Optimization, Structure-Activity Relationship, and Mode of Action of Nortopsentin Analogues Containing Thiazole and Oxazole Moieties

Design, synthesis, and biological evaluation of novel derivatives of marine natural product ammosamide B

BACKGROUND

Plant diseases caused by plant viruses and pathogens seriously affect crop yield and quality, making prevention and control extremely difficult and posing a serious threat to human health. Marine alkaloids have novel structures and diverse biological activities, which are an important source of the development of new pesticides. In this work, ammosamide alkaloids were selected as the parent structure. A series of derivatives based on the chlorinated pyrrolo[4,3,2-de]quinoline were synthesized and evaluated for their antiviral and fungicidal activities.

RESULT

Ammosamide B was efficiently synthesized from commercially available 6-chloroisatin in seven steps with an overall yield of 18.5%. A series of pyrrolo[4,3,2-de]quinoline derivatives were synthesized and evaluated for their antiviral and fungicidal activities. Most of these compounds were demonstrated to have higher antiviral activities than ribavirin. Particularly, compounds 7, 8a, 11, and 13a exhibited higher inhibitory activities against tobacco mosaic virus (TMV) and potato virus Y (PVY) than ningnanmycin. Antiviral mechanism research revealed that compound 13a can inhibit the assembly of TMV particles by interfering with the formation of the 20S coat protein (CP) disk. We further validated the interaction between this type of compound and TMV CP through molecular docking. This type of compounds also exhibits broad-spectrum inhibitory effects against eight common plant fungi, especially Physalospora piricola.

CONCLUSION

An improved method for synthesis of ammosamide B and its derivatives from commercially available 6-chloroisatin was explored. Systematic structural modifications based on ammosamide alkaloids were carried out and the structure-activity relationship of anti-TMV activity was summarized. Compound 13a, exhibiting excellent antiviral activity, has emerged as a new candidate for antiviral agents. The current work provides a valuable reference for exploring the potential applications of ammosamide derivatives in plant protection. © 2025 Society of Chemical Industry.

Discovery of N-((2-Arylthiazol-4-yl)methyl)oxazole-5-carboxamide Derivatives as SDHi for Fungicidal Candidates

A series of oxazole-5-carboxamide derivatives were designed as succinate dehydrogenase (SDH) inhibitors and synthesized, characterized, and evaluated for their fungicidal activities. Among these compounds, SEZA18 and SEZC7 displayed 0.17 and 0.50 mg/L EC50 values against Magnaporthe grisea in vitro, respectively, whose anti-M. grisea activities were closed to prochloraz (0.15 mg/L) and surpassed hymexazol (45.5 mg/L). Moreover, SEZA18 and SEZC7 exhibited 45.3 and 49.5% protective effects against M. grisea at the dose of 200 mg/L in vivo, whose preventive effects were about double times less than that of azoxystrobin (93%). In addition, SEZA14 possessed an EC50 value of 2.33 mg/L against Penicillium digitatum in vitro and showed 77.9% prevention effects from P. digitatum at the dose of 100 mg/L in a potted experiment, whose fungicidal activity was comparable to that of boscalid (75.5%). SEZC7 demonstrated SDH inhibitory activity (IC50 = 16.6 μM), exhibiting activity levels similar to those of boscalid (IC50 = 12.9 μM). Molecular docking results further revealed that SEZA14, SEZA18, SEZC7, and boscalid possessed a mode of action similar to that of SDH. Transcriptome analysis suggested that C7 interfered with the energy metabolism by inhibiting SDH activity, thereby affecting cellular sugar metabolism processes of M. grisea. In summary, our finding gave SDH inhibitors featuring novel structures, which provided a potential candidate for the management of plant pathogenic fungi.

Synthesis of a Natural Product-Based 5H-Thiazolo[5',4':5,6]pyrido[2,3-b]indole Derivative via Solid-Phase Synthesis

The solid-phase synthesis method is optimized for building chemical libraries. Furthermore, chemical libraries are essential tools in drug discovery used to identify hit compounds. We constructed a 5H-thiazolo[5',4':5,6]pyrido[2,3-b]indole derivative library using solid-phase synthesis. The indole insertion reaction at the benzylic position using a Lewis acid and the oxidative cyclization reaction using iodine were used for synthesis. Using optimized solution-phase reaction conditions, a solid-phase synthesis method comprising a total of eight steps was employed to build a 5H-thiazolo[5',4':5,6]pyrido[2,3-b]indole derivative library. In addition, we found an efficient compound library synthesis route with each synthetic step having a yield of 62-82%.

Synthesis, Characterization, and Evaluation of the Antifungal Properties of 3-Indolyl-3-Hydroxy Oxindole Derivatives Against Plant Pathogenic Fungi

To discover novel fungicides with good inhibitory effects on plant fungal diseases, twenty-five 3-indolyl-3-hydroxy oxindole derivatives (3a-3y) were synthesized. These newly derivatives were characterized by NMR and HRMS. Their antifungal activities against five plant pathogenic fungi were assessed in vitro. Most of the compounds exhibited moderate to excellent antifungal activities against the five pathogenic fungi. Notably, compounds 3t, 3u, 3v, and 3w displayed remarkable and broad-spectrum antifungal activities comparable to or superior to those of the fungicides carvacrol (CA) and phenazine-1-carboxylic acid (PCA). Among them, compound 3u displayed the most excellent antifungal activity against Rhizoctonia solani Kühn (R. solani), with an EC50 of 3.44 mg/L, which was superior to CA (7.38 mg/L) and PCA (11.62 mg/L). Preliminary structure-activity relationship (SAR) results indicated that the introduction of I, Cl, or Br substituents at position 5 of the 3-hydroxy-2-oxindole and indole rings is crucial for compounds to exhibit good antifungal activity. The in vivo antifungal activity assay showed that compound 3u has good curative effects against R. solani. The current results suggest that these compounds are capable of serving as promising lead compounds.

