Recent Developments in the Synthesis, Chemistry and Applications of the Fully Unsaturated 1,2,4-Oxadiazoles

Hybrids of Benzimidazole-oxadiazole: A New Avenue for Synthesis, Pharmacological Activity and Recent Patents for the Development of More Effective Ligands

BACKGROUND

Two significant families of compounds i.e. 1,3,4-oxadiazole and benzimidazole, have undergone extensive investigation into their pharmacological characteristics and possible therapeutic applications. Both classes have shown their potential in a variety of applications, and because of their synergistic interactions, they may have an even better therapeutic impact when combined.

OBJECTIVES

To produce a specific molecule with potent therapeutic properties, it is now common methods to combine at least two pharmacophores. This facilitates interaction with several targets, enhances biological functions, or eliminates adverse effects associated with them.

CONCLUSION

The synthesis of benzimidazole-1,3,4-oxadiazole hybrid compounds has recently involved the use of several synthetic techniques, all of which are detailed in the literature along with the advantages and disadvantages. It has been noted that the structure-activity relationship relates their pharmacological actions to their molecular structure. In order to set the stage for future research, the study aims to provide researchers with an effective toolbox and an understanding of benzimidazole and 1,3,4-oxadiazole hybrid compounds.

Design, Synthesis and Antifungal/Nematicidal Activity of Novel 1,2,4-Oxadiazole Derivatives Containing Amide Fragments

Plant diseases that are caused by fungi and nematodes have become increasingly serious in recent years. However, there are few pesticide chemicals that can be used for the joint control of fungi and nematodes on the market. To solve this problem, a series of novel 1,2,4-oxadiazole derivatives containing amide fragments were designed and synthesized. Additionally, the bioassays revealed that the compound F15 demonstrated excellent antifungal activity against Sclerotinia sclerotiorum (S. sclerotiorum) in vitro, and the EC₅₀ value of that was 2.9 μg/mL, which is comparable with commonly used fungicides thifluzamide and fluopyram. Meanwhile, F15 demonstrated excellent curative and protective activity against S. sclerotiorum-infected cole in vivo. The scanning electron microscopy results showed that the hyphae of S. sclerotiorum treated with F15 became abnormally collapsed and shriveled, thereby inhibiting the growth of the hyphae. Furthermore, F15 exhibited favorable inhibition against the succinate dehydrogenase (SDH) of the S. sclerotiorum (IC₅₀ = 12.5 μg/mL), and the combination mode and binding ability between compound F15 and SDH were confirmed by molecular docking. In addition, compound F11 showed excellent nematicidal activity against Meloidogyne incognita at 200 μg/mL, the corrected mortality rate was 93.2%, which is higher than that of tioxazafen.

Recent updates on Synthetic Strategies and Biological Potential of 1,3,4-oxadiazole: Review

Abstract:

Among the large variety of nitrogen and oxygen-containing heterocycles, 1,3,4-oxadiazole, the scaffold, has attracted considerable attention owing to its ability to show an extensive range of pharmacological actions. According to literature investigations, prepared 1,3,4-oxadiazole and its derivative are pharmacologically significant and consist of a variety of activities, such as anticonvulsant, anticancer, antioxidant, anti-inflammatory, antibacterial, antidiabetic, etc. These heterocyclics are formed mainly by the cyclization reactions of various reactants under diverse reaction circumstances. Therefore, significant efforts of organic chemists have been directed towards the synthesis of new drug candidates containing 1,3,4-oxadiazole subunits connected to an established potential pharmacophore to improve the efficacy and potency. This article aims to highlight recent publications on the various synthesis techniques of 1,3,4-oxadiazole and related compounds over the previous ten years (2011–2021). The purpose of this review is to help researchers by summarizing several synthetic strategies for synthesizing oxadiazole.

Synthesis of 5-arylacetylenyl-1,2,4-oxadiazoles and their transformations under superelectrophilic activation conditions

Acetylene derivatives of 1,2,4-oxadiazoles, i.e., 5-(2-arylethynyl)-3-aryl-1,2,4-oxadiazoles, have been obtained, for the first time reported, from 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles by their bromination at the carbon–carbon double bond followed by di-dehydrobromination with NaNH₂ in liquid NH₃. The reaction of the acetylenyl-1,2,4-oxadiazoles with arenes in neat triflic acid TfOH (CF₃SO₃H) at room temperature for 1 h resulted in the formation of E/Z-5-(2,2-diarylethenyl)-3-aryl-1,2,4-oxadiazoles as products of regioselective hydroarylation of the acetylene bond. The addition of TfOH to the acetylene bond of these oxadiazoles quantitatively resulted in E/Z-vinyl triflates. The reactions of the cationic intermediates have been studied by DFT calculations and the reaction mechanisms are discussed.

Synthesis of 1,2,4-Oxadiazoles by Tandem Reaction of Nitroalkenes with Arenes and Nitriles in the Superacid TfOH

The tandem reaction of nitroalkenes with arenes and nitriles in the superacid CF₃SO₃H (TfOH) results in the formation of 1,2,4-oxadiazole derivatives in yields up to 96%. This reaction proceeds via the consequent interaction of arenes and nitriles, as nucleophiles, with intermediate cationic species derived by the protonation of nitroalkenes in TfOH. This is novel and general synthesis of 1,2,4-oxadiazoles, which are very important compounds for medicinal chemistry.

