o-Iodoxybenzoic Acid Mediated Oxidative Desulfurization Initiated Domino Reactions for Synthesis of Azoles

Iodine(V)-Based Oxidants in Oxidation Reactions

The chemistry of hypervalent iodine reagents has now become quite valuable due to the reactivity of these compounds under mild reaction conditions and their resemblance in chemical properties to transition metals. The environmentally friendly nature of these reagents makes them suitable for Green Chemistry. Reagents with a dual nature, such as iodine(III) reagents, are capable electrophiles, while iodine(V) reagents are known for their strong oxidant behavior. Various iodine(V) reagents including IBX and DMP have been used as oxidants in organic synthesis either in stoichiometric or in catalytic amounts. In this review article, we describe various oxidation reactions induced by iodine(V) reagents reported in the past decade.

Synthesis of 2-Imino-1,3,4-oxadiazolines from Acylhydrazides and Isothiocyanates via Aerobic Oxidation and a DMAP-Mediated Annulation Sequence

In this work, an efficient synthesis of 2-imino-1,3,4-oxadiazolines from acylhydrazides and isothiocyanates is described. In the presence of 4-dimethylaminopyridine (DMAP) and molecular oxygen, various 2-imino-1,3,4-oxadiazolines were produced in good to high yields. The developed method showed a broad substrate scope and was effective on the gram scale. On the basis of the mechanistic studies and previous literature, it was proposed that the mechanism consists of an aerobic oxidation of acylhydrazides facilitated by DMAP and isothiocyanates, followed by a DMAP-mediated annulation of the in situ generated acyldiazenes with isothiocyanates.

Discovery of Substituted Di(pyridin-2-yl)-1,2,4-thiadiazol-5-amines as Novel Macrofilaricidal Compounds for the Treatment of Human Filarial Infections

Filarial diseases, including lymphatic filariasis and onchocerciasis, are considered among the most devastating of all tropical diseases, affecting about 145 million people worldwide. Efforts to control and eliminate onchocerciasis are impeded by a lack of effective treatments that target the adult filarial stage. Herein, we describe the discovery of a series of substituted di(pyridin-2-yl)-1,2,4-thiadiazol-5-amines as novel macrofilaricides for the treatment of human filarial infections.

Tetrazole: A privileged scaffold for the discovery of anti-cancer agents

Carcinoma, characterized by abnormal growth of cells and tissue, is a ubiquitously leading cause of mortality across the globe due to some carcinogenic factors. Currently, several anticancer agents are commercially available in the global market. However, due to their resistance and cost, researchers are gaining more interest in developing newer novel potential anticancer agents. In the search for new drugs for clinical use, the tetrazole ring system has emerged as an exciting prospect in the optimization studies of promising lead molecules. Among the various heterocyclic agents, tetrazole-containing compounds have shown significant promise in the treatment of a wide range of diseases, particularly cancer. Here, in this review, we focused on several synthetic approaches for the synthesis of tetrazole analogues, their targets for treating cancer along with the biological activity of some of the recently reported tetrazole-containing anticancer agents.

Sulfur–DMSO promoted oxidative coupling of active methylhetarenes with amines: access to amides

The sulfur–DMSO couple was found to promote the coupling of active methylhetarenes with amines to yield amides under heating conditions. When 2-methylquinoline was used as the methylhetarene component, the reaction could be catalyzed by Fe, Co and Ni salts.

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.

