Synthesis of pyrazole-4-carboxamides as potential fungicide candidates

Design, synthesis, bioactivity and action mechanism of N-substituted N'-phenylpicolinohydrazides against phytopathogenic fungi

N'-phenylpicolinohydrazide has been proven to be a promising lead compound for research and development of novel fungicides for agriculture in our previous study. As our continuing research, in this study, a series of N-substituted derivatives of N'-phenylpicolinohydrazide were synthesized and explored for the inhibition activity on nine phytopathogenic fungi and action mechanism. The results found that eleven of the compounds had excellent antifungal activity with more than 80% inhibition rates at 50 µg/mL on part or most of the fungi, especially A. solani and P. piricola. Compounds 5i, 5j and 5k showed EC50 values of < 8.0 µg/mL against A. solani superior to positive control carbendazim (EC50 = 36.0 µg/mL) while 5p and 5q exhibited the highest activity with EC50 values of 2.72 and 2.80 µg/mL against P. piricola superior to positive control boscalid (EC50 > 50.0 µg/mL). Furthermore, 5k also showed significant protective effect against A. solani infection on tomatoes in a concentration-dependent manner. Action mechanism research showed that 5k was able to increase the intracellular ROS level, change both MMP and permeability of cell membrane and damage mycelial morphology. Molecular docking studies showed that 5k could bind into ubiquinone-binding region of succinate dehydrogenase by hydrogen bonds, π-cation, π-π stacked, π-alkyl, and alkyl interactions. Additionally, the antibacterial activity was also investigated. Thus, N-substituted derivatives of N'-phenylpicolinohydrazide were emerged as novel and highly promising antifungal molecular skeletons to develop new fungicides for crop protection.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Applications of Bioisosteres in the Design of Biologically Active Compounds

The design of bioisosteres represents a creative and productive approach to improve a molecule, including by enhancing potency, addressing pharmacokinetic challenges, reducing off-target liabilities, and productively modulating physicochemical properties. Bioisosterism is a principle exploited in the design of bioactive compounds of interest to both medicinal and agricultural chemists, and in this review, we provide a synopsis of applications where this kind of molecular editing has proved to be advantageous in molecule optimization. The examples selected for discussion focus on bioisosteres of carboxylic acids, applications of fluorine and fluorinated motifs in compound design, some applications of the sulfoximine functionality, the design of bioisosteres of drug-H2O complexes, and the design of bioisosteres of the phenyl ring.

On the Question of the Formation of Nitro-Functionalized 2,4-Pyrazole Analogs on the Basis of Nitrylimine Molecular Systems and 3,3,3-Trichloro-1-Nitroprop-1-Ene

Experimental and theoretical studies on the reaction between (E)-3,3,3-trichloro-1-nitroprop-1-ene and N-(4-bromophenyl)-C-arylnitrylimine were performed. It was found that the title process unexpectedly led to 1-(4-bromophenyl)-3-phenyl-5-nitropyrazole instead of the expected Δ²-pyrazoline molecular system. This was the result of a unique CHCl₃ elimination process. The observed mechanism of transformation was explained in the framework of the molecular electron density theory (MEDT). The theoretical results showed that both of the possible channels of [3 + 2] cycloaddition were favorable from a kinetic point of view, due to which the creation of 1-(4-bromophenyl)-3-aryl-4-tricholomethyl-5-nitro-Δ²-pyrazoline was more probable. On the other hand, according to the experimental data, the presented reactions occurred with full regioselectivity.

Design, Synthesis, and Evaluation of Novel 4-Chloropyrazole-Based Pyridines as Potent Fungicide Candidates

A rational molecular design approach was developed in our laboratory to guide the discovery of novel sterol biosynthesis inhibitors. Based on the application of bioactivities of heterocyclic rings and molecular docking targeting the sterol biosynthesis 14α-demethylase, a series of 4-chloropyrazole-based pyridine derivatives were rationally designed, synthesized, and characterized and their fungicidal activities were also evaluated. Bioassay results showed that 7e, 7f, and 7m exhibited commendable, diverse antifungal actions that are comparable to those of the positive controls imazalil and triadimefon. The active compounds' mode of action was further studied by microscopy observations, Q-PCR, and enzyme inhibition assay and discovered that target compounds affect fungal sterol biosynthesis via disturbing RcCYP51 enzyme system. These findings support that their fungicidal mode of action still targets the cytochrome P450-dependent 14α-demethylase as the molecular design did at first. The above results strongly suggest that our rational molecular design protocol is not only practical but also efficient.

