Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Synthesis of 2,5-Dialkyl-1,3,4-oxadiazoles Bearing Carboxymethylamino Groups

A series of new symmetrical 2,5-dialkyl-1,3,4-oxadiazoles containing substituted alkyl groups at the terminal positions with substituents, such as bromine, isopropyloxycarbonylmethylamino, and carboxymethylamino, were successfully synthesized. The developed multistep method employed commercially available acid chlorides differing in alkyl chain length and terminal substituent, hydrazine hydrate, and phosphorus oxychloride. The intermediate bromine-containing 2,5-dialkyl-1,3,4-oxadiazoles were easily substituted with diisopropyl iminodiacetate, followed by hydrolysis in aqueous methanol solution giving the corresponding 1,3,4-oxadiazoles bearing carboxymethylamino substituents. The structure of all products was confirmed by conventional spectroscopic methods including ¹H NMR, ¹³C NMR, and HRMS.

Development of 1,3,4-Oxadiazole Derived Antifungal Agents and Their Application in Maize Diseases Control

Maize is an important food crop and its fungal disease has become a limiting factor to improve the yield and quality of maize. In the control of plant pathogens, commercial fungicides have no obvious effect on corn diseases due to the emergence of drug resistance. Therefore, it is of great significance to develop new fungicides with novel structure, high efficiency, and low toxicity to control maize diseases. In this paper, a series of 1,3,4-oxadiazole derivatives were designed and synthesized from benzoyl hydrazine and aromatic aldehydes through condensation and oxidation cyclization reaction. The antifungal activity of oxadiazole derivatives against three maize disease pathogens, such as Rhizoctonia solani (R. solani), Gibberella zeae (G. zeae), and Exserohilum turcicum (E. turcicum), were evaluated by mycelium growth rate method in vitro. The results indicated that most of the synthesized derivatives exhibited positive antifungal activities. Especially against E. turcicum, several compounds demonstrated significant antifungal activities and their EC ₅₀ values were lower than positive control carbendazim. The EC ₅₀ values of compounds 4k, 5e, and 5k were 50.48, 47.56, 32.25 μg/ml, respectively, and the carbendazim was 102.83 μg/ml. The effects of active compounds on E. turcicum microstructure were observed by scanning electron microscopy (SEM). The results showed that compounds 4k, 5e, and 5k could induce the hyphae of E. turcicum to shrink and collapse obviously. In order to elucidate the preliminary mechanism of oxadiazole derivatives, the target compounds 5e and 5k were docked with the theoretical active site of succinate dehydrogenase (SDH). Compounds 5e and 5k could bind to amino acid residues through hydrophobic contact and hydrogen bonds, which explained the possible mechanism of binding between the inhibitor and target protein. In addition, the compounds with antifungal activities had almost no cytotoxicity to MCF-7. This study showed that 1,3,4-oxadiazole derivatives were worthy for further attention as potential antifungal agents for the control of maize diseases.

Synthesis and Biological Activity of 1,3,4-Oxadiazoles Used in Medicine and Agriculture

Biologically active compounds play a key role in the fight against diseases affecting both human and animal living organisms, as well as plants. Finding out about new molecules with a potential biological effect, not yet described in the literature, is one of the most important aspects in the development of medicine and agriculture. Compounds showing desirable biological activity include heterocyclic moieties such as 1,3,4-oxadiazoles. The oxadiazole molecule is composed of two nitrogen atoms and one oxygen atom, forming a five-membered heterocyclic ring. Structures of this type have been successfully used in the treatment of various diseases in humans and animals, and play an important role in modern agriculture. It has been proven that many oxadiazole derivatives exhibit antibacterial, antiviral, blood pressure lowering, antifungal, antineoplastic, anticancer, antioxidant, anti-inflammatory and analgesic properties. In addition, compounds based on 1,3,4-oxadiazole can act as plant protection agents due to their herbicidal, insecticidal and fungicidal activity. Due to the constantly growing interest in heterocyclic systems of this nature, new methods of obtaining complex structures containing oxadiazole rings are sought. This article discusses various methods of synthesis of 1,3,4-oxadiazole derivatives exhibiting biological activity. Based on these techniques, these compounds could be used in the future in medicine and agriculture.

