigated.

Synthesis and Biological Activity of Novel Succinate Dehydrogenase Inhibitor Derivatives as Potent Fungicide Candidates

In searching for novel fungicidal leads, the novel bioactive succinate dehydrogenase inhibitor (SDHI) derivatives were designed and synthesized by the inversion of carbonyl and amide groups. Bioassay indicated that compound 5i stood out with a broad spectrum of in vitro activity against five fungi. Its EC₅₀ value (0.73 μg/mL) was comparable to that of boscalid (EC₅₀ of 0.51 μg/mL) and fluxapyroxad (EC₅₀ of 0.19 μg/mL) against Sclerotinia sclerotiorum. For Rhizoctonia cerealis, 5i and 5p with EC₅₀ values of 4.61 and 6.48 μg/mL, respectively, showed significantly higher activity than fluxapyroxad with the EC₅₀ value of 16.99 μg/mL. In vivo fungicidal activity of 5i exhibited an excellent inhibitory rate (100%) against Puccinia sorghi at 50 μg/mL, while the positive control boscalid showed only a 70% inhibitory rate. Moreover, 5i showed promising fungicidal activity with a 60% inhibitory rate against Rhizoctonia solani at 1 μg/mL, which was better than that of boscalid (30%). Compound 5i possessed better in vivo efficacy against P. sorghi and R. solani than boscalid. Molecular docking showed that even the carbonyl oxygen atom of 5i was far from the pyrazole ring. It could also form hydrogen bonds toward the hydroxyl hydrogen and amino hydrogen of TYR58 and TRP173 on SDH, respectively, which consisted of the positive control fluxapyroxad. Fluorescence quenching analysis and SDH enzymatic inhibition studies also validated its mode of action. Our studies showed that 5i was worthy of further investigation as a promising fungicide candidate.

Late-Stage C-H Functionalization of Nicotinamides for the Expedient Discovery of Novel Antifungal Leads

Encouraged by the successful flexible modifications of the succinate dehydrogenase inhibitors, antifungal activity guided by the divergent synthesis of nicotinamides of the prevalidated pharmacophore 2-(2-oxazolinyl)aniline was conducted. The work highlighted the first utilization of the late-stage C-H functionalization assisted by the innate pharmacophore for the discovery of promising agrochemicals. New synthetic methodology and antifungal exploration of alkoxylated nicotinamides were accomplished. Fifty-five functionalized nicotinamides of 7 types were rationally designed and efficiently prepared through C-H functionalization, which facilitated the acquirement of four N-para aryloxylated nicotinamides (E3, E13, E19, and E22) as potential antifungal candidates against Botrytis cinerea, with the EC₅₀ values lower than 5 mg/L. In vivo/vitro biotest, molecular docking, and structural analysis reconfirmed the novelty and practical potential of the antifungal candidates E3 and E19. This operationally simple platform will provide various "polar parts" and offer intriguing opportunities for the optimization of the carboxamide fungicides and structure-related pharmaceuticals.

The potential of respiration inhibition as a new approach to combat human fungal pathogens

The respiratory chain has been proposed as an attractive target for the development of new therapies to tackle human fungal pathogens. This arises from the presence of fungal-specific electron transport chain components and links between respiration and the control of virulence traits in several pathogenic species. However, as the physiological roles of mitochondria remain largely undetermined with respect to pathogenesis, its value as a potential new drug target remains to be determined. The use of respiration inhibitors as fungicides is well developed but has been hampered by the emergence of rapid resistance to current inhibitors. In addition, recent data suggest that adaptation of the human fungal pathogen, Candida albicans, to respiration inhibitors can enhance virulence traits such as yeast-to-hypha transition and cell wall organisation. We conclude that although respiration holds promise as a target for the development of new therapies to treat human fungal infections, we require a more detailed understanding of the role that mitochondria play in stress adaption and virulence.

Design, Synthesis, and Antifungal Activities of Novel Aromatic Carboxamides Containing a Diphenylamine Scaffold

A series of novel N-(2-(phenylamino)-4-fluorophenyl)-pyrazole-4-carboxamides 1-15 and aromatic carboxamides with a diphenylamine scaffold 16-29 were designed, synthesized, and evaluated for their antifungal activities. In vitro experiments showed that compound 6 (EC₅₀ = 0.03 mg/L) was superior to bixafen (EC₅₀ = 0.04 mg/L) against Rhizoctoinia solani and compound 6 (IC₅₀ = 1.41 mg/L) was close to bixafen (IC₅₀ = 1.22 mg/L) against succinate dehydrogenase from R. solani. Additionally, in vivo pot experiments showed that compound 6 (EC₅₀ = 1.93 mg/L) was better than bixafen (EC₅₀ = 3.72 mg/L) and close to thifluzamide (EC₅₀ = 1.83 mg/L) against R. solani. In vivo field trials showed that compound 6 at 200 g ai ha^-1 had 64.10% control efficacy against rice sheath blight after 21 days with two sprayings, close to thifluzamide (71.40%). Furthermore, molecular docking showed that compound 6 anchors in the binding site of SDH.