Studies on gold-nitrone systems

A series of novel Au(i)-nitrone complexes with specific catalytic properties were prepared. Furthermore, Au(i)- and Au(iii)-oxadiazole complexes were formed by a novel Au-generated nitrile-nitrone [3 + 2] cycloaddition, and the crystal structures of Au(i)-nitrones as well as the Au(i)- and Au(iii)-oxadiazole complexes were studied (X-ray). A useful one-pot Au(iii)-mediated cycloaddition method was developed for the formation of a number of dihydro-1,2,4-oxadiazoles, involving in situ formation of Au(iii)-oxadiazole complexes. The observed Au(i) and Au(iii) dual selective reactivity gives new understanding about the Au(i)- and Au(iii)-nitrone chemistry.

Metal-free hydroarylation of the side chain carbon-carbon double bond of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles in triflic acid

The metal-free reaction of 5-(2-arylethenyl)-3-aryl-1,2,4-oxadiazoles with arenes in neat triflic acid (TfOH, CF3SO3H), both under thermal and microwave conditions, leads to 5-(2,2-diarylethyl)-3-aryl-1,2,4-oxadiazoles. The products are formed through the regioselective hydroarylation of the side chain carbon-carbon double bond of the starting oxadiazoles in yields up to 97%. According to NMR data and DFT calculations, N4,C-diprotonated forms of oxadiazoles are the electrophilic intermediates in this reaction.

Novel biphenyl-substituted 1,2,4-oxadiazole ferroelectric liquid crystals: synthesis and characterization

Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.C^*C_n compounds are synthesized and characterized. An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule. A biphenyl moiety prepared by Suzuki coupling reaction is directly attached to the oxadiazole core at C-5 position. Investigations for the phase behavior revealed that the series with a benzyl group on one end of the oxadiazole core exhibits an 1D orthogonal smectic-A phase while the second series with dodecyl flexible end chain shows orthogonal smectic-A and tilted chiral smectic-C (SmC*) phases over a wide range of temperatures. The smectic-C phase exhibits ferroelectric (FE) polarization switching. The mesomorphic thermal stabilities of these compounds are discussed in the domain of the symmetry and the flexibility of the alkyloxy end chain length attached to the chiral center.

Synthesis of phidianidines A and B

Reaction of a substituted indole-3-acetyl chloride with N-5-azidopentyl-N'-hydroxyguanidine generated a substituted 3-(5-azidopentylamino)-5-((indol-3-yl)methyl)-1,2,4-oxadiazole. Reduction of the azide with zinc and ammonium formate afforded the amine, which was elaborated to the guanidine, completing short and efficient syntheses of the cytotoxic natural products phidianidines A and B in 19% overall yield by a convergent route that will make analogues readily available for biological evaluation. Initial screening in the NCI 60 cell line at 10(-5) M indicated that the bromine on the indole is necessary for activity and that the amine precursor to phidianidine A is more potent than phidianidine A.

Improved microwave-mediated synthesis of 3-(3-aryl-1,2,4-oxadiazol-5-yl)propionic acids and their larvicidal and fungal growth inhibitory properties

The synthesis of 3-(3-aryl-1,2,4-oxadiazol-5-yl)propionic acids from arylamidoximes and succinic anhydride under focused microwave irradiation conditions is described. The new synthetic method furnished the desired products in 2-3 min and good yields. Furthermore, the previously complicated purification procedure has been simplified in a manner which is quick, eco-friendly and cost-effective. Larvicidal bioassay and fungal growth inhibitory tests were performed using several 3-(3-aryl-1,2,4-oxadiazol-5-yl)propionic acids. These acids presented strong larvicidal activity against L4 larvae of Aedes aegypti. The results suggest that larvicidal activity might be correlated with the presence of electron-withdrawing substituents in the para position of the phenyl ring except the fluorine atom. The alterations observed in the larvae spiracular valves of the siphon and anal papillae by 1,2,4-oxadiazoles in the larvicidal bioassay are responsible for larvae's death. Furthermore, all acids inhibited the fungal growth of five different types of fungi, viz., Fusarium solani, F. oxysporum, F. moniliforme, F. decemcellulare and F. lateritium in a preliminary evaluation. Both of these activities are being disclosed for the first time for 1,2,4-oxadiazole-5-yl ring linked at C-3 of propionic acid.

Synthesis of glycosyl-triazole linked 1,2,4-oxadiazoles

The synthesis of four different types of oxadiazoles containing a terminal acetylenic group is described. Reaction of these oxadiazoles with various azidoglycosides via a copper-catalyzed [3+2] cycloaddition ('click chemistry') afforded the corresponding glycosyl-triazole linked 1,2,4-oxadiazoles in good yields.

A handy and solventless direct route to primary 3-[3-aryl)-1,2,4-oxadiazol-5-yl]propionamides using microwave irradiation

A one-step, simple and straightforward synthesis of the title amides from the corresponding carboxylic acids, urea and imidazole under microwave irradiation is described.