Synthesis, bioactivity and structure-activity relationships of new 2-aryl-8-OR-3,4-dihydroisoquinolin-2-iums salts as potential antifungal agents

Design, bioactivity and mechanism of N'-phenyl pyridylcarbohydrazides with broad-spectrum antifungal activity

Succinate dehydrogenase inhibitors (SDHIs) as one of the fastest-growing fungicide categories for plant protection. In this study, a series of N'-phenyl pyridylcarbohydrazides as analogues of commercial SDHIs were designed and evaluated for inhibition activity on phytopathogenic fungi to search for potential novel SDHIs. The determination of antifungal activity in vitro and in vivo led to the discovery of a series of compounds with high activity and broad-spectrum property. Especially, N'-(4-fluorophenyl)picolinohydrazide (1c) and N'-(3,4-fluorophenyl)picolinohydrazide (1ae) showed 0.041-1.851 μg/mL of EC50 values on twelve fungi, superior to positive controls carbendazim and boscalid. In vivo activity, 1c at 50 μg/mL showed 61% of control efficacy at the post-treatment 9th day for the infection of P. piricola on apples, slightly smaller than 70% of carbendazim. In terms of action mechanism, 1c showed strong inhibition activity with IC50 of 0.107 μg/mL on SDH in Alternaria brassicae, superior to positive SDHI boscalid (IC50 0.182 μg/mL). Molecular docking indicated that 1c can well bind with the ubiquinone-binding region of SDH mainly by hydrogen bond, carbon hydrogen bond, π-alkyl, amide-π stacking, F-N and F-H interactions. Furthermore, scanning and transmission electron micrographs showed that 1c was able to obviously change the structure of mycelia and cell membrane. Fluorescence staining analysis showed that 1c could increase both the intracellular reactive oxygen species level and mitochondrial membrane potential. Finally, seed germination test, seedling growth test and cytotoxicity assay showed that 1c had very low toxicity to plant growth and mammalian cells. Thus, N'-phenyl pyridylcarbohydrazides especially 1c and 1ae can be considered promising fungicide alternatives for plant protection.

Quinoline Derivatives in Discovery and Development of Pesticides

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

Novel Flavonol Derivatives Containing 1,3,4-Thiadiazole as Potential Antifungal Agents: Design, Synthesis, and Biological Evaluation

In order to discover novel compounds with excellent agricultural activities, novel flavonol derivatives containing 1,3,4-thiadiazole were synthesized and evaluated for their antifungal activities. The bioassay results showed that some of the target compounds had good antifungal activities against Botrytis cinerea, Phomopsis sp. and Sclerotinia sclerotiorum in vitro. It is worth noting that the half-effective concentration (EC50) value of Y18 against B. cinerea was 2.4 μg/mL, which was obviously superior to that of azoxystrobin (21.7 μg/mL). The curative activity of Y18 at 200 μg/mL (79.9%) was better than that of azoxystrobin (59.1%), and its protective activity (90.9%) was better than that of azoxystrobin (83.9%). Morphological studies by using scanning electron microscopy and fluorescence microscopy revealed that Y18 could affect the normal growth of B. cinerea mycelium. In addition, the mechanism of action studies indicated that Y18 could affect the integrity of cell membranes by inducing the production of endogenous reactive oxygen species and the release of the malondialdehyde content, leading to membrane lipid peroxidation and the release of cell contents. The inhibitory activity of flavonol derivatives containing 1,3,4-thiadiazole on plant fungi is notable, offering significant potential for the development of new antifungal agents.

CBr4 as a Mild Oxidant-Enabled Oxidation of a sp3 C-H Bond: A Facile Synthesis of the Persistent Iminium Salts of Tetrahydroisoquinolines

Using CBr4 as a mild oxidant, the direct C-H bond oxidation of N-aryltetrahydroisoquinolines was achieved, giving a series of the corresponding iminium salts in high yields under metal- and photo-free reaction conditions. This reaction is superior in high yields and good functional group tolerance, and the late-stage derivatization showed that these iminium salts can readily be applied to the synthesis of the functionalized THIQs.

