Antimicrobial activities of pyridinium-tailored pyrazoles bearing 1,3,4-oxadiazole scaffolds

Design, Synthesis and Bioactivity Evaluation of Novel 2-(pyrazol-4-yl)-1,3,4-oxadiazoles Containing an Imidazole Fragment as Antibacterial Agents

Imidazole alkaloids, a common class of five-membered aromatic heterocyclic compounds, exist widely in plants, animals and marine organisms. Because of imidazole's extensive and excellent biological and pharmacological activities, it has always been a topic of major interest for researchers and has been widely used as an active moiety in search of bioactive molecules. To find more efficient antibacterial compounds, a series of novel imidazole-fragment-decorated 2-(pyrazol-4-yl)-1,3,4-oxadiazoles were designed and synthesized based on our previous works via the active substructure splicing principle, and their bioactivities were systematically evaluated both in vitro and in vivo. The bioassays showed that some of the target compounds displayed excellent in vitro antibacterial activity toward three virulent phytopathogenic bacteria, including Xanthomonas oryzae pv. oryzae (Xoo), Xanthomonas axonopodis pv. citri (Xac) and Pseudomonas syringae pv. actinidiae (Psa), affording the lowest EC50 values of 7.40 (7c), 5.44 (9a) and 12.85 (9a) μg/mL, respectively. Meanwhile, compound 7c possessed good in vivo protective and curative activities to manage rice bacterial leaf blight at 200 μg/mL, with control efficacies of 47.34% and 41.18%, respectively. Furthermore, compound 9a showed commendable in vivo protective and curative activities to manage kiwifruit bacterial canker at 200 μg/mL, with control efficacies of 46.05% and 32.89%, respectively, which were much better than those of the commercial bactericide TC (31.58% and 17.11%, respectively). In addition, the antibacterial mechanism suggested that these new types of title compounds could negatively impact the cell membranes of phytopathogenic bacteria cells and cause the leakage of the intracellular component, thereby leading to the killing of bacteria. All these findings confirm that novel 2-(pyrazol-4-yl)-1,3,4-oxadiazoles containing an imidazole fragment are promising lead compounds for discovering new bactericidal agents.

Potential Synthetic Routes and Metal-Ion Sensing Applications of 1,3,4-Oxadiazoles: An Integrative Review

Oxadiazoles, especially 1,3,4-oxadiazole scaffolds, stand among the foremost heterocyclic fragments with a broad spectrum of applications in diverse fields, including pharmacology, polymers, material science, and organic electronics, among others. In this comprehensive review, we summarize the pivotal synthetic strategies for 1,3,4-oxadiazole derivatives including dehydrogenative cyclization of 1,2-diacylhydrazines, oxidative cyclization of acylhydrazones, condensation cyclization, C-H activation of oxadiazole ring, decarboxylative cyclization and oxidative annulation along with plausible mechanisms. The set of 1,3,4-oxadiazoles selected from the literature and discussed herein epitomize the ease of synthesis as well as the possibility of linking π-conjugated groups; thereby encouraging the use of these molecules as important starting building blocks for a wide variety of fluorescent frameworks, particularly in the development of potential chemosensors. High photoluminescent quantum yield, excellent thermal and chemical stability, and the presence of potential coordination (N and O donor atoms) sites make these molecules a prominent choice for metal-ions sensors. An overview of selective metal-ion sensing, the detection limit along with the sensing mechanisms (photo-induced electron transfer, excited-state intramolecular proton transfer, and complex formation) is also included.

The antimicrobial and immunomodulatory effects of Ionophores for the treatment of human infection

Ionophores are a diverse class of synthetic and naturally occurring ion transporter compounds which demonstrate both direct and in-direct antimicrobial properties against a broad panel of bacterial, fungal, viral and parasitic pathogens. In addition, ionophores can regulate the host-immune response during communicable and non-communicable disease states. Although the clinical use of ionophores such as Amphotericin B, Bedaquiline and Ivermectin highlight the utility of ionophores in modern medicine, for many other ionophore compounds issues surrounding toxicity, bioavailability or lack of in vivo efficacy studies have hindered clinical development. The antimicrobial and immunomodulating properties of a range of compounds with characteristics of ionophores remain largely unexplored. As such, ionophores remain a latent therapeutic avenue to address both the global burden of antimicrobial resistance, and the unmet clinical need for new antimicrobial therapies. This review will provide an overview of the broad-spectrum antimicrobial and immunomodulatory properties of ionophores, and their potential uses in clinical medicine for combatting infection.

