Plant Flavonoids with Antimicrobial Activity against Methicillin-Resistant Staphylococcus aureus (MRSA).

Methicillin-resistant Staphylococcus aureus (MRSA) has become a serious threat to human public health and global economic development, and there is an urgent need to develop new antimicrobial agents. Flavonoids are the largest group of plant secondary metabolites, and the anti-S. aureus and anti-MRSA activities of flavonoids have now been widely reported. The aim of this Review is to describe plant-derived flavonoid active ingredients and their effects and mechanisms of inhibitory activity against MRSA in order to provide insights for screening novel antimicrobial agents. Here, 85 plant-derived flavonoids (14 flavones, 21 flavonols, 26 flavanones, 9 isoflavones, 12 chalcones, and 3 other classes) with anti-MRSA activity are reviewed. Among these flavonoids, flavones and isoflavones generally showed the most significant anti-MRSA activity (MICs: 1-8 µg/mL). The results of the present Review display that most of the flavonoids with excellent anti-MRSA activity were derived from Morus alba L. and Paulownia tomentosa (Thunb.) Steud. The antibacterial mechanism of flavonoids against MRSA is mainly achieved by disruption of membrane structures, inhibition of efflux pumps, and inhibition of ß-lactamases and bacterial virulence factors. We hope this Review can provide insights into the development of novel antimicrobials based on natural products for treating MRSA infections.

Novel natural osthole-inspired amphiphiles as membrane targeting antibacterials against methicillin-resistant Staphylococcus aureus (MRSA).

Methicillin-resistant Staphylococcus aureus (MRSA) is a widespread pathogen causing clinical infections and is multi-resistant to many antibiotics, making it urgent need to develop novel antibacterials to combat MRSA. Herein, we designed and prepared a series of novel osthole amphiphiles 6a-6ad by mimicking the structures and function of antimicrobial peptides (AMPs). Antibacterial assays showed that osthole amphiphile 6aa strongly inhibited S. aureus and 10 clinical MRSA isolates with MIC values of 1-2 µg/mL, comparable to that of the commercial antibiotic vancomycin. Additionally, 6aa had the advantages of rapid bacteria killing without readily developing drug resistance, low toxicity, good membrane selectivity, and good plasma stability. Mechanistic studies indicated that 6aa possesses good membrane-targeting ability to bind to phosphatidylglycerol (PG) on the bacterial cell membranes, thereby disrupting the cell membranes and causing an increase in intracellular ROS as well as leakage of proteins and DNA, and accelerating bacterial death. Notably, in vivo activity results revealed that 6aa exhibits strong anti-MRSA efficacy than vancomycin as well as a substantial reduction in MRSA-induced proinflammatory cytokines, including TNF-α and IL-6. Given the impressive in vitro and in vivo anti-MRSA efficacy of 6aa, which makes it a potential candidate against MRSA infections.

Semisynthesis and biological evaluation of novel honokiol thioethers against colon cancer cells HCT116 via inhibiting the transcription and expression of YAP protein.

Honokiol, derived from Magnolia officinalis (a traditional Chinese medicine), has been reported to have anticancer activity. Here, a series of novel honokiol thioethers bearing a 1,3,4-oxadiazole moiety were prepared and evaluated for their anticancer activities against three types of digestive system tumor cells. Biological evaluation showed that honokiol derivative 3k exhibited the best antiproliferative activity against HCT116 cells with an IC50 value of 6.1 µmol/L, superior to the reference drug 5-fluorouracil (IC50: 9.63 ± 0.27 µmol/L). The structure-activity relationships (SARs) indicated that the introduction of -(4-NO2)Ph, 3-pyridyl, -(2-F)Ph, -(4-F)Ph, -(3-F)Ph, -(4-Cl)Ph, and -(3-Cl)Ph groups was favorable for enhancing the anticancer activity of the title honokiol thioethers. Further study revealed that honokiol thioether 3k can well inhibit the proliferation of colon cancer cells HCT116, arresting the cells in G1 phase and inducing cell death. Moreover, a preliminary mechanism study indicated that 3k directly inhibits the transcription and expression of YAP protein without activating the Hippo signaling pathway. Thus, honokiol thioether 3k could be deeply developed for the development of honokiol-based anticancer candidates.

Development of Membrane-Targeting Fluorescent 2-Phenyl-1H-phenanthro[9,10-d]imidazole-Antimicrobial Peptide Mimic Conjugates against Methicillin-Resistant Staphylococcus aureus.