A Review of the Recent Development in the Synthesis and Biological Evaluations of Pyrazole Derivatives

Pyrazoles are five-membered heterocyclic compounds that contain nitrogen. They are an important class of compounds for drug development; thus, they have attracted much attention. In the meantime, pyrazole derivatives have been synthesized as target structures and have demonstrated numerous biological activities such as antituberculosis, antimicrobial, antifungal, and anti-inflammatory. This review summarizes the results of published research on pyrazole derivatives synthesis and biological activities. The published research works on pyrazole derivatives synthesis and biological activities between January 2018 and December 2021 were retrieved from the Scopus database and reviewed accordingly.

Discovery of Novel Pyrazole Amides as Potent Fungicide Candidates and Evaluation of Their Mode of Action

A rational molecule design strategy based on scaffold hopping was applied to discover novel leads, and then a series of novel pyrazole amide derivatives were designed, synthesized, characterized, and evaluated for their antifungal activities. Bioassay results indicated that some target compounds such as S3, S12, and S26 showed good in vivo antifungal activities; among them, S26 exhibited commendable in vivo protective activity with an 89% inhibition rate against Botrytis cinerea on cucumber at 100 μg/mL that is comparable to positive controls boscalid, isopyrazam, and fluxapyroxad. Microscopy observations suggested that S26 affects the normal fungal growth. Fluorescence quenching analysis and SDH (succinate dehydrogenase) enzymatic inhibition studies validated that S26 may not be an SDH inhibitor. Based on induction of plant defense responses testing, S26 enhanced the accumulation of RBOH, WRKY6, WRKY30, PR1, and PAL defense-related genes expression and the defense-associated enzyme phenylalanine ammonia lyase (PAL) expression on cucumber. These findings support that S26 not only displayed direct fungicidal activity but also exhibited plant innate immunity stimulation activity, and it could be used as a promising plant defense-related fungicide candidate.

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.

Design, Synthesis, and Bioactivity Evaluation of New Thiochromanone Derivatives Containing a Carboxamide Moiety

In this study, using the botanical active component thiochromanone as the lead compound, a total of 32 new thiochromanone derivatives containing a carboxamide moiety were designed and synthesized and their in vitro antibacterial activities against Xanthomonas oryzae pv. oryzae (Xoo), Xanthomonas oryzae pv. oryzicolaby (Xoc), and Xanthomonas axonopodis pv. citri (Xac) were determined, as well as their in vitro antifungal activities against Botryosphaeria dothidea (B. dothidea), Phomopsis sp., and Botrytis cinerea (B. cinerea). Bioassay results demonstrated that some of the target compounds exhibited moderate to good in vitro antibacterial and antifungal activities. In particular, compound 4e revealed excellent in vitro antibacterial activity against Xoo, Xoc, and Xac, and its EC50 values of 15, 19, and 23 μg/mL, respectively, were superior to those of Bismerthiazol and Thiodiazole copper. Meanwhile, compound 3b revealed moderate in vitro antifungal activity against B. dothidea at 50 μg/mL, and the inhibition rate reached 88%, which was even better than that of Pyrimethanil, however, lower than that of Carbendazim. To the best of our knowledge, this is the first report on the antibacterial and antifungal activities of this series of novel thiochromanone derivatives containing a carboxamide moiety.

The antimicrobial potential and pharmacokinetic profiles of novel quinoline-based scaffolds: synthesis and in silico mechanistic studies as dual DNA gyrase and DHFR inhibitors

The resistance of pathogenic microbes to currently available antimicrobial agents has been considered a global alarming concern.