Advances in isoxazole chemistry and their role in drug discovery

Isoxazoles are a class of five-membered heterocyclic compounds that have gained significant attention in medicinal chemistry due to their diverse biological activities and therapeutic potential. Recent advances in isoxazole chemistry have led to the development of novel synthetic strategies, enabling the creation of a wide array of isoxazole derivatives with enhanced bioactivity and selectivity. This review explores the latest progress in isoxazole synthesis, highlighting key methodologies such as transition metal-catalyzed cycloadditions, green chemistry approaches, and regioselective functionalization techniques. These advances have not only improved the efficiency of isoxazole synthesis but have also facilitated the design of more complex and bioactive derivatives. In addition to their synthetic advances, isoxazoles have demonstrated a broad spectrum of biological activities, including antimicrobial, anticancer, anti-inflammatory, and neuroprotective effects, making them attractive candidates in drug discovery. This review discusses the structural modifications that enhance their pharmacological properties and their potential for developing therapies for diseases such as cancer, neurodegenerative disorders, and infections. Moreover, we examine the emerging trends in isoxazole-based drug discovery, such as the development of multi-targeted therapies and personalized medicine approaches. The evolving role of isoxazoles in drug discovery underscores their continued importance in modern pharmaceutical research and their potential to address unmet medical needs.

Cu-Catalyzed Electrochemical Activation of Nitromethane to Access Aldoxime-Substituted Nitrile Oxide for Huisgen Reaction

Here, we report an electrochemical approach to generate an aldoxime-substituted nitrile oxide via the activation of nitromethane. The Cu-catalyzed Huisgen reaction of this 1,3-dipole with alkynes enables successful preparation of 48 new isoxazole aldoximes, which are typically challenging to synthesize by other methods, in 52 to 97% yields with excellent regioselectivity and chemoselectivity in a single step. Moreover, 20 3,3'-bisisoxazoles are prepared from the isoxazole aldoxime products in good yields via a two-step sequence.

Sustainable and solvent-free synthesis of molecules of pharmaceutical importance by ball milling

The solvent-free mechanochemical reactions under ball milling have emerged as a promising alternative to traditional solution-based chemistry. This approach not only eliminates the necessity for large quantities of solvents and minimizes waste production, but it also facilitates a unique reaction environment that enables strategies, reactions, and compound syntheses that are typically unattainable in solution. This solvent-less synthetic strategy under ball-milling has been well employed in synthetic organic chemistry in accessing various potential organic molecules including pharmaceutically important molecules and pharmaceuticals or drug-molecules. This review highlights the potential of ball milling in the synthesis of pharmaceutically important classes of molecules without using any solvent (solvent-free conditions).

5-Nitroisoxazoles in SNAr Reactions: A Novel Chemo- and Regioselective Approach to Isoxazole-Based Bivalent Ligands of AMPA Receptors

An efficient regioselective approach to novel functionalized bis(isoxazoles) with a variety of aromatic and aliphatic linkers was elaborated, based on the heterocyclization reaction of electrophilic alkenes under the treatment with tetranitromethane-triethylamine complex affording 3-EWG-5-nitroisoxazoles. The subsequent SNAr reactions of 5-nitroisoxazoles with various O,O-, N,N- and S,S-bis(nucleophiles) provide a wide range of bis(isoxazole) derivatives in good isolated yields. Employing an elaborated method, a series of novel bis(3-EWG-isoxazoles) as the promising allosteric modulators of AMPA receptors were designed and synthesized. The effect of the compounds on the kainate-induced currents was studied in the patch clamp experiments, revealing modulator properties for several of them. The best positive modulator potency was found for dimethyl 5,5'-(ethane-1,2-diylbis(sulfanediyl))bis(isoxazole-3-carboxylate), which potentiated the kainate-induced currents in a wide concentration range (10-12-10-6 M) with maximum potentiation of 77% at 10-10 M. The results were rationalized using molecular docking and molecular dynamics simulations of modulator complexes with the dimeric ligand-binding domain of the GluA2 AMPA receptor. The predicted physicochemical, ADMET, and PAINS properties confirmed that the AMPA receptor modulators based on the bis(isoxazole) scaffold may serve as potential lead compounds for the development of neuroprotective drugs.

Heterocyclic Compounds as Hsp90 Inhibitors: A Perspective on Anticancer Applications

Heat shock proteins (Hsps) have garnered special attention in cancer therapy as molecular chaperones with regulatory/mediatory effects on folding, maintenance/stability, maturation, and conformation of proteins as well as their effects on prevention of protein aggregation. Hsp90 ensures the stability of various client proteins needed for the growth of cells or the survival of tumor cells; therefore, they are overexpressed in tumor cells and play key roles in carcinogenesis. Accordingly, Hsp90 inhibitors are recognized as attractive therapeutic agents for investigations pertaining to tumor suppression. Natural Hsp90 inhibitors comprising geldanamycin (GM), reclaimed analogs of GM including 17-AAG and DMAG, and radicicol, a natural macrocyclic antifungal, are among the first potent Hsp90 inhibitors. Herein, recently synthesized heterocyclic compounds recognized as potent Hsp90 inhibitors are reviewed along with the anticancer effects of heterocyclic compounds, comprising purine, pyrazole, triazine, quinolines, coumarin, and isoxazoles molecules.

An isoxazole strategy for the synthesis of 4-oxo-1,4-dihydropyridine-3-carboxylates

A method has been developed for the preparation of 2-alkyl-6-aryl-, 2-aryl-6-aryl and 2,6-diaryl-5-aryl/hetaryl-substituted methyl 4-oxo-1,4-dihydropyridine-3-carboxylates by Mo(CO)₆-mediated ring expansion of methyl 2-(isoxazol-5-yl)-3-oxopropanoates. The high reactivity of 4-oxo-1,4-dihydropyridine-3-carboxylates synthesized provide easy access to 2,4,6-triaryl-substituted and 1,2,5,6-tetrasubstituted nicotinates.

Efficient Catalytic Synthesis of Condensed Isoxazole