New 7-Chloro-9-methyl-2-phenyl-3,4-dihydro-β-carbolin-2-iums as Promising Fungicide Candidates: Design, Synthesis, and Bioactivity

As our further research, a series of new 7-chloro-9-methyl-2-phenyl-3,4-dihydro-β-carbolin-2-iums were designed and synthesized. Twelve compounds were found with excellent inhibition activity in vitro on three to five out of six phytopathogenic fungi, superior to standard drugs thiabendazole and/or azoxystrobin. Especially, 18 displayed the highest activity against three out of the fungi and the highest comprehensive activity for all of the fungi. The test in vivo revealed that 18 at 50 μg/mL was able to completely control Physalospora piricola infections in apples over 8 days. Scanning/transmission electron microscopic observations found that 18 could damage the hyphal integrity and cell membrane structure of P. piricola. The safety evaluation showed that 18 had no effect on the germination rate of cowpea seed at ≤200 μg/mL. The SAR revealed that the combination of 7-Cl and 2'- or 4'-alkyl is conducive to improvement of the activity. Thus, 7-chloro-9-methyl-2-phenyl-3,4-dihydro-β-carbolin-2-ium is a promising antifungal lead scaffold.

Simple Analogues of Quaternary Benzo[c]phenanthridine Alkaloids: Discovery of a Novel Antifungal 2-Phenylphthalazin-2-ium Scaffold with Excellent Potency against Phytopathogenic Fungi

Inspired by sanguinarine and chelerythrine, a novel antifungal 2-phenylphthalazin-2-ium scaffold as a simple analogue was designed. Most of the 30 compounds showed excellent inhibition activity against almost all eight phytopathogenic fungi, far superior to sanguinarine and chelerythrine. A third of the compounds were more active than azoxystrobin in most cases. Compounds 26 and 27 showed the highest total activity against all the fungi with EC₅₀ means of ca. 4.6 μg/mL. Fusarium solani showed the highest susceptibility with an EC₅₀ mean of 3.62 μg/mL to 19 compounds. A concentration of 25.0 μg/mL 27 can fully control the Colletotrichum gloeosporioides infection in apples over 9 days. Electron microscopic observations showed that 27 was able to damage the structures of the hypha and cell membrane. The structure-activity relationship showed that the presence of electron-withdrawing groups on the C-ring increases the activity against most of the fungi. Thus, 2-phenylphthalazin-2-ium compounds represent promising leads for the development of novel fungicides.

Simple analogues of natural product chelerythrine: Discovery of a novel anticholinesterase 2-phenylisoquinolin-2-ium scaffold with excellent potency against acetylcholinesterase

As simple analogues of the natural compound chelerythrine, a novel anti-cholinesterase 2-phenylisoquinolin-2-ium scaffold was designed by structure imitation. The activity evaluation led to the discovery of seven compounds with potent anti-acetylcholinesterase activity with IC₅₀ values of ≤0.72 μM, superior to chelerythrine and standard drugs galantamine. Particularly, compound 8y showed the excellent dual acetylcholinesterase-butyrylcholinesterase inhibition activity, superior to rivastigmine, a dual cholinesterase inhibitor drug. Furthermore, the compounds displayed a competitive anti-acetylcholinesterase mechanism with the substrate and low cytotoxicity. Molecular docking showed that the isoquinoline moiety is embedded in a cavity surrounded by four aromatic residues of acetylcholinesterase by the π-π action. Structure-activity relationship showed that the p-substituents on the C-ring can dramatically improve the anti-acetylcholinesterase activity, while 8-OMe can increase the activity against the two cholinesterases simultaneously. Thus, the title compounds emerged as promising lead compounds for the development of novel cholinesterase inhibitor agents.