The escalation of multidrug-resistant bacterial infections, especially infections caused by methicillin-resistant Staphylococcus aureus (MRSA), underscores the urgent need for novel antimicrobial drugs. Here, we synthesized a series of amphiphilic 2-phenyl-1H-phenanthro[9,10-d]imidazole-antimicrobial peptide (AMP) mimic conjugates (III1-30). Among them, compound III13 exhibited excellent antibacterial activity against G+ bacteria and clinical MRSA isolates (MIC = 0.5-2 µg/mL), high membrane selectivity, and low toxicity. Additionally, compared with traditional clinical antibiotics, III13 demonstrated rapid bactericidal efficacy and was less susceptible to causing bacterial resistance. Mechanistic studies revealed that III13 targets phosphatidylglycerol (PG) on bacterial membranes to disrupt membrane integrity, leading to an increase in intracellular ROS and leakage of proteins and DNA, ultimately causing bacterial cell death. Furthermore, III13 possessed good fluorescence properties with potential for further dynamic monitoring of the antimicrobial process. Notably, III13 showed better in vivo efficacy against MRSA compared to vancomycin, suggesting its potential as a promising candidate for anti-MRSA medication.

Novel membrane-targeting isoxanthohumol-amine conjugates for combating methicillin-resistant Staphylococcus aureus (MRSA) infections.

Methicillin-resistant Staphylococcus aureus (MRSA) is a widespread pathogen causing clinical infections and is multi-resistant to many antibiotics, making it urgent need to develop novel antibacterials to combat MRSA. Here, a series of novel isoxanthohumol-amine conjugates were synthesized as antibacterials. After bioactivity evaluation, a compound E2 was obtained, which showed excellent antibacterial activity against S. aureus and clinical MRSA isolates (MICs = 0.25-1 µg/mL), superior to vancomycin, and with negligible hemolysis and good membrane selectivity. Additionally, E2 exhibited fast bacterial killing, less susceptible to resistance, relatively low cytotoxicity, and good plasma stability. Mechanism investigation revealed that E2 can disrupt bacterial membranes by specifically binding to phosphatidylglycerol on the bacterial membrane, thus causing elevated intracellular ROS and leakage of DNA and proteins, and ultimately killing bacteria. Noticeably, E2 displayed a good in vivo safety profile and better in vivo therapeutic efficacy than the same dose of vancomycin, allowing it to be a potential antibacterial to conquer MRSA infections.

Current Development of Thiazole-Containing Compounds as Potential Antibacterials against Methicillin-Resistant Staphylococcus aureus.

The emergence of multi-drug-resistant bacteria is threatening to human health and life around the world. In particular, methicillin-resistant Staphylococcus aureus (MRSA) causes fatal injuries to human beings and serious economic losses to animal husbandry due to its easy transmission and difficult treatment. Currently, the development of novel, highly effective, and low-toxicity antimicrobials is important to combat MRSA infections. Thiazole-containing compounds with good biological activity are widely used in clinical practice, and appropriate structural modifications make it possible to develop new antimicrobials. Here, we review thiazole-containing compounds and their antibacterial effects against MRSA reported in the past two decades and discuss their structure-activity relationships as well as the corresponding antimicrobial mechanisms. Some thiazole-containing compounds exhibit potent antibacterial efficacy in vitro and in vivo after appropriate structural modifications and could be used as antibacterial candidates. This Review provides insights into the development of thiazole-containing compounds as antimicrobials to combat MRSA infections.

Discovery of Amphiphilic Xanthohumol Derivatives as Membrane-Targeting Antimicrobials against Methicillin-Resistant Staphylococcus aureus.

Infections caused by multidrug-resistant (MDR) bacteria are increasing worldwide, and with limited clinically available antibiotics, it is urgent to develop new antimicrobials to combat these MDR bacteria. Here, a class of novel amphiphilic xanthohumol derivatives were prepared using a building-block approach. Bioactivity assays showed that the molecule IV15 not only exhibited a remarkable antibacterial effect against clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates (MICs: 1-2 µg/mL) but also had the advantages of rapid bactericidal properties, low toxicity, good plasma stability, and not readily inducing bacterial resistance. Mechanistic studies indicated that IV15 has good membrane-targeting ability and can bind to phosphatidylglycerol and cardiolipin in bacterial membranes, thus disrupting the bacterial cell membranes and causing increased intracellular reactive oxygen species and leakage of proteins and DNA, eventually resulting in bacterial death. Notably, IV15 exhibited remarkable in vivo anti-MRSA efficacy, superior to vancomycin, making it a potential candidate to combat MRSA infections.