Design, Synthesis, and Study of the Insecticidal Activity of Novel Steroidal 1,3,4-Oxadiazoles

A series of novel steroidal derivatives with a substituted 1,3,4-oxadiazole structure was designed and synthesized, and the target compounds were evaluated for their insecticidal activity against five aphid species. Most of the tested compounds exhibited potent insecticidal activity against Eriosoma lanigerum (Hausmann), Myzus persicae, and Aphis citricola. Compounds 20g and 24g displayed the highest activity against E. lanigerum, showing LC₅₀ values of 27.6 and 30.4 μg/mL, respectively. Ultrastructural changes in the midgut cells of E. lanigerum were detected by transmission electron microscopy, indicating that these steroidal oxazole derivatives might exert their insecticidal activity by destroying the mitochondria and nuclear membranes in insect midgut cells. Furthermore, a field trial showed that compound 20g exhibited effects similar to those of the positive controls chlorpyrifos and thiamethoxam against E. lanigerum, reaching a control rate of 89.5% at a dose of 200 μg/mL after 21 days. We also investigated the hydrolysis and metabolism of the target compounds in E. lanigerum by assaying the activities of three insecticide-detoxifying enzymes. Compound 20g at 50 μg/mL exhibited inhibitory action on carboxylesterase similar to the known inhibitor triphenyl phosphate. The above results demonstrate the potential of these steroidal oxazole derivatives to be developed as novel pesticides.

Novel Ferulic Amide Ac6c Derivatives: Design, Synthesis, and Their Antipest Activity

Thirty-eight novel ferulic amide 1-aminocyclohexane carboxylic acid (Ac6c) derivatives D1-D19 and E1-E19 were designed and synthesized, and their antibacterial, antifungal, and insecticidal activities were tested. Most of the synthesized compounds displayed excellent activity againstXanthomonas oryzae pv. oryzae (Xoo), with EC₅₀ values ranging from 11.6 to 83.1 μg/mL better than that of commercial bismerthiazol (BMT, EC₅₀ = 84.3 μg/mL), as well as much better performance compared to that of thiediazole copper (TDC, EC₅₀ = 137.8 μg/mL). D6 (EC₅₀ = 17.3 μg/mL), D19 (EC₅₀ = 29.4 μg/mL), E3 (EC₅₀ = 29.7 μg/mL), E9 (EC₅₀ = 27.0 μg/mL), E10 (EC₅₀ = 18.6 μg/mL), and E18 (EC₅₀ = 20.8 μg/mL) showed much higher activity on Xanthomonas oryzae pv. oryzicola compared with BMT (EC₅₀ = 80.1 μg/mL) and TDC (EC₅₀ = 124.7 μg/mL). In relation to controlling the fungus, Rhizoctonia solani, E1, E10, and E13 had much lower EC₅₀ values of 0.005, 0.140, and 0.159 μg/mL compared to hymexazol at 74.8 μg/mL. Further in vivo experiments demonstrated that E6 and E12 controlled rice bacterial leaf blight disease better than BMT and TDC did. Scanning electron microscopy (SEM) studies revealed that E12 induced the Xoo cell membrane collapse. Moreover, D13 (73.7%), E5 (80.6%), and E10 (73.4%) also showed moderate activity against Plutella xylostella. These results indicated that the synthesized ferulic amide Ac6c derivatives showed promise as candidates for treating crop diseases.

Novel 1,3,4-Oxadiazole-2-carbohydrazides as Prospective Agricultural Antifungal Agents Potentially Targeting Succinate Dehydrogenase

A novel simple 1,3,4-oxadiazole-2-carbohydrazide was reported to discover low-cost and versatile antifungal agents. Bioassay results suggested that a majority of the designed compounds were extremely bioactive against four types of fungi and two kinds of oomycetes. This extreme bioactivity was highlighted by the applausive inhibitory effects of compounds 4b, 4h, 5c, 5g, 5h, 5i, 5m, 5p, 5t, and 5v against Gibberella zeae, affording EC₅₀ values ranging from 0.486 to 0.799 μg/mL, which were superior to that of fluopyram (2.96 μg/mL) and comparable to those of carbendazim (0.947 μg/mL) and prochloraz (0.570 μg/mL). Meanwhile, compounds 4g, 5f, 5i, and 5t showed significant actions against Fusarium oxysporum with EC₅₀ values of 0.652, 0.706, 0.813, and 0.925 μg/mL, respectively. Pharmacophore exploration suggested that the N'-phenyl-1,3,4-oxadiazole-2-carbohydrazide pattern is necessary for the bioactivity. Molecular docking of 5h with succinate dehydrogenase (SDH) indicated that it can completely locate the inside of the binding pocket via hydrogen-bonding and hydrophobic interactions, revealing that this novel framework might target SDH. This result was further verified by the significant inhibitory effect on SDH activity. In addition, scanning electron microscopy patterns were performed to elucidate the anti-G. zeae mechanism. Given these features, this type of framework is a suitable template for future exploration of alternative SDH inhibitors against plant microbial infections.