Oxazole and isoxazole-containing pharmaceuticals: targets, pharmacological activities, and their SAR studies

Oxazole, a five-membered aromatic heterocycle featuring a nitrogen and an oxygen atom separated by a carbon atom, and its isomer isoxazole, with directly attached oxygen and nitrogen atoms, have been pivotal in medicinal chemistry. Over the past few decades, the U.S. Food and Drug Administration (FDA) has approved more than 20 drugs containing these nuclei for various clinical conditions, including Tafamidis and Oxaprozin. Due to their unique physicochemical properties, these drugs often exhibit superior pharmacokinetic profiles and pharmacological effects compared to those with similar heterocycles. This review provides a comprehensive overview of all FDA-approved drugs containing oxazole and isoxazole nuclei, focusing on their pharmacological activities and structure-activity relationships.

Thiazole and Isothiazole Chemistry in Crop Protection

Thiazole and isothiazole are types of five-membered heterocycles that contain both sulfur and nitrogen atoms. They have gained attention in the field of green pesticide research due to their low toxicity, strong biological activity, and ability to undergo diverse structural modifications. By incorporating thiazole and isothiazole groups into various compounds, researchers have been able to create a wide range of pesticides with broad-spectrum effectiveness. Understanding the relationship between the structure of these compounds and their activities is crucial for the development of new and highly potent pesticides. This review highlights thiazole and isothiazole derivatives with various biological activities and aims to inspire the development of innovative pesticide based on these structures.

Chemical and Biological Investigations of Antiviral Agents Against Plant Viruses Conducted in China in the 21st Century

Research into the biology of plant viruses, their mechanisms of pathogenicity, and the induction of host resistance has laid a solid foundation for the discovery of antiviral agents and their targets and the development of effective control technologies. Additionally, recent advancements in fields such as chemical biology, cheminformatics, bioinformatics, and synthetic biology have provided valuable methods and tools for the design of antiviral drugs, the synthesis of drug molecules, assessment of their activity, and investigation of their modes of action. Compared with drug development for human viral diseases, the control of plant viral diseases presents greater challenges, including the cost-benefit of agents, simplification of control technologies, and the effectiveness of treatments. Therefore, in the current context of complex outbreaks and severe damage caused by plant viral diseases, it is crucial to delve deeper into the research and development of antiviral agents. This review provides a detailed overview of the biological characteristics of current targets for antiviral agents, the mode of interaction between plant virus targets and antivirals, and insights for future drug development. We believe this review will not only facilitate the in-depth analysis of the development of antivirals for crops but also offer valuable perspectives for the development of antiviral agents for use in human and veterinary medicine.

Design, synthesis, and biological activities of arecoline derivatives containing 1,3,4-oxadiazole structure

Pesticides play an important role in the development of agriculture, as they can prevent and control crop diseases and pests, improve crop yield and quality. However, the abuse and improper use of pesticides can lead to negative impacts such as environmental pollution and pest resistance issues. There is an urgent need to develop green, safe, and efficient pesticides. In this work, natural product arecoline was selected as parent structure, a series of arecoline derivatives were designed, synthesized, and systematically investigated antiviral activities against tobacco mosaic virus (TMV). These compounds were found to have good to excellent anti-TMV activities for the first time. The antiviral activities of 4a, 4 h, 4 l, 4p, 6a, 6c, and 6f are higher than that of ningnanmycin. Compounds 4 h (EC50 value 146 µg/mL) and 4p (EC50 value 161 µg/mL) with simple structures and excellent activities emerged as new antiviral candidates. We chose 4 h to further investigate the antiviral mechanism, which revealed that it can cause virus fragmentation by acting on the viral coat protein (CP). We further validated this result through molecular docking. These compounds also displayed broad-spectrum fungicidal activities against 8 plant pathogenic fungi. This work lays the theoretical foundation for the application of arecoline derivatives in the agricultural field.

Discovery of Novel Chiral Indole Derivatives Containing the Oxazoline Moiety as Potential Antiviral Agents for Plants

Potato virus Y (PVY) is an important plant virus that has spread worldwide, causing significant economic losses. To search for novel structures as potent antiviral agents, a series of chiral indole derivatives containing oxazoline moieties were designed and synthesized and their anti-PVY activities were evaluated. Biological activity tests demonstrated that many chiral compounds exhibited promising anti-PVY activities and that their absolute configurations exhibited obvious distinctions in antiviral bioactivities. Notably, compound (S)-4v displayed excellent curative and protective efficacy against PVY, with EC50 values of 328.6 and 256.1 μg/mL, respectively, which were superior to those of commercial virucide ningnanmycin (NNM, 437.4 and 397.4 μg/mL, respectively). The preliminary antiviral mechanism was investigated to determine the difference in antiviral activity between the two enantiomers of 4v chiral compounds. Molecular docking indicated a stronger binding affinity between the coating proteins of PVY (PVY-CP) and (S)-4v (-6.5 kcal/mol) compared to (R)-4v (-6.2 kcal/mol). Additionally, compound (S)-4v can increase the chlorophyll content and defense-related enzyme activities more effectively than its enantiomer. Therefore, this study provides an important basis for the development of chiral indole derivatives containing oxazoline moieties as novel agricultural chemicals.

The evolutionary trajectories of specialized metabolites towards antiviral defense system in plants

Viral infections in plants pose major challenges to agriculture and global food security in the twenty-first century. Plants have evolved a diverse range of specialized metabolites (PSMs) for defenses against pathogens. Although, PSMs-mediated plant-microorganism interactions have been widely discovered, these are mainly confined to plant-bacteria or plant-fungal interactions. PSM-mediated plant-virus interaction, however, is more complicated often due to the additional involvement of virus spreading vectors. Here, we review the major classes of PSMs and their emerging roles involved in antiviral resistances. In addition, evolutionary scenarios for PSM-mediated interactions between plant, virus and virus-transmitting vectors are presented. These advancements in comprehending the biochemical language of PSMs during plant-virus interactions not only lay the foundation for understanding potential co-evolution across life kingdoms, but also open a gateway to the fundamental principles of biological control strategies and beyond.