Preparation and Biological Evaluation of Novel Osthole-Derived N-Benzoylthioureas as Insecticide Candidates.

To discover novel natural-product-based insecticide candidates, herein, a variety of osthole-derived N-benzoylthioureas were synthesized and assessed for their insecticidal activities against three insect pests. An insecticidal assay showed that most of the target osthole-derived N-benzoylthioureas displayed a more potent and broad-spectrum insecticidal effect than the parent osthole after the introduction of N-benzoylthioureas on the C-3' position of osthole. Compound B24 displayed the most potent growth inhibitory (GI) effect on Mythimna separata Walker, with a final corrected mortality rate of 82.1% when treated with a concentration of 1 mg/mL, which was 1.64- and 1.53-fold higher in comparison to osthole and the botanical insecticide toosendanin, respectively. Compounds B22, B23, and B25 displayed a more promising aphicidal effect on Myzus persicae Sulzer, and their LD50 values were 0.015, 0.017, and 0.019 µg/larvae, respectively, superior to the commercially available insecticide rotenone (0.024 µg/larvae). Derivatives B19, B20, B23, and B25 displayed more potent larvicidal activity against Plutella xylostella Linnaeus, with LC50 values of 0.22, 0.26, 0.15, and 0.30 mg/mL, respectively, exceeding that of rotenone (0.37 mg/mL). Furthermore, both compounds B19 and B23 against P. xylostella were found to be more effective than rotenone in a control efficacy assay under greenhouse conditions. The structure-activity relationship (SAR) suggested that osthole-derived N-benzoylthioureas are more active in most cases when the R group is an electron-withdrawing group than when it is an electron-donating group, especially for halogenated groups. Additionally, the potent compounds B19 and B23 possessed good selectivity and were less toxic to non-target organisms. This study suggests that these osthole-derived N-benzoylthioureas could be further studied in depth as eco-friendly natural product pesticides in crop protection.

Membrane-Targeting Neolignan-Antimicrobial Peptide Mimic Conjugates to Combat Methicillin-Resistant Staphylococcus aureus (MRSA) Infections.

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) continue to endanger public health. Here, we report the synthesis of neolignan isomagnolone (I) and its isomer II, and the preparation of a series of novel neolignan-antimicrobial peptide (AMP) mimic conjugates. Notably, conjugates III5 and III15 exhibit potent anti-MRSA activity in vitro and in vivo, comparable to that of vancomycin, a current effective treatment for MRSA. Moreover, III5 and III15 display not only fast-killing kinetics and low resistance frequency but also low toxicity as well as effects on bacterial biofilms. Mechanism studies reveal that III5 and III15 exhibit rapid bactericidal effects through binding to the phosphatidylglycerol (PG) and cardiolipin (CL) of the bacterial membrane, thereby disrupting the cell membranes and allowing increased reactive oxygen species (ROS) as well as protein and DNA leakage. The results indicate that these neolignan-AMP mimic conjugates could be promising antimicrobial candidates for combating MRSA infections.

Semisynthesis, biological activities, and mechanism studies of Mannich base analogues of magnolol/honokiol as potential α-glucosidase inhibitors.

Magnolol and honokiol, derived from a Magnolia officinalis Rehd. et Wils, are a class of natural biphenolic lignans. Currently, the discovery of new α-glucosidase inhibitors from natural analogues is of interest. Here, four series of thirty new Mannich base analogues of magnolol/honokiol were prepared and evaluated for their α-glucosidase inhibitory activities. Among these Mannich base analogues of magnolol/honokiol, 3k and 3l exhibited more potent inhibitory effects on α-glucosidase than the reference drug acarbose, and their IC50 values were 14.94 ± 0.17 µM and 13.78 ± 1.42 µM, respectively. Some interesting structure-activity relationships (SARs) were also analyzed. The enzyme inhibition kinetics indicated that 3k and 3l were noncompetitive inhibitors. This result was in agreement with molecular docking studies, where the binding sites of 3k and 3l to α-glucosidase were different from that of the competitive inhibitor acarbose to α-glucosidase. Moverover, compounds 3k and 3l exhibited low toxicity to normal cells (LO2). Thus, analogues 3k and 3l could be deeply developed for the discovery of natural products based antidiabetic candidates.

