Synthesis And Antifungal Activity Of 1,3,4-and 1,2,4-oxadiazole Derivatives

Plant diseases caused by plant pathogens have seriously threatened crop production and food security,and caused huge economic losses to global agriculture.The prevention and control of fungal diseases has become an urgent problem in agricultural production.At present,chemical control was still an effective method to prevent plant fungal diseases.However,multidrug resistance of plant pathogens to existing fungicides is becoming more and more common with the extensive use of chemical fungicides.Therefore,it is urgent to develop new fungicides with high efficiency,novel action mechanism and overcoming resistance.In the research of novel fungicides,heterocyclic compounds containing oxadiazole have attracted attention due to their extensive biological activities against bacteria and fungi.On this basis,a series of oxadiazole derivatives were designed and synthesized with 1,3,4-oxadiazole and 1,2,4-oxadiazole as lead structures by the principle of pharmacophore splicing.The antifungal activities of the synthesized compounds were evaluated in vitro and in vivo,and oxadiazole derivatives with significant antifungal activities were obtained.and the antifungal mechanism of these compounds was discussed to evaluate their potential as fungicides.The antifungal mechanisms of these compounds were analyzed in order to evaluate their potential for drug preparation.The specific research contents of this paper are as follows:(1)Three series of thirty 2,5-disubstituted 1,3,4-oxadiazole derivatives(4a41,5a-51 and 6a-6f)were designed and synthesized by introducing active structures at the C(2)and C(5)sites of 1,3,4-oxadiazole using three benzoyl hydrazides and a variety of aromatic aldehydes as feedstock through a two-step reaction.The highest yield of oxadiazole derivatives was 90%.The structure and purity of these compounds were determined by 1H-NMR,13C-NMR and MS.This synthesis method has the advantages of simple operation,mild reaction conditions,cheap and readily available raw materials,and wide applicability of substrates.It can be used for the synthesis of various 2,5-di-substituted-1,3,4-oxadiazole derivatives,and has certain practical value.(2)The antifungal activities of thirty 2,5-disubstituted-1,3,4-oxadiazole derivatives against five plant pathogenic fungi were evaluated.The results showed that compound 5k had an effective inhibitory effect on all five plant pathogenic fungi,followed by compound 5i.Compound 5k showed better inhibitory activity against E.turcicum and B.cinerea than Carbendazim.In vivo,compound 5k still maintained the inhibitory effect against B.cinerea and C.capsica,with the inhibition effect of 66.10%and 82.34%,respectively.The antifungal mechanism of the preferred compounds was preliminarily discussed.The results of scanning electron microscopy(SEM)showed that the active compounds could cause the mycelium to shrink,showing the morphology of twisted and folded mycelium.Molecular docking results showed that compound 5k could bind to amino acid residues of succinate dehydrogenase(SDH)through hydrophobic interaction and hydrogen bond.In addition,it can affect the synthesis of soluble protein in B.cinerea and the accumulation of malondialdehyde(MDA)in lipid peroxidation metabolites.By inhibiting the activity of SDH,the cells of B.cinerea were damaged.Finally,the growth of mycelium cells was inhibited by hindering their energy metabolism.The results of physicochemical properties and cytotoxicity showed that these active compounds basically conformed to the principle of"Lipinski",and had low cytotoxicity.These compounds have the potential to be used as plant fungicides.(3)The antifungal activities of 35 3,5-disubstituted 1,2,4-oxadiazole derivatives(7a-7f,8a-8s and 9a-9f)and intermediates against five plant pathogenic fungi were evaluated.The results showed that compound 8e had significant antifungal,followed by compound 7e.Among them,compound 8e showed superior antifungal activity against E.turcicum than carbendazim,and had the lowest EC50 value(8.87 μg/mL)against C.capsica,showing the antifungal effect at lower doses.The antifungal effect of compound 8e was further verified in vivo,and its control effect against C.capsica was 100%.The antifungal mechanism of the highly active compound was discussed.The SEM results showed that the active compounds could also cause the mycelium to shrink and become twisted mycelium.In addition,compound 8k caused mycelium death by affecting the synthesis of soluble protein,the accumulation of MDA and the inhibition of SDH activity in C.capsica.These compounds all conform to the principle of "Lipinski" and had low cytotoxicity,indicating that these compounds theoretically have better pharmacokinetic properties and bioavailability.Especially the preferred compound 8e has the potential to be developed as a plant fungicide,and its antifungal mechanism can be further studied.(4)Thirty-three 1,2,4-oxadiazole derivatives(10a-10w)containing pyrazolamide or pyridinamide groups and their intermediates were synthesized and purified from three aromatic acids containing N-heterocycles.Their highest yield was 85%.The structures of the intermediates and target products were characterized by melting point measurement,1H-NMR and 13C-NMR.The synthetic process of oxadiazole derivatives containing Nheterocyclic substitutions was optimized and established.This method has the advantages of high yield,simple operation,high safety performance and fast reaction speed.Moreover,most of the steps omit the extraction and column chromatography separation process,so it has a certain practical value and economic prospect.Finally,active structures such as pyrazolyl,pyridinyl,amide bond,1,2,4-oxadiazole,halogen and trifluoromethyl were introduced into the structure of the synthesized compound,which basically conform to the principle of "Lipinski".It can provide preferable candidate compounds for the further study of antifungal activity.In conclusion,the antifungal activities of series 1,3,4-and 1,2,4-oxadiazole derivatives were studied in this paper,which confirmed the potential of oxadiazole derivatives as antifungal agents,and provided the research basis for the development of novel plant fungicides.