Design, Synthesis, Antibacterial, and Antifungal Evaluation of Phenylthiazole Derivatives Containing a 1,3,4-Thiadiazole Thione Moiety

To effectively control the infection of plant pathogens, we designed and synthesized a series of phenylthiazole derivatives containing a 1,3,4-thiadiazole thione moiety and screened for their antibacterial potencies against Ralstonia solanacearum, Xanthomonas oryzae pv. oryzae, as well as their antifungal potencies against Sclerotinia sclerotiorum, Rhizoctonia solani, Magnaporthe oryzae and Colletotrichum gloeosporioides. The chemical structures of the target compounds were characterized by 1H NMR, 13C NMR and HRMS. The bioassay results revealed that all the tested compounds exhibited moderate-to-excellent antibacterial and antifungal activities against six plant pathogens. Especially, compound 5k possessed the most remarkable antibacterial activity against R. solanacearum (EC50 = 2.23 μg/mL), which was significantly superior to that of compound E1 (EC50 = 69.87 μg/mL) and the commercial agent Thiodiazole copper (EC50 = 52.01 μg/mL). Meanwhile, compound 5b displayed the most excellent antifungal activity against S. sclerotiorum (EC50 = 0.51 μg/mL), which was equivalent to that of the commercial fungicide Carbendazim (EC50 = 0.57 μg/mL). The preliminary structure-activity relationship (SAR) results suggested that introducing an electron-withdrawing group at the meta-position and ortho-position of the benzene ring could endow the final structure with remarkable antibacterial and antifungal activity, respectively. The current results indicated that these compounds were capable of serving as promising lead compounds.

Design, Synthesis, and Antifungal Activity of Some Novel Phenylthiazole Derivatives Containing an Acylhydrazone Moiety

Crop fungal diseases pose a serious threat to global crop production and quality. Developing new and efficient fungicides is an important measure to control crop diseases. Phenylthiazole was found to be an excellent antifungal skeleton based on our previous study on the structural optimization and biological activity of the natural product thiasporine A. To find new fungicides, 45 phenylthiazole derivatives containing an acylhydrazone moiety were designed and synthesized by the principle of active substructure splicing. Forty-two of the forty-five compounds are novel, except for compounds E1, E14, and E33. Their structures were structurally characterized by 1H NMR, 13C NMR, and HRMS. The antifungal activities of the target compounds against Magnaporthe oryzae Colletotrichum camelliaet, Bipolaris maydis, and Sclerotinia sclerotiorum were evaluated at 25 μg/mL. The bioassay results revealed that most of these compounds exhibited excellent antifungal activities against M. oryzae and C. camelliaet at 25 μg/mL. In particular, compounds E4, E10, E14, E17, E23, E26, and E27 showed the inhibition rate of more than 80% against M. oryzae, with EC50 values of 1.66, 2.01, 2.26, 1.45, 1.50, 1.29, and 2.65 μg/mL, respectively, which were superior to that of the commercial fungicides Isoprothiolane (EC50 = 3.22 μg/mL) and Phenazine-1-carboxylic acid (EC50 = 27.87 μg/mL). The preliminary structure-activity relationship (SAR) results suggested that introducing methyl, halogen, or methoxy at the ortho-position of R1 and the para-position of R2 can endow the final structure with excellent antifungal activity against M. oryzae. The current results provide useful data for developing phenylthiazole derivatives as new fungicides for controlling rice blast caused by M. oryzae.

Design, Synthesis, and Biological Evaluation of Novel Derivatives of the Marine Natural Product Laurene

Plant pathogenic fungi and viruses are seriously threatening agricultural production. There is an urgent need to develop novel fungicides and antiviral agents with low toxicity and high efficiency. In this study, we designed and synthesized 32 thiazole-, hydrazone-, and amide-containing derivatives of laurene and systematically evaluated their antiviral activities and fungicidal activities. Structure-simplified compounds 5a-5c, 5i, 5k, 5l, 11a, 11j, and 12c displayed higher antiviral activities than that of ningnanmycin. Compound 11a with a simple chemical structure, convenient synthetic route, and excellent antiviral activity emerged as a secondary lead compound. The docking results show that compounds 5i, 5k, and 11a have strong interactions with the tobacco mosaic virus coat protein (TMV CP). These compounds also exhibited significant fungicidal activities. Compounds 5g, 5k, 11j, and 11l displayed 9.15-17.45 μg/mL EC50 values against Pyricularia grisea, and compounds 5h (EC50: 8.01 μg/mL) and 11i (EC50: 15.23 μg/mL) exhibited a similar level of EC50 values with chlorothalonil (EC50: 7.33 μg/mL) against Physalospora piricola. Preliminary fungicidal mechanism research indicated that compound 5h has a certain destructive effect on the hyphae of P. piricola. This work lays a foundation for the application of laurene derivatives in plant protection.

Design, synthesis and antifungal evaluation of phenylthiazole-1,3,4-oxadiazole thione (ketone) derivatives inspired by natural thiasporine A

BACKGROUND

Plant pathogenic fungal infections have become a severe threat to the yield and quality of agricultural products, and new green antifungal agents with high efficiency and low toxicity are needed. In this study, a series of thiasporine A derivatives containing phenylthiazole-1,3,4-oxadiazole thione (ketone) structures were designed and synthesized, and their antifungal activities against six invasive and highly destructive phytopathogenic fungi were evaluated.

RESULTS

The results found that all compounds showed moderate to potent antifungal activity against six phytopathogenic fungi, and most of the E series compounds showed remarkable antifungal activity against Sclerotinia sclerotiorum and Colletotrichum camelliaet. In particular, compounds E1-E5, E7, E8, E13, E14, E17, and E22 showed more significant antifungal activity against S. sclerotiorum, with half-maximal effective concentration (EC50 ) values of 0.22, 0.48, 0.56, 0.65, 0.51, 0.39, 0.60, 0.56, 0.60, 0.63, and 0.45 μg mL-1 , respectively, which were superior to that of carbendazim (0.70 μg mL-1 ). Further activity studies showed that compound E1 possessed superior curative activities against S. sclerotiorum in vivo and better inhibitory effects on sclerotia germination and the formation of S. sclerotiorum compared with those of carbendazim.