Design, Synthesis, and Biological Evaluation of Novel Osthole-Based Isoxazoline Derivatives as Insecticide Candidates.

Natural products are an abundant and environmentally friendly source for controlling plant pathogens and insect pests. Toward the development of new natural product-based pesticides, here, a series of osthole-based isoxazoline derivatives were prepared by [3 + 2] annulation and evaluated for their insecticidal activities and toxicities. The structures of all osthole-based isoxazoline derivatives were characterized by various spectral analyses, and derivative B13 was further confirmed by X-ray crystallography. Among all the osthole derivatives, B2 displayed the most promising growth inhibitory effect on Mythimna separata with a final corrected mortality rate of 96.4% ± 3.3, which was 1.80 times higher than those of both osthole and toosendanin. Derivative B13 displayed the most promising larvicidal activity against Plutella xylostella with an LC50 value of 0.220 mg/mL, which was superior to rotenone. Furthermore, both B13 and B21 also exhibited better control efficacy against P. xylostella than rotenone in the pot experiments. Additionally, the toxicity evaluation suggested that these osthole-based isoxazoline derivatives showed relatively low toxicity toward nontarget organisms. Given these results, osthole derivatives B2, B13, and B21 could be deeply developed as natural insecticidal agents in agriculture.

Semisynthesis and neurotrophic activity studies of novel neomajucin/majucin derivatives as neurotrophin small molecule mimetics.

Majucin-type Illicium sesquiterpenes with potent neurotrophic activity are considered to be promising candidates for the treatment of various neurodegenerative disease. Owing to the low-abundance metabolites in Illicium genus, there are few studies on their structural modifications, structure-activity relationships, and pharmacophoric motif. Herein, structural modifications were conducted on the hydroxyl groups at C-3 and C-6 positions of two majucin-type compounds neomajucin (1) and majucin (2), and 39 neomajucin/majucin based esters were synthesized and evaluated for their neurite outgrowth-promoting activities. Among all the target derivatives, compounds 1a, 1j, 1r, 2b, 2d, 3a, 3b, 3d and 3h displayed more potent neurite outgrowth-promoting activity than their precursors. Some interesting structure-activity relationships (SARs) were also observed. Moreover, compound 1a showed good neuroprotective effect on MPP+-induced PC12 cell damage. Finally, compounds 1a and 3a exhibited relatively no cytotoxicity to normal human H9C2 cardiac cells. This work will shed light on the development of neomajucin/majucin derivatives as potential neurotrophic agents.

Discovery of Natural Product-Based Fungicides (III): Semisynthesis and Biological Activity of N-Phenylpyrazole Sarisan Hybrids as Antifungal Agents.

Many phytopathogenic fungi can easily infect crops, resulting in crop yield reductions. In continuation of our efforts to develop natural product (NP)-based antifungal agents, a series of N-phenylpyrazole sarisan hybrids 6a-v were prepared via I2 -mediated oxidative cyclization, and their structures were determined by various spectral analyses including IR, 1 H-NMR and ESI-MS. Among all N-phenylpyrazole sarisan hybrids, compounds 6a, 6b, 6e, 6i, 6j and 6r exhibited more encouraging antifungal action against at least two phytopathogenic fungi than the reference fungicide hymexazol. Especially, 6a displayed really encouraging and broad-spectrum antifungal activity against F. graminearum, V. mali, and F. oxysporum f.sp.niveum with the EC50 values of 12.6±0.9, 18.5±0.2, and 37.4±1.8 µg/mL, respectively. Moreover, the structure-activity relationships (SARs) were also observed. Additionally, compounds 6a and 6e also exhibited relative low toxicity on normal LO2 cells. This study indicates that these N-phenylpyrazole sarisan hybrids would shed light on developing novel NP-based antifungal agents.

Development of Novel (+)-Nootkatone Thioethers Containing 1,3,4-Oxadiazole/Thiadiazole Moieties as Insecticide Candidates against Three Species of Insect Pests.