New 2-aryl-6-methyl-3,4-dihydro-β-carbolin-2-iums as potential antifungal agents: Synthesis, bioactivity and structure-activity relationship

Thirty new title compounds along with five known analogues were prepared from commercially available 2-arylhydrazin-1-ium chlorides and α-ketoglutaric acid. The mycelium growth rate method was used to evaluate inhibition activity against six strains of plant pathogenic fungi. Most of the compounds displayed the activity for each the fungi at 150 μΜ, higher than azoxystrobin, a positive drug. Compound 6-2 showed the lowest average IC₅₀ value of 4.58 μg/mL for all the fungi where F. solani exhibited the highest susceptibility to most of the compounds. For F. solani, some compounds were more active with IC₅₀ values of 2.67-8.48 μM than thiabendazole (IC₅₀ = 9.30 μM) and/or carbendazim (IC₅₀ = 3.36 μM). The SAR showed that the activity is significantly affected by substituents on the A-ring and/or D-ring along with the degree of unsaturation of the C-ring. Thus, a series of new β-carboline compounds with potent antifungal potential were found.

New 2-aryl-7,8-dimethoxy-3,4-dihydroisoquinolin-2-ium salts as potential antifungal agents: synthesis, bioactivity and structure-activity relationships

The title compounds can be considered as simple analogues of quaternary benzo[c]phenanthridine alkaloids (QBAs). In order to develop potent QBA-like antifungal agents, as our continuing study, a series of new title compounds were synthesized and evaluated for bioactivity against five plant pathogenic fungi by the mycelium growth rate method in this study. The SAR were also derived. The majority of the compounds showed good to excellent inhibition activity with average EC₅₀ values of 7.87–20.0 μM for the fungi, superior to sanguinarine and cherythrine (two QBAs) and the commercial fungicide azoxystrobin. Part of the compounds were more active than commercial fungicides thiabendazole or carbendazim against F. solani, F. graminearum and C. gloeosporioides. Six compounds with average EC₅₀ of 3.5–5.1 μg/mL possessed very great potential for development of new antifungal agents. SAR found that substitution patterns of the two aryl-rings significantly affect the activity. There exists a complex interaction effect between substituents of the two aryl-rings on the activity. Generally, the presence of electron-withdrawing groups on the C-ring can significantly increase the activity. These findings will be of great importance for the design of more potent antifungal isoquinoline agents.

Cytotoxic activity, apoptosis induction and structure-activity relationship of 8-OR-2-aryl-3,4-dihydroisoquinolin-2-ium salts as promising anticancer agents

As our continuing research, a series of 2-aryl-8-OR-3,4-dihydroisoquinolin-2-ium bromides were evaluated for cytotoxic activity on cancer cells and apoptosis induction in the present study. SAR was derived also. Among them, 23 compounds showed the higher cytotoxicity on MKN-45 cells with IC₅₀ values of 1.99-11.3μM than a standard anticancer drug cis-platinum (IC₅₀=11.4μM) or their natural model compound chelerythrine (IC₅₀=12.7μM); 16 compounds possessed the medium to high activity on NB4 cells with IC₅₀ values of 1.67-4.62μM. SAR analysis showed that both substitution patterns of the N-aromatic ring and the type of 8-OR significantly impact the activity. AO/EB staining and flow cytometry analysis with Annexin V/PI double staining showed that the compounds were able to induce apoptosis in a concentration-dependent manner. The results above suggested that the title compounds are a class of promising compounds for the development of new anti-cancer drugs.

Synthesis and Biological Evaluation of Benzimidazole Phenylhydrazone Derivatives as Antifungal Agents against Phytopathogenic Fungi

A series of benzimidazole phenylhydrazone derivatives (6a–6ai) were synthesized and characterized by ¹H-NMR, ESI-MS, and elemental analysis. The structure of 6b was further confirmed by single crystal X-ray diffraction as (E)-configuration. All the compounds were screened for antifungal activity against Rhizoctonia solani and Magnaporthe oryzae employing a mycelium growth rate method. Compound 6f exhibited significant inhibitory activity against R. solani and M. oryzae with the EC₅₀ values of 1.20 and 1.85 μg/mL, respectively. In vivo testing demonstrated that 6f could effectively control the development of rice sheath blight (RSB) and rice blast (RB) caused by the above two phytopathogens. This work indicated that the compound 6f with a benzimidazole phenylhydrazone scaffold could be considered as a leading structure for the development of novel fungicides.