CONCLUSIONS

This study indicates that these thiasporine A derivatives containing phenylthiazole-1,3,4-oxadiazole thione structures might be used as antifungal agents against S. sclerotiorum. © 2023 Society of Chemical Industry.

Natural Products for Pesticides Discovery: Structural Diversity Derivation and Biological Activities of Naphthoquinones Plumbagin and Juglone

Plant diseases and insect pests seriously affect the yield and quality of crops and are difficult to control. Natural products are an important source for the discovery of new pesticides. In this work, naphthoquinones plumbagin and juglone were selected as parent structures, and a series of their derivatives were designed, synthesized and evaluated for their fungicidal activities, antiviral activities and insecticidal activities. We found that the naphthoquinones have broad-spectrum anti-fungal activities against 14 types of fungus for the first time. Some of the naphthoquinones showed higher fungicidal activities than pyrimethanil. Compounds I, I-1e and II-1a emerged as new anti-fungal lead compounds with excellent fungicidal activities (EC₅₀ values: 11.35-17.70 µg/mL) against Cercospora, arachidicola Hori. Some compounds also displayed good to excellent antiviral activities against the tobacco mosaic virus (TMV). Compounds I-1f and II-1f showed similar level of anti-TMV activities with ribavirin, and could be used as new antiviral candidates. These compound also exhibited good to excellent insecticidal activities. Compounds II-1d and III-1c displayed a similar level of insecticidal activities with matrine, hexaflumuron and rotenone against Plutella xylostella. In current study, plumbagin and juglone were discovered as parent structures, which lays a foundation for their application in plant protection.

Discovery of Barakacin and Its Derivatives as Novel Antiviral and Fungicidal Agents

Pesticides are essential for the development of agriculture. It is urgent to develop green, safe and efficient pesticides. Bisindole alkaloids have unique and concise structures and broad biological activities, which make them an important leading skeleton in the creation of new pesticides. In this work, we synthesized bisindole alkaloid barakacin in a simple seven-step process, and simultaneously designed and synthesized a series of its derivatives. Biological activity research indicated that most of these compounds displayed good antiviral activities against tobacco mosaic virus (TMV). Among them, compound 14b exerted a superior inhibitory effect in comparison to commercially available antiviral agent ribavirin, and could be expected to become a novel antiviral candidate. Molecular biology experiments and molecular docking research found that the potential target of compound 14b was TMV coat protein (CP). These compounds also showed broad-spectrum anti-fungal activities against seven kinds of plant fungi.

Design, Synthesis, Antiviral and Fungicidal Activities of Novel Polycarpine Simplified Analogues

Fungal and viral diseases account for 70-80% of agricultural production losses caused by microbial diseases. Synthetic fungicides and antiviral agents have been used to treat plant diseases caused by plant pathogenic fungi and viruses, but their use has been criticized due to their adverse side effects. As alternative strategies, natural fungicides and antiviral agents have attracted many researchers' interest in recent years. Herein, we designed and synthesized a series of novel polycarpine simplified analogues. Antiviral activity research against tobacco mosaic virus (TMV) revealed that most of the designed compounds have good antiviral activities. The virucidal activities of 4, 6d, 6f, 6h, and 8c are higher than that of polycarpine and similar to that of ningnanmycin. The structure simplified compound 8c was selected for further antiviral mechanism research which showed that compound 8c could inhibit the formation of 20S protein discs by acting on TMV coat protein. These compounds also displayed broad-spectrum fungicidal activities against 7 kinds of plant fungi. This work lays the foundation for the application of polycarpine simplified analogues in crop protection.

Discovery of Flavone Derivatives Containing Carboxamide Fragments as Novel Antiviral Agents

Plant virus diseases seriously affect the yield and quality of agricultural products, and their prevention and control are difficult. It is urgent to develop new and efficient antiviral agents. In this work, a series of flavone derivatives containing carboxamide fragments were designed, synthesized, and systematically evaluated for their antiviral activities against tobacco mosaic virus (TMV) on the basis of a structural-diversity-derivation strategy. All the target compounds were characterized by 1H-NMR, 13C-NMR, and HRMS techniques. Most of these derivatives displayed excellent in vivo antiviral activities against TMV, especially 4m (inactivation inhibitory effect, 58%; curative inhibitory effect, 57%; and protection inhibitory effect, 59%), which displayed similar activity to ningnanmycin (inactivation inhibitory effect, 61%; curative inhibitory effect, 57%; and protection inhibitory effect, 58%) at 500 μg mL-1; thus, it emerged as a new lead compound for antiviral research against TMV. Antiviral mechanism research by molecular docking demonstrated that compounds 4m, 5a, and 6b could interact with TMV CP and disturb virus assembly.

Recent Research Progress: Discovery of Anti-Plant Virus Agents Based on Natural Scaffold

Plant virus diseases, also known as “plant cancers”, cause serious harm to the agriculture of the world and huge economic losses every year. Antiviral agents are one of the most effective ways to control plant virus diseases. Ningnanmycin is currently the most successful anti-plant virus agent, but its field control effect is not ideal due to its instability. In recent years, great progress has been made in the research and development of antiviral agents, the mainstream research direction is to obtain antiviral agents or lead compounds based on structural modification of natural products. However, no antiviral agent has been able to completely inhibit plant viruses. Therefore, the development of highly effective antiviral agents still faces enormous challenges. Therefore, we reviewed the recent research progress of anti-plant virus agents based on natural products in the past decade, and discussed their structure-activity relationship (SAR) and mechanism of action. It is hoped that this review can provide new inspiration for the discovery and mechanism of action of novel antiviral agents.