Rational Optimization and Action Mechanism of Novel Imidazole (or Imidazolium)-Labeled 1,3,4-Oxadiazole Thioethers as Promising Antibacterial Agents against Plant Bacterial Diseases

The emergence and widespread occurrence of plant bacterial diseases that cause global production constraints have become major challenges to agriculture worldwide. To promote the discovery and development of new bactericides, imidazole-labeled 1,3,4-oxadiazole thioethers were first fabricated by integrating the crucially bioactive scaffolds of the imidazole motif and 1,3,4-oxadiazole skeleton in a single molecular architecture. Subsequently, a superior antibacterial compound A₆ was gradually discovered possessing excellent competence against plant pathogens Xanthomonas oryzae pv oryzae and Xanthomonas axonopodis pv citri with EC₅₀ values of 0.734 and 1.79 μg/mL, respectively. These values were better than those of commercial agents bismerthiazol (92.6 μg/mL) and thiodiazole copper (77.0 μg/mL). Further modifying the imidazole moiety into the imidazolium scaffold led to the discovery of an array of potent antibacterial compounds providing the corresponding minimum EC₅₀ values of 0.295 and 0.607 μg/mL against the two strains. Moreover, a plausible action mechanism for attacking pathogens was proposed based on the concentration dependence of scanning electron microscopy, transmission electron microscopy, and fluorescence microscopy images. Given the simple molecular structures, easy synthetic procedure, and highly efficient bioactivity, imidazole (or imidazolium)-labeled 1,3,4-oxadiazole thioethers can be further explored and developed as promising indicators for the development of commercial drugs.

Synthesis and antifungal activities of novel thiophene-based stilbene derivatives bearing an 1,3,4-oxadiazole unit

BACKGROUND

Natural stilbenes (especially resveratrol and its derivatives) are well-known phytoalexins that are active against many plant diseases. However, oxidative degradation and low bioavailability limit their exogenous application as fungicides on crops. In this study, a new class of resveratrol-inspired thiophene-based stilbene derivatives bearing an 1,3,4-oxadiazole unit was synthesized and the derivatives' antifungal activities against phytopathogenic fungi were investigated.

RESULTS

The results revealed that compounds 5h and 5j exhibited improved antifungal activity against Botrytis cinerea with median effective concentrations (EC₅₀ ) of 168.5 and 155.4 µg mL^-1 , respectively, which were superior to the EC₅₀ of resveratrol (263.1 µg mL^-1 ). Compound 5j was shown to effectively control disease development in B. cinerea-infected tomatoes in vivo. Notably, considerably abnormal mycelial morphology and increased cell membrane conductivity were observed in the presence of compound 5j.

CONCLUSION

A new class of thiophene-containing stilbene derivatives was designed and synthesized. Bioassay results showed that compound 5j exhibited promising antifungal activity, suggesting practical potential for fungal disease control. © 2018 Society of Chemical Industry.

Synthesis and three-dimensional quantitative structure-activity relationship study of quinazoline derivatives containing a 1,3,4-oxadiazole moiety as efficient inhibitors against Xanthomonas axonopodis pv. citri

A series of quinazoline derivatives containing a 1,3,4-oxadiazole moiety were synthesized and evaluated for their antibacterial activities against Xanthomonas axonopodis pv. citri (Xac) and Ralstonia solanacearum (Rs). Antibacterial bioassays indicated that most of target compounds exhibited significant antibacterial activities against Xac and Rs in vitro. Strikingly, compounds 6d-6i, 6m-6r and 6u-6x showed antibacterial activity against Xac, with [Formula: see text] values ranging from 14.42 to 38.91 [Formula: see text]g/mL, which are better than that of bismerthiazol (39.86 [Formula: see text]g/mL). Based on the antibacterial activity against Xac, comparative molecular filed analysis and comparative molecular similarity index analysis models were generated to investigate the structure-activity relationship of title compounds against Xac. The analytical results indicated that the above models exhibited good predictive accuracy and could be used as practical tools for guiding the design and synthesis of more potent quinazoline derivatives containing a 1,3,4-oxadiazole moiety.

Synthesis and Pharmacological Activities of Pyrazole Derivatives: A Review

Pyrazole and its derivatives are considered a pharmacologically important active scaffold that possesses almost all types of pharmacological activities. The presence of this nucleus in pharmacological agents of diverse therapeutic categories such as celecoxib, a potent anti-inflammatory, the antipsychotic CDPPB, the anti-obesity drug rimonabant, difenamizole, an analgesic, betazole, a H2-receptor agonist and the antidepressant agent fezolamide have proved the pharmacological potential of the pyrazole moiety. Owing to this diversity in the biological field, this nucleus has attracted the attention of many researchers to study its skeleton chemically and biologically. This review highlights the different synthesis methods and the pharmacological properties of pyrazole derivatives. Studies on the synthesis and biological activity of pyrazole derivatives developed by many scientists around the globe are reported.

Synthesis and Pharmacological Activities of Pyrazole Derivatives: A Review

Pyrazole and its derivatives are considered a pharmacologically important active scaffold that possesses almost all types of pharmacological activities. The presence of this nucleus in pharmacological agents of diverse therapeutic categories such as celecoxib, a potent anti-inflammatory, the antipsychotic CDPPB, the anti-obesity drug rimonabant, difenamizole, an analgesic, betazole, a H2-receptor agonist and the antidepressant agent fezolamide have proved the pharmacological potential of the pyrazole moiety. Owing to this diversity in the biological field, this nucleus has attracted the attention of many researchers to study its skeleton chemically and biologically. This review highlights the different synthesis methods and the pharmacological properties of pyrazole derivatives. Studies on the synthesis and biological activity of pyrazole derivatives developed by many scientists around the globe are reported.