Indolyl-α-keto-1,3,4-oxadiazoles: Synthesis, anti-cell proliferation activity, and inhibition of tubulin polymerization

A series of novel indolyl-α-keto-1,3,4-oxadiazole derivatives have been synthesized by employing molecular iodine-mediated oxidative cyclization of acylhydrazones. In vitro anti cell proliferation activity of these derivatives against various cancer cells lines such as human lymphoblast (U937), leukemia (Jurkat & SB) and human breast (BT474) was investigated. Among the synthesized indolyl-α-keto-1,3,4-oxadiazoles 19a-p, only one compound (19e) exhibited significant antiproliferative activity against a panel of cell lines. The compound 19e with 3,4,5-trimethoxyphenyl motif, endowed strong cytotoxicity against U937, Jurkat, BT474 and SB cancer cells with IC₅₀ values of 7.1, 3.1, 4.1, and 0.8 µM, respectively. Molecular docking studies suggested a potential binding mode for 19e in the colchicine binding site of tubulin. When tested for in vitro tubulin polymerizaton, 19e inhibited tubulin polymezations (IC₅₀ = 10.66 µM) and induced apoptosis through caspase 3/7 activation. Further, the derivative 19e did not cause necrosis when measured using lactate dehydrogenase assay.

The thioamidation of gem-dibromoalkenes in an aqueous medium

The direct integration of sulphur and amine groups with 1,1-dibromoalkenes for thioamide synthesis has been achieved in an aqueous medium. The presented green protocol emphasizes the suitability of aqueous media for the thioamidation reaction and enables greater selectivity with synthetic utility. A wide range of thioamides in moderate to excellent yields has been achieved using readily available starting materials, with the use of no organic solvents, catalysts, or additives.

Development of (4-methoxyphenyl)-1H-tetrazol-5-amine regioisomers as a new class of selective antitubercular agents

A series of halogenated (4-methoxyphenyl)-1H-tetrazol-5-amine regioisomers (1a-9a, 1b-9b) were synthesized from their corresponding thiourea analogues (1-9). The synthesis pathway was confirmed by an X-ray crystallographic studies of 1a, 1b and 5a. Title derivatives were tested for their in vitro antitubercular activity against standard, "wild-type" and atypical mycobacteria. The highest therapeutic potential was attributed to isomeric N-(bromophenyl)tetrazoles 8a and 9a. Their growth-inhibitory effect against multidrug-resistant Mycobacterium tuberculosis Spec. 210 was 8-16-fold stronger than that of the first-line tuberculostatics. Other new tetrazole-derived compounds were also more or equally effective towards that pathogen comparing to the established pharmaceuticals. Among non-tuberculous strains, Mycobacterium scrofulaceum was the most susceptible to the presence of the majority of tetrazole derivatives. The synergistic interaction was found between 9a and streptomycin, as well as the additivity of both 8a and 9a in pairs with isoniazid, rifampicin and ethambutol. None of the studied compounds displayed antibacterial or cytotoxic properties against normal and cancer cell lines, which indicated their highly selective antimycobacterial effects.

One-Pot Parallel Synthesis of 5-(Dialkylamino)tetrazoles

Two protocols for the combinatorial synthesis of 5-(dialkylamino)tetrazoles were developed. The best success rate (67%) was shown by the method that used primary and secondary amines, 2,2,2-trifluoroethylthiocarbamate, and sodium azide as the starting reagents. The key steps included the formation of unsymmetrical thiourea, subsequent alkylation with 1,3-propane sultone and cyclization with azide anion. A 559-member aminotetrazole library was synthesized by this approach; the overall readily accessible (REAL) chemical space covered by the method exceeded 7 million feasible compounds.

Micelle-Enabled One-Pot Guanidine Synthesis in Water Directly from Isothiocyanate using Hypervalent Iodine(III) Reagents under Mild Conditions

In this work, we developed a one-pot synthesis of guanidine directly from isothiocyanate using DIB (diacetoxyiodobenzene) as a desulfurizing agent under micellar conditions in water. Our optimization study revealed that the use of 1 % TPGS-750-M as a surfactant with NaOH as an additive base at room temperature can convert a variety of isothiocyanates and amines into corresponding guanidines in excellent yields (69-95 %). This synthetic process in water can be applied to prepare guanidine at gram-scale quantity. Our aqueous micellar medium also demonstrated high reusability as the reaction can be performed for several cycles without losing its efficiency. The reaction is metal-free, utilizes water as solvent and practical (room temperature and open flask).