Marine Sesquiterpenes for Plant Protection: Discovery of Laurene Sesquiterpenes and Their Derivatives as Novel Antiviral and Antiphytopathogenic Fungal Agents

The unreasonable use or long-term use of a single variety of pesticide has led to drug resistance and made the pesticides ineffective. Therefore, the creation of new, efficient, and low-risk pesticides is imminent. Marine natural products play a vital role in serving as new lead compounds. In this work, we realized the efficient preparation of nine marine sesquiterpenes with the Stork-Danheiser reaction as the key step and designed and synthesized a series of their derivatives. The antiviral activity and antifungal activity research showed that these compounds exhibited good to excellent biological activities. Compounds 7b and 8e displayed significantly higher antiviral activities against tobacco mosaic virus (TMV) than ribavirin and could be used as new antiviral candidates. The antiviral mode of action research indicated that compound 8e inhibited the formation of the 20S protein disk by acting on the TMV coat protein and therefore inhibited the assembly of TMV particles. This work provides a new idea for the development of new pesticides based on marine sesquiterpenes.

Role of Indole Derivatives in Agrochemistry: Synthesis and Future Insights

Abstract:

Heterocycles constitute a wider class of organic compounds which contribute significantly in every facet of pure and applied chemistry. Indole, one of the bicyclic heterocyclic compounds containing nitrogen atom, witnessed unparalleled biological activity such as antiviral, antibacterial, anticancer, anti-depressant and antifungal activities. Different biological activities exhibited by indole derivatives provide the impulsion to explore its activity against anti-phytopathogenic microbes to save the plants from pests and disease, as food security will once again become a rigid demand. This review mainly focuses on various methods related to the synthesis of indole derivatives and its role in agriculture.

Discovery of Phytoalexin Camalexin and Its Derivatives as Novel Antiviral and Antiphytopathogenic-Fungus Agents

In response to the invasion of plant viruses and pathogenic fungi, higher plants produce defensive allelochemicals. Finding candidate varieties of botanical pesticides based on allelochemicals is one of the important ways to create efficient and green pesticides. Here, a series of camalexin derivatives based on a phytoalexin camalexin scaffold were designed, synthesized, and assessed for their antiviral and fungicidal activities systematically. Most of these camalexin derivatives exhibited better antiviral activities against tobacco mosaic virus (TMV) than the control antiviral agent ribavirin. Under the same test conditions, the anti-TMV activities of compounds 3d, 5a, 5d, and 10f-10h were found to be equivalent to or better than that of ningnanmycin, an agricultural cytosine nucleoside antibiotic with excellent protective effect. The antiviral mechanism research showed that compound 5a could cause 20S CP disk fusion and disintegration, thus affecting the assembly of virus particles. The results of molecular docking indicate that there were obvious hydrogen bonds between compounds 3d, 5a, and 10f and TMV CP. The binding constants of compounds 5a and 10f to TMV CP were also calculated using fluorescence titration. These camalexin derivatives also presented broad spectrum fungicidal activities, especially for Rhizoctonia solani and Physalospora piricola. In this work, the design, synthesis, structure optimization, and mode of action of camalexin derivatives were carried out progressively. This work provides a reference for using defensive chemical compounds as novel pesticide lead compounds.

Discovery of glyantrypine-family alkaloids as novel antiviral and antiphytopathogenic-fungus agents

BACKGROUND

Plant diseases caused by viruses and fungi have caused great losses to crop quality and yield. The discovery of novel and efficient antiviral and antiphytopathogenic-fungus agents is urgently needed. It is the most important pesticide innovation strategy to find active compounds from natural products. Here, glyantrypine-family alkaloids were taken as the parent structures and a series of their derivatives were designed through molecular splicing, ring expansion, and ring contraction strategies, and synthesized. The anti-tobacco mosaic virus (TMV) activities and antifungal activities of these alkaloids were systematically investigated for the first time.

RESULT

The antiviral activities of compounds 7bb, 7bc, 11c, 18b, 18d, 28d, and 28e are equivalent to or better than that of ribavirin (inhibitory rates 39%, 37%, and 40% at 500 μg mL^-1 for inactivation, curative, and protection activity in vivo, respectively). Compounds 18d and 28d with good antiviral activities were selected for antiviral mode of action studies, which indicated that these alkaloids could achieve good antiviral effects by inhibiting TMV particle extension during assembly. These compounds also exhibited broad-spectrum fungicidal activities.

CONCLUSION

Glyantrypine-family alkaloids and their derivatives were synthesized and evaluated for anti-TMV and fungicidal activities for the first time. Compounds 18d and 28d with excellent antiviral activities and compound 7bc with remarkable fungicidal activity emerged as novel lead compounds. This study lays a foundation for the application of glyantrypine alkaloids in plant protection.

Design, Synthesis, and Bioactivities of Phthalide and Coumarin Derivatives Based on the Biosynthesis and Structure Simplification of Gossypol

Because gossypol and hemigossypol show antiviral activity but are structurally complex, we designed and synthesized a series of structurally simpler phthalide and coumarin derivatives. The phthalide derivatives were synthesized by opening the naphthalene ring of hemigossypol, and the coumarin derivatives were synthesized by ring-opening reactions of the phthalide derivatives with the goal of investigating the effect of the lactone ring size on bioactivity. The bioassay results showed that the two series of target compounds possessed moderate to good activities against tobacco mosaic virus, One of the compounds showed in vivo inactivation, curative, and protection activities of 50 ± 1, 53 ± 3, and 48 ± 2% at 500 mg/L, values which are higher than those of gossypol (32 ± 1, 35 ± 1, 29 ± 1%, respectively) and comparable to those of hemigossypol (55 ± 1, 49 ± 1, and 48 ± 1%, respectively) and the commercial antiviral agent ningnanmycin (56 ± 2, 54 ± 1, 58 ± 1%) at the same dose. Thus, this compound is a promising candidate for the development of new anti-plant-virus agents. In addition, most of the synthesized compounds showed broad-spectrum activity when tested against 14 kinds of phytopathogenic fungi and showed selectivity against Sclerotinia sclerotiorum, Physalospora piricola, and Rhizoctonia cerealis. Moreover, some of the compounds exhibited activity against Plutella xylostella larvae; the two most active compounds exhibited larvicidal activities (LC₅₀) of 4.10 and 5.47 mg/L, respectively. Further studies showed that these compounds also exhibited insecticidal activities against Mythimna separata, Helicoverpa armigera, and Pyrausta nubilalis larvae.