To improve the insecticidal activity of (+)-nootkatone, a series of 42 (+)-nootkatone thioethers containing 1,3,4-oxadiazole/thiadiazole moieties were prepared to evaluate their insecticidal activities against Mythimna separata Walker, Myzus persicae Sulzer, and Plutella xylostella Linnaeus. Insecticidal evaluation revealed that most of the title derivatives exhibited more potent insecticidal activities than the precursor (+)-nootkatone after the introduction of 1,3,4-oxadiazole/thiadiazole on (+)-nootkatone. Among all of the (+)-nootkatone derivatives, compound 8c (1 mg/mL) exhibited the best growth inhibitory (GI) activity against M. separata with a final corrected mortality rate (CMR) of 71.4%, which was 1.54- and 1.43-fold that of (+)-nootkatone and toosendanin, respectively; 8c also displayed the most potent aphicidal activity against M. persicae with an LD50 value of 0.030 µg/larvae, which was closer to that of the commercial insecticidal etoxazole (0.026 µg/larvae); and 8s showed the best larvicidal activity against P. xylostella with an LC50 value of 0.27 mg/mL, which was 3.37-fold that of toosendanin and slightly higher than that of etoxazole (0.28 mg/mL). Furthermore, the control efficacy of 8s against P. xylostella in the pot experiments under greenhouse conditions was better than that of etoxazole. Structure-activity relationships (SARs) revealed that in most cases, the introduction of 1,3,4-oxadiazole/thiadiazole containing halophenyl groups at the C-13 position of (+)-nootkatone could obtain more active derivatives against M. separata, M. persicae, and P. xylostella than those containing other groups. In addition, toxicity assays indicated that these (+)-nootkatone derivatives had good selectivity to insects over nontarget organisms (normal mammalian NRK-52E cells and C. idella and N. denticulata fries) with relatively low toxicity. Therefore, the above results indicate that these (+)-nootkatone derivatives could be further explored as new lead compounds for the development of potential eco-friendly pesticides.

Triphenyl-sesquineolignan analogues derived from Illicium simonsii Maxim exhibit potent antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) by disrupting bacterial membranes.

Infections caused by clinical methicillin-resistant Staphylococcus aureus (MRSA) are a serious public problem. Triphenyl-sesquineolignans from Illicium genus possess antibacterial activity, but few researches have reported their antibacterial spectrums, structure-activity relationships (SARs) and antibacterial mechanism. In this study, three triphenyl-sesquineolignans, dunnianol (1), macranthol (2) and isodunnianol (3) were isolated from the stems and leaves of I. simonsii Maxim, and seven dunnianol derivatives were prepared through esterification, etherification and halogenation reactions. Among all triphenyl-sesquineolignan analogues, compound 6 showed the best antibacterial activity against four Gram-positive bacteria (MICs = 1-2 µg/mL) and ten clinical MRSA strains (MICs = 2-8 µg/mL), and also exhibited characteristics of killing MRSA more rapidly than tigecycline. Meanwhile, compound 6 did not only show a low probability of drug resistance development, but also exhibited relatively low hemolysis, and good stability in 50% plasma. Further mechanism studies revealed that 6 could kill bacterial strains by disrupting bacterial membranes. These results suggested that 6 may be developed into a new antibacterial candidate for combating MRSA infections.

Discovery, semisynthesis and neurite outgrowth-promoting activity of novel merrillianone/cycloparviforalone based esters as neurotrophic agents.

Natural products (NPs) are very important sources for the development of new drugs. Merrillianone and cycloparvifloralone, isolated from the roots, stems, and fruits of Illicium henryi Diels, are two natural sesquiterpene compounds. In continuation of our effort to discovery more effective neurotrophic compounds from NPs, a series of novel merrillianone/cycloparviforalone based esters 2a-i, 3a-g and 3i-q were prepared and their structures were characterized by 1H NMR, 13C NMR and IR spectral analyses. Furthermore, the spatial structure of compound 2h was unambiguously confirmed by X-ray crystallography. The neurite outgrowth-promoting activity results indicated that most of the target derivatives exhibited more potent neurite outgrowth-promoting activity than merrillianone and cycloparviforalone. Among all target derivatives, the neurite outgrowth-promoting activity of compounds 2a, 3a and 3b was about 2-fold stronger than that of their precursors merrillianone and cycloparviforalone, respectively. Besides, compounds 2a and 3a displayed relatively low cytotoxicity to normal GES-1 cells. Moreover, these derivatives had good hydrolytic stability. Finally, some interesting results of the structure-activity relationships (SARs) were also discussed. This work will pave the way for the development of merrillianone/cycloparviforalone derivatives as potential neurotrophic agents.

Discovery of Natural Product-Based Fungicides (II): Semisynthesis and Biological Activity of Sarisan Attached 3-Phenylisoxazolines as Antifungal Agents.

Many phytopathogenic fungi cause severe damage to crop yields. In continuation of our research aimed at the discovery and development of natural products-based fungicides, a series of thirty-one sarisan attached 3-phenylisoxazolines were synthesized and evaluated for their antifungal activities against five phytopathogenic fungi (B. cinerea, C. lagenarium, A. solani, F. solani, and F. graminearum). Among all title sarisan derivatives, compounds IV2, IV14 and IV23 showed potent antifungal activity against some phytopathogenic fungi. In particular, compound IV2 exhibited a broad-spectrum and more potent antifungal activity against A. solani, F. solani, and F. graminearum than the commercial fungicide Hymexazol. In addition, compounds IV2, IV14 and IV23 also displayed relative low toxicity on normal NRK-52E cells. This work will give some insights into the development of sarisan derivatives as new fungicide candidates in plant protection.

Discovery, biological evaluation and docking studies of novel N-acyl-2-aminothiazoles fused (+)-nootkatone from Citrus paradisi Macf. as potential α-glucosidase inhibitors.

Nowadays, the discovery and development of α-glucosidase inhibitors from natural products or their derivatives represents an attractive approach. Here we reported studies on a series of novel N-acyl-2-aminothiazoles fused (+)-nootkatone and evaluation for their α-glucosidase inhibitory activities. Most of (+)-nootkatone derivatives exhibited more potent α-glucosidase inhibitory ability than the positive drug acarbose. In particular, compounds II7 and II14 showed the most promising α-glucosidase inhibitory ability with IC50 values of 13.2 and 13.8 µM. II7 and II14 also exhibited relatively low cytotoxicities towards normal LO2 cells. Kinetic study indicated that compounds II7 and II14 inhibited the α-glucosidase in a noncompetitive manner, and molecular docking results were in line with the noncompetitive characteristics that II7 and II14 did not bind to the known active sites (Asp214, Glu276 and Asp349). Based on our findings, these (+)-nootkatone derivatives could be used as antidiabetic candidates.

Application of natural products as insecticide candidates: Semisynthesis and biological evaluation of some novel osthole-based esters.

Natural products are very important sources for the development of new pesticides. Osthole, derived from many medical plants such as Cnidium, Angelica and Citrus plants, is a naturally occurring coumarin compound. To discover the new natural products-based insecticides, thirty-one osthole-based esters containing O-acyl-hydroxylamine groups were prepared, and their structures were identified by different spectral analysis methods. Derivatives A7, A17, A20 and A25 displayed more potent growth inhibitory (GI) activity than the botanical insecticide, toosendanin. Over half of target osthole derivatives had more effective larvicidal effect on P. xylostella than toosendanin. Among all title derivatives, compound A18 displayed more pronounced larvicidal activity (LC50 = 0.64 µmol mL-1) when compared with toosendanin (LC50 = 0.94 µmol mL-1). Some interesting results of structure-activity relationships (SARs) of these osthole derivatives were also discussed. In addition, the hemolysis and cytotoxicity assays indicated that these osthole derivatives showed very low toxicity toward normal mammalian cells.

Natural Product-Based Fungicides Discovery: Design, Synthesis and Antifungal Activities of Some Sarisan Analogs Containing 1,3,4-Oxadiazole Moieties.

A series of sarisan analogs containing 1,3,4-oxadiazole moieties were synthesized by iodine-mediated oxidative cyclization and screened in vitro for their antifungal activities at 50 µg/mL against five phytopathogenic fungi such as Valsa mali, Curvularia lunata, Alternaria alternate, Fusarium solani and Fusarium graminearum. 1,3,4-Oxadiazole derivatives 7e, 7p, 7r, 7t and 7u exhibited potent and a broad spectrum of antifungal activities against at least three phytopathogenic fungi at the concentration of 50 µg/mL. Especially, compound 7r displayed more potent antifungal activities against five phytopathogenic fungi than the positive control hymexazol. The EC50 of 7r against V. mali, C. lunata and A. alternate were 12.6, 14.5 and 17.0 µg/mL, respectively. Additionally, some interesting results of structure-activity relationships (SARs) were also observed.