Construction of 5-Aminotetrazoles via in Situ Generation of Carbodiimidium Ions from Ketones Promoted by TMSN3/TfOH

A novel synthetic approach for the synthesis of 5-aminotetrazoles has been developed by employing simple ketones as substrates. This methodology involved the N₂-extrusion/aryl migration of azido complexes as the key step for the in situ generation of carbodiimidium ion, which could further react with hydrazoic acid and cyclize intramolecularly to provide 5-aminotetrazoles in good to excellent yields. In addition, the regioselectivity of the reaction was studied and rationalized by density functional theory calculations.

A Useful Synthesis of 2-Acylamino-1,3,4-oxadiazoles from Acylthiosemicarbazides Using Potassium Iodate and the Discovery of New Antibacterial Compounds

A useful method for the synthesis of 2-acylamino-1,3,4-oxadiazoles was developed. By using potassium iodate as an oxidant in water at 60 °C, a wide range of 2-acylamino-1,3,4-oxadiazoles were afforded in moderate to excellent yields within two hours. This method could provide a facile shortcut to generate a series of 2-acylamino-1,3,4-oxadiazoles in medicinal chemistry. Interestingly, some highly potent antibiotic compounds were found through this synthetic method, and some of them displayed a significant improvement in activity compared with the corresponding 1,4-diacylthiosemicarbazides. Compound 2n was the most active against Staphylococcus aureus with MIC (minimum inhibitory concentration) of 1.56 mg/mL, and compounds 2m and 2q were the most active against Bacillus subtilis with MIC of 0.78 mg/mL. The preliminary cytotoxic activities of the most potent compounds 2m, 2n, and 2q against the androgen-independent (PC-3) prostate cancer cell line were more than 30 μM (IC₅₀ > 30 μM).

Hetero-bimetallic cooperative catalysis for the synthesis of heteroarenes

Review covering the synthesis of 5- and 6-membered as well as condensed heteroarenes, focussing on the combinations in cooperative catalytic systems in strategies used to achieve selectivity and also highlights the mode of action for the cooperative catalysis leading to the synthesis of commercially and biologically relevant heteroarenes.

Design and synthesis of novel 1H-tetrazol-5-amine based potent antimicrobial agents: DNA topoisomerase IV and gyrase affinity evaluation supported by molecular docking studies

A total of 14 of 1,5-disubstituted tetrazole derivatives were prepared by reacting appropriate thiourea and sodium azide in the presence of mercury (II) chloride and triethylamine. All compounds were evaluated in vitro for their antimicrobial activity. Derivatives 10 and 11 showed the highest inhibition against Gram-positive and Gram-negative strains (standard and hospital strains). The observed minimal inhibitory concentrations values were in the range of 1-208 μM (0.25-64 μg/ml). Inhibitory activity of 1,5-tetrazole derivatives 10 and 11 against gyrase and topoisomerase IV isolated from S. aureus was studied. Evaluation was supported by molecular docking studies for all synthesized derivatives and reference ciprofloxacin. Moreover, selected tetrazoles (2, 3, 5, 6, 8, 9, 10 and 11) were evaluated for their cytotoxicity. All tested compounds are non-cytotoxic against HaCaT and A549 cells (CC₅₀ ≤ 60 μM).

Current Parallel Solid-Phase Synthesis of Drug-like Oxadiazole and Thiadiazole Derivatives for Combinatorial Chemistry

Solid-phase organic synthesis is a powerful tool in the synthesis of small organic molecules and building of libraries of compounds for drug discovery. Heterocyclic compounds are important components of the drug discovery field as well and serve as a core for hundreds of marketed drugs. In particular, oxadiazole and thiadiazole cores are compounds of great interest due to their comprehensive biological activities and structural features. Therefore, a plethora of oxadiazole and thiadiazole synthesis methodologies have been reported to date, including solution and solid-phase synthesis methodologies. In this review, we concentrate on and summarize solid-phase synthetic approaches of the oxadiazole and thiadiazole derivatives.