Synthesis, Molecular Docking, and Biofilm Formation Inhibitory Activity of Bis(Indolyl)Pyridines Analogues of the Marine Alkaloid Nortopsentin

An efficient and simple protocol for the synthesis of a new class of diverse bis(indolyl)pyridines analogues of the marine alkaloid nortopsentin has been reported. A one-pot four-component condensation of 3-cyanocarbomethylindole, various aldehyde, 3-acetylindole, and ammonium acetate in glacial acetic acid led to the formation of 2,6-bis(1H-indol-3-yl)-4-(substituted-phenyl)pyridine-5-carbonitriles. Additionally, 2,6-bis(1H-indol-3-yl)-4-(benzofuran) pyridine-5-carbonitriles were prepared via a one-pot four-component condensation of 3-cyanocarbomethylindole, various N-substituted-indole-3-aldehydes, 2-acetylbenzofuran, and ammonium acetate. The synthesized compounds were evaluated for their ability to inhibit biofilm formation against the Gram-positive bacterial reference strains Staphylococcus aureus ATCC 6538 and the Gram-negative strain Escherichia coli ATCC 25922. Some of the new compounds showed a marked selectivity against the Gram-positive and Gram-negative strains. Remarkably, five compounds 4b, 7a, 7c, 7d and 8e demonstrated good antibiofilm formation against S. aureus and E. coli. On the other hand, the release of reducing sugars and proteins from the treated bacterial strains over the untreated strains was considered to explain the disruption effect of the selected compound on the contact cells of S. aureus and E. coli. Out of all studied compounds, the binding energies and binding mode of bis-indole derivatives 7c and 7d were theoretically the best thymidylate kinase, DNA gyrase B and DNA topoisomerase IV subunit B inhibitors.

Enhancing stability by trapping palladium inside N-heterocyclic carbene-functionalized hypercrosslinked polymers for heterogeneous C-C bond formations

Catalyst deactivation caused by the aggregation of active metal species in the reaction process poses great challenges for practical applications of supported metal catalysts in solid-liquid catalysis. Herein, we develop a hypercrosslinked polymer integrated with N-heterocyclic carbene (NHC) as bifunctional support to stabilize palladium in heterogeneous C-C bond formations. This polymer supported palladium catalyst exhibits excellent stability in the one-pot fluorocarbonylation of indoles to four kinds of valuable indole-derived carbonyl compounds in cascade or sequential manner, as well as the representative Suzuki-Miyaura coupling reaction. Investigations on stabilizing effect disclose that this catalyst displays a molecular fence effect in which the coordination of NHC sites and confinement of polymer skeleton contribute together to stabilize the active palladium species in the reaction process. This work provides new insight into the development of supported metal catalysts with high stability and will also boost their efficient applications in advanced synthesis.

Catalyst deactivation caused by the aggregation of active metal species poses great challenges for supported metal catalyzed solid-liquid reactions. Here, the authors develop a hypercrosslinked polymer integrated with N-heterocyclic carbene (NHC) as bifunctional support to stabilize palladium in heterogeneous C-C bond formations.

Iron-Catalyzed Cycloaddition of Amides and 2,3-Diaryl-2H-azirines To Access Oxazoles via C-N Bond Cleavage

A novel and efficient iron-catalyzed cycloaddition reaction using readily available 2,3-diaryl-2H-azirines and primary amides is reported. A wide range of trisubstituted oxazoles could be achieved in good yields with good functional group compatibility. In this transformation, two C-N bonds were cleaed and new C-N and C-O bonds were formed.

Design, synthesis and anti-TMV activity of novel α-aminophosphonate derivatives containing a chalcone moiety that induce resistance against plant disease and target the TMV coat protein

Plant viral diseases, known as "plant cancer", with high contagiosity can substantially reduce crop quality and yield. To identify potential anti-tobacco mosaic virus (TMV) agents with different mechanisms, a series of novel α-aminophosphonate derivatives containing a chalcone moiety were designed and synthesized. Bioassay results revealed that some target compounds exhibited improved curative activity against TMV in vivo, and the EC50 value of compound B3 was 356.7 mg L-1. The activities of the defensive enzymes POD and CAT from tobacco leaves treated with B3 and B17 showed that these target compounds could improve the photosynthetic ability of the leaves and activate plant host resistance against TMV infection. The binding constant between B3 and TMV Coat Protein (CP) (2.51 × 108 M-1), calculated by the fluorescence titration experiment and docking results, revealed that B3 has a strong interaction with TMV CP. Further docking analysis revealed that B3 was embedded between two layers of the TMV CP, which was consistent with the 2:1 binding mode of TMV CP and B3 determined by the binding affinity experiment. The TEM morphological study of TMV treated with B3 and B17 indicated that this series of target compounds may trigger the disassembly of TMV by interacting directly with TMV CP. This study provides new insight for the discovery of antiviral compounds with two different mechanisms of action.

Discovery of Cysteine and Its Derivatives as Novel Antiviral and Antifungal Agents

Based on the structure of the natural product cysteine, a series of thiazolidine-4-carboxylic acids were designed and synthesized. All target compounds bearing thiazolidine-4-carboxylic acid were characterized by 1H-NMR, 13C-NMR, and HRMS techniques. The antiviral and antifungal activities of cysteine and its derivatives were evaluated in vitro and in vivo. The results of anti-TMV activity revealed that all compounds exhibited moderate to excellent activities against tobacco mosaic virus (TMV) at the concentration of 500 μg/mL. The compounds cysteine (1), 3-4, 7, 10, 13, 20, 23, and 24 displayed higher anti-TMV activities than the commercial plant virucide ribavirin (inhibitory rate: 40, 40, and 38% at 500 μg/mL for inactivation, curative, and protection activity in vivo, respectively), especially compound 3 (inhibitory rate: 51%, 47%, and 49% at 500 μg/mL for inactivation, curative, and protection activity in vivo, respectively) with excellent antiviral activity emerged as a new antiviral candidate. Antiviral mechanism research by TEM exhibited that compound 3 could inhibit virus assembly by aggregated the 20S protein disk. Molecular docking results revealed that compound 3 with higher antiviral activities than that of compound 24 did show stronger interaction with TMV CP. Further fungicidal activity tests against 14 kinds of phytopathogenic fungi revealed that these cysteine derivatives displayed broad-spectrum fungicidal activities. Compound 16 exhibited higher antifungal activities against Cercospora arachidicola Hori and Alternaria solani than commercial fungicides carbendazim and chlorothalonil, which emerged as a new candidate for fungicidal research.

Synthesis and Cytotoxic Activity of Novel Mono- and Bis-Indole Derivatives: Analogues of Marine Alkaloid Nortopsentin

BACKGROUND

The oceans cover more than 70% of the earth's surface, which represents over 95% of the biosphere. Therefore, oceans provide a wealth of marine invertebrates, especially sponges, ascidians, bryozoans and molluscs that produce structurally unique bioactive metabolites such as alkaloids. The bioactive scaffolds of marine alkaloids exhibit cytotoxic activities against human cancer cell lines.

OBJECTIVE

To prepare analogues of the marine alkaloid nortopsentin [having 2,4-bis(3'- indolyl)imidazole scaffold] as cytotoxic agents via structural modification of the core imidazole ring and one of the side indole rings.

METHODS

Four series of nortopsentin analogues were synthesized in which the imidazole ring was replaced by pyrazole, pyrido[2,3-d]pyrimidinone and pyridine rings. Furthermore, one of the side indole rings was replaced by substituted phenyl moiety. The target compounds were tested for their in vitro cytotoxic activity against HCT-116 cell-line and the most potent compound was subjected to further investigation on its effect on HCT-116 cell cycle progression.

RESULTS

The cytotoxic screening of the synthesized compounds revealed that bis-indolylpyridinedicarbonitriles 8a-d exhibited the most potent cytotoxic activity with IC50=2.6-8.8 μM. Compound 8c was further tested by flow cytometry analysis to explore its effect on HCT-116 cell cycle progression that, in turn, indicated its anti-proliferative effect.

CONCLUSION

Marine-derived bis-indole alkaloids (nortopsentins) have emerged as a new class of indole-based antitumor agents. The design of new analogues involved several modifications in order to obtain more selective and potent cytotoxic agents. Indole derivatives bearing a pyridine core displayed more potent cytotoxic activity than those containing pyrido[2,3-d]pyrimidin-4(1H)-one moiety.

Design, Synthesis and Biological Evaluation of N-((2-phenyloxazol-4-yl)methyl) Pyrimidine Carboxamide Derivatives as Potential Fungicidal Agents

Twelve N-((2-phenyloxazol-4-yl)methyl) pyrimidine carboxamide derivatives were designed, synthesized, and characterized by 1H NMR, 13C NMR, and HRMS. The fungicidal activities of these new compounds against Sclerotinia sclerotiorum, Botrytis cinereal, and Colletotrichum fragariae were evaluated. The results indicated that compounds 5b, 5f, and 5g displayed potential fungicidal activities against tested fungi, especially 5f exhibited IC50 value of 28.9 mg/L against S. sclerotiorum. Moreover, the compounds 5f and 5g showed IC50 values of 54.8 mg/L and 62.2 mg/L against C. fragariae respectively, which shows that they were more active than the commercial fungicide hymexazol. The superficial structure-activity relationships were discussed, which may be of benefit for the development of fungicides and discovery of novel fungicides.

Marine Natural Product for Pesticide Candidate: Pulmonarin Alkaloids as Novel Antiviral and Anti-Phytopathogenic-Fungus Agents

Plant diseases are seriously endangering agricultural production. The emergence of drug resistance has brought great challenges to the prevention and control of plant diseases. There is an urgent need for the emergence of new drug candidates. In this work, we achieved the efficient synthesis of pulmonarins A and B in 64% and 59% overall yield, respectively. Pulmonarins A and B were found to have good antiviral activities against tobacco mosaic virus (TMV) for the first time. A series of pulmonarin derivatives were designed, synthesized, and evaluated for their antiviral and fungicidal activities systematically. Most compounds displayed higher anti-TMV activities than commercial ribavirin. Compounds 6a, 6c, and 6n with better inactivation effects than ningnanmycin emerged as new antiviral candidates. We selected 6c for further antiviral mechanism research, which revealed that it could inhibit virus assembly by interacting with TMV coat protein (CP). The molecular docking results further confirmed that these compounds could interact with CP through hydrogen bonding. These compounds also displayed broad spectrum fungicidal activities. Especially compound 6u with prominent antifungal activity emerged as a new fungicidal candidate for further research. The current work provides a reference for understanding the application of pulmonarin alkaloids in plant protection.

Luotonin A and Its Derivatives as Novel Antiviral and Antiphytopathogenic Fungus Agents

Plant diseases caused by viruses and fungi have posed a serious threat to global agricultural production. The discovery of new leads based on natural products is an important way to innovate pesticides. In this work, natural product luotonin A was found to have good antiviral activity against tobacco mosaic virus (TMV) for the first time. A series of luotonin A derivatives were designed, synthesized, and evaluated for their antiviral activities and fungicidal activities systematically. Most compounds displayed better antiviral activities against TMV than commercial ribavirin. Compounds 9k, 12b, and 12d displayed about similar inhibitory effects as ningnanmycin (inhibitory rates of 55, 57, and 59% at 500 μg/mL for inactivation, curative, and protection activities in vivo, respectively), the best antiviral agent at present, and emerged as novel antiviral leads for further research. We selected 9k for further antiviral mechanism research via transmission electron microscopy and molecular docking, which revealed that compound 9k can interact with TMV coat protein through the hydrogen bond, leading to its polymerization, thus preventing virus assembly. Further fungicidal activity tests showed that these compounds also showed broad-spectrum fungicidal activities against 14 kinds of phytopathogenic fungi. Especially, compound 14 with a 100% antifungal effect against Botrytis cinereal emerged as a lead for further research. This work provides a reference for the development of agricultural active ingredients based on Chinese medicine plants.

Streptindole and Its Derivatives as Novel Antiviral and Anti-Phytopathogenic Fungus Agents

Plant diseases caused by plant viruses and pathogens seriously affect the production and storage of food crops. With the emergence of drug resistance, it is very difficult to control. Natural products are the source of new drug discovery. Here, the natural product streptindole was found to have good antiviral activity against tobacco mosaic virus (TMV) and fungicidal activities against 14 kinds of phytopathogenic fungi. A series of derivatives of streptindole were designed, synthesized, and evaluated for their antiviral and fungicidal activities. Compounds 4, 5, 11, 12c, 12d, 13d, and 13i-13l showed higher anti-TMV activities than ribavirin (inhibitory rate: 38, 37, and 40% at 500 μg/mL for inactivation, curative, and protection activity in vivo, respectively), among which compound 12d (inhibitory rate: 57, 55, and 53% at 500 μg/mL for inactivation, curative, and protection activity in vivo, respectively) with excellent antiviral activity was further evaluated for the mode of action. The mechanism research revealed that 12d can break the three-dimensional structure of TMV coat protein (CP) through hydrogen bonds, thus inhibiting the assembly of virus particles. The molecular docking result showed that compound 12d did exhibit strong interaction with TMV CP. The derivatives of streptindole also displayed broad-spectrum fungicidal activities. The current study provided valuable insights into the antiviral and fungicidal activities of streptindole derivatives.

Discovery of Tryptanthrins as Novel Antiviral and Anti-Phytopathogenic-Fungus Agents

Plant diseases seriously affect the yield and quality of crops and are difficult to control. Tryptanthrin and its derivatives (tryptanthrins) were synthesized and evaluated for their antiviral activities and fungicidal activities. We found that tryptanthrins have good antiviral activities against tobacco mosaic virus (TMV) for the first time. Most of the tryptanthrins showed higher anti-TMV activities than that of ribavirin (inhibitory rates of 40, 37, and 38% at 500 μg/mL for inactivation, curative, and protection activities in vivo, respectively). Compound 3n (inhibitory rates of 52, 49, and 54% at 500 μg/mL for inactivation, curative, and protection activities in vivo, respectively) and compound 14 (inhibitory rates of 51, 48, and 53% at 500 μg/mL for inactivation, curative, and protection activities in vivo, respectively) emerged as new antiviral lead compounds with excellent antiviral activities. Compound 16 was selected for further antiviral mechanism research, which revealed that compound 16 could inhibit virus assembly by decomposing 20S coat protein (CP) disk. Molecular docking results showed that compounds 3n and 14, which have higher antiviral activities in vivo than that of compound 16, do show stronger interaction with TMV CP. Further fungicidal activity tests showed that tryptanthrins displayed broad-spectrum fungicidal activities, especially for compound 16. These compounds showed good selectivity to Physalospora piricola. In the current study, a small molecular library of tryptanthrin was constructed and the bioactivity spectrum of these compounds was broadened, which lays a foundation for their application in plant protection.

Synthesis and Antibacterial Evaluation of N-phenylacetamide Derivatives Containing 4-arylthiazole Moieties

A series of new N-phenylacetamide derivatives containing 4-arylthiazole moieties was designed and synthesized by introducing the thiazole moiety into the amide scaffold. The structures of the target compounds were confirmed by ¹H-NMR, ¹³C-NMR and HRMS. Their in vitro antibacterial activities were evaluated against three kinds of bacteria-Xanthomonas oryzae pv. Oryzae (Xoo), Xanthomonas axonopodis pv. Citri (Xac) and X.oryzae pv. oryzicola (Xoc)-showing promising results. The minimum 50% effective concentration (EC₅₀) value of N-(4-((4-(4-fluoro-phenyl)thiazol-2-yl)amino)phenyl)acetamide (A₁) is 156.7 µM, which is superior to bismerthiazol (230.5 µM) and thiodiazole copper (545.2 µM). A scanning electron microscopy (SEM) investigation has confirmed that compound A₁ could cause cell membrane rupture of Xoo. In addition, the nematicidal activity of the compounds against Meloidogyne incognita (M. incognita) was also tested, and compound A₂₃ displayed excellent nematicidal activity, with mortality of 100% and 53.2% at 500 μg/mL and 100 μg/mL after 24 h of treatment, respectively. The preliminary structure-activity relationship (SAR) studies of these compounds are also briefly described. These results demonstrated that phenylacetamide derivatives may be considered as potential leads in the design of antibacterial agents.

Synthesis of Four Optical Isomers of Antiviral Agent NK0209 and Determination of Their Configurations and Activities against a Plant Virus

Previously, we reported for the first time that harmala alkaloids harmine and tetrahydroharmine exhibit activity against plant viruses, and we developed an analogue, designated NK0209, that efficiently prevents and controls plant virus diseases. Here, to investigate the influence of the spatial configuration of NK0209 on its antiviral activities, we synthesized its four optical isomers, determined their configurations, and evaluated their activities against tobacco mosaic virus. All four isomers were significantly more active than ningnanmycin, which is one of the most successful commercial antiviral agents, with in vivo inactivation, cure, and protection rates of 57.3 ± 1.9, 54.2 ± 3.3, and 55.0 ± 4.1% at 500 μg/mL. Furthermore, analysis of structure-activity relationships demonstrated for the first time that the spatial conformation of NK0209 is an important determinant of its antiviral activity, and our results provide information about the possible optimum configuration for interaction of this molecule with its target protein.