Flow synthesis of oxadiazoles coupled with sequential in-line extraction and chromatography

An efficient continuous flow process is reported for the synthesis of various 1,3,4-oxadiazoles via an iodine-mediated oxidative cyclisation approach. This entails the use of a heated packed-bed reactor filled with solid K₂CO₃ as a base. Using DMSO as solvent, this flow method generates the target heterocycles within short residence times of 10 minutes and in yields up to 93%. Scale-up of this flow process was achieved (34 mmol/h) and featured an integrated quenching and extraction step. Lastly, the use of an automated in-line chromatography system was exploited to realise a powerful flow platform for the generation of the heterocyclic targets.

Cobalt(II) nitrate promoted cyclization of benzoyl hydrazone for the synthesis of 2,5-diphenyl-1,3,4-oxadiazole derivatives

A Co(NO3)2 method to promote cyclization of benzoyl hydrazone for the formation of 2,5-diphenyl-1,3,4-oxadiazoles has been developed. The reaction proceeded smoothly and was promoted by Co(NO3)2 under air at 110 °C in DCE; 16 examples of products were obtained.

The preparation, characterization and catalytic activity of Ni NPs supported on porous alginate-g-poly(p-styrene sulfonamide-co-acrylamide)

Herein, we report the synthesis of nickel nanoparticles under mild conditions using porous alginate-g-poly(p-styrene sulfonamide-co-acrylamide) as a protecting/stabilizing agent and sodium borohydride as a reducing agent. The porous cross-linked polymeric support was prepared via combining the use of sol–gel, nanocasting, and crosslinking techniques, in which the p-styrene sulfonamide monomer (PSSA) and N,N′-methylene-bis (acrylamide) (MBA) cross-linker underwent copolymerization on the surface of sodium alginate in the presence of a SiO₂ nanoparticle (NP) template (Alg–PSSA-co-ACA). The prepared catalyst (Alg–PSSA-co-ACA@Ni) showed high catalytic activity for the one-step synthesis of 1,3,4-oxadiazoles from the reaction of hydrazides and aryl iodides through isocyanide insertion/cyclization.

Ni NPs supported on novel porous alginate-g-poly(p-styrene sulfonamide-co-acrylamide) was investigated for the synthesis of 1,3,4-oxadiazoles.

DNA-Encoded Libraries: Hydrazide as a Pluripotent Precursor for On-DNA Synthesis of Various Azole Derivatives

DNA-encoded combinatorial chemical library (DEL) technology, an approach that combines the power of genetics and chemistry, has emerged as an invaluable tool in drug discovery. Skeletal diversity plays a fundamental importance in DEL applications, and relies heavily on novel DNA-compatible chemical reactions. We report herein a phylogenic chemical transformation strategy using DNA-conjugated benzoyl hydrazine as a common versatile precursor in azole chemical expansion of DELs. DNA-compatible reactions deriving from the common benzoyl hydrazine precursor showed excellent functional group tolerance with exceptional efficiency in the synthesis of various azoles, including oxadiazoles, thiadiazoles, and triazoles, under mild reaction conditions. The phylogenic chemical transformation strategy provides DELs a facile way to expand into various unique chemical spaces with privileged scaffolds and pharmacophores.

Synthesis of 3-oxadiazole-substituted imidazo[1,2-a]pyridines by nickel immobilized on multifunctional amphiphilic porous polysulfonamide–melamine

A novel efficient and environmentally friendly approach for the synthesis of 3-oxadiazole-substituted imidazo[1,2-a]pyridines in the presence of nickel immobilized on amphiphilic polysulfonamide–melamine is described.

Iodine mediated oxidative cross coupling of 2-aminopyridine and aromatic terminal alkyne: a practical route to imidazo[1,2-a]pyridine derivatives

A novel, transition-metal free route leading to imidazo[1,2-a]pyridine derivatives via iodine mediated oxidative coupling between 2-aminopyridine and aromatic terminal alkyne has been demonstrated. This newly developed method discloses an operationally simple way for the construction of imidazoheterocycles. Commercially available antiulcer drug zolimidine may readily be synthesized employing this method.

A transition metal-free approach towards synthesis of β-carboline tethered 1,3,4-oxadiazoles via oxidative C–O bond formation

An efficient protocol has been developed for one-pot synthesis of biologically interesting β-carboline substituted 1,3,4-oxadiazoles via an I₂-assisted oxidative C–O bond formation strategy.

Oxidant- and hydrogen acceptor-free palladium catalyzed dehydrogenative cyclization of acylhydrazones to substituted oxadiazoles

An efficient method for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles has been developed through palladium(0) catalyzed dehydrogenative cyclization ofN-arylidenearoylhydrazides without oxidants and hydrogen acceptors.

Ligand-free Cu(ii)-mediated aerobic oxidations of aldehyde hydrazones leading to N,N'-diacylhydrazines and 1,3,4-oxadiazoles

A Cu(ii)-mediated synthesis of N,N'-diacylhydrazines and 1,3,4-oxadiazoles from aldehyde hydrazones has been developed. This is the first time that the synthesis of N,N'-diacylhydrazines and 1,3,4-oxadiazoles using N,N-dimethylamides as the acylation reagent and O₂ in air as the oxidation reagent is reported. These reactions offered several advantages including simple workups, ligand-free inexpensive metal salts as mediators, high yields, and wide scope of substrates.

Synthesis of novel profluorescent nitroxides as dual luminescent-paramagnetic active probes

Profluorescent nitroxides containing 2,5-diaryl-1,3,4-oxadiazoles as fluorophore units were synthesized and characterized as actively responsive probes to reducing agents.

Palladium-catalyzed one-pot synthesis of diazoles via tert-butyl isocyanide insertion

An efficient one-pot palladium-catalyzed reaction for the synthesis of diazoles from readily available hydrazides and aryl halide via isocyanide insertion/cyclization sequence has been developed. This methodology efficiently constructs diazoles in good to excellent yields with the advantages of wide functional group tolerance and operational simplicity.

I2 mediated synthesis of 5-substituted-3-methyl/benzyl-1,3,4-oxadiazol-2(3H)-ones via sequential condensation/oxidative cyclization and rearrangement

A simple and efficient iodine-assisted protocol for synthesis of 5-substituted-3-methyl/benzyl-1,3,4-oxadiazol-2(3H)-ones has been developed.

A convenient access to β-iodo sulfone by the iodine-mediated iodosulfonylation of alkenes

A novel iodine-mediated intermolecular iodosulfonylation reaction of alkenes for dual C–S and C–I bond formation was achieved.

Intramolecular decarboxylative coupling as the key step in copper-catalyzed domino reaction: facile access to 2-(1,3,4-oxadiazol-2-yl)aniline derivatives

Novel access to diverse 2-(1,3,4-oxadiazol-2-yl)aniline derivatives has been developed by introducing copper-catalyzed decarboxylative coupling for intramolecular C–O bond formation.

Tuning the thermotropic properties of liquid crystalline p-substituted aroylhydrazones

Lamellar smectic A to columnar mesophase crossover is observed in p-substituted aroylhydrazones by variation of the alkoxy chain density at peripherals.

Transition metal free intramolecular selective oxidative C(sp3)–N coupling: synthesis of N-aryl-isoindolinones from 2-alkylbenzamides

A transition metal-free intramolecular selective oxidative coupling reaction of C(sp³)–H and N–H bonds has been presented utilizing iodine, potassium carbonate and di-tert-butyl peroxide in acetonitrile at 110–140 °C.

IBX works efficiently under solvent free conditions in ball milling

Under ball-milling, the organo-oxidant IBX efficiently works at room temperature, in solvent free conditions and in milligram to gram scale.