New lipophilic isoniazid derivatives and their 1,3,4-oxadiazole analogues: Synthesis, antimycobacterial activity and investigation of their mechanism of action

The development of novel drugs is essential for the treatment of tuberculosis and other mycobacterial infections in future. A series of N-alkyl-2-isonicotinoylhydrazine-1-carboxamides was synthesized from isoniazid (INH) and then cyclized to N-alkyl-5-(pyridin-4-yl)-1,3,4-oxadiazole-2-amines. All derivatives were characterised spectroscopically. The compounds were screened for their in vitro antimycobacterial activity against susceptible and multidrug-resistant Mycobacterium tuberculosis (Mtb.) and nontuberculous mycobacteria (NTM; M. avium, M. kansasii). The most active carboxamides were substituted by a short n-alkyl, their activity was comparable to INH with minimum inhibitory concentrations (MICs) against Mtb. of 0.5-2 μM. Moreover, they are non-toxic for HepG2, and some of them are highly active against INH-resistant NTM (MICs ≥4 μM). Their cyclization to 1,3,4-oxadiazoles did not increase the activity. The experimentally proved mechanism of action of 2-isonicotinoylhydrazine-1-carboxamides consists of the inhibition of enoyl-ACP reductase (InhA) in a way similar to INH, which is blocking the biosynthesis of mycolic acids. N-Dodecyl-5-(pyridin-4-yl)-1,3,4-oxadiazol-2-amine as the most efficacious oxadiazole inhibits growth of both susceptible and drug-resistant Mtb. strains with uniform MIC values of 4-8 μM with no cross-resistance to antitubercular drugs including INH. The mechanism of action is not elucidated but it is different from INH. Obtained results qualify these promising derivatives for further investigation.

o-Iodoxybenzoic acid mediated generation of aryl free radicals: synthesis of stilbenes through C–C cross-coupling with β-nitrostyrenes

The generation of an aryl free radicals in the presence of nitrostyrenes through a combination of aryl hydrazines and o-iodoxybenzoic acid led to the synthesis of stilbenes by forming a new carbon–carbon bond after subsequent elimination of a nitrosyl radical.

Isothiocyanyl Alanine as a Synthetic Intermediate for the Synthesis of Thioureayl Alanines and Subsequent Aminotetrazolyl Alanines

The synthesis of unnatural amino acids with small side-chain functionalities usable for further transformations is highly demanding for the expansion of the genetic code and other possible biotechnological applications. To this end, we wanted to report the utility of an unexplored unnatural amino acid, isothiocyanyl alanine (^NCSAla = Ita), for the synthesis of another class of unnatural amino acids, thioureayl alanines (^TUAla = Tua). The synthesis of a third class of unnatural amino acids, amino tetrazolyl alanines (^ATzAla = Ata), in a very good yield was subsequently achieved utilizing thioureayl alanines. Thus, a variety of aliphatic- and aromatic-substituted thioureayl alanines and aromatic-substituted amino tetrazolyl alanines were successfully synthesized in good to excellent yields. The photophysical properties of three of the fluorescent unnatural amino acids from two classes were also studied and presented herein.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Synthesis of 2-amino-1,3,4-oxadiazoles and 2-amino-1,3,4-thiadiazoles via sequential condensation and I2-mediated oxidative C–O/C–S bond formation

2-Amino-substituted 1,3,4-oxadiazoles and 1,3,4-thiadiazoles were synthesized via condensation of semicarbazide/thiosemicarbazide and the corresponding aldehydes followed by I2-mediated oxidative C–O/C–S bond formation. This transition-metal-free sequential synthesis process is compatible with aromatic, aliphatic, and cinnamic aldehydes, providing facile access to a variety of diazole derivatives bearing a 2-amino substituent in an efficient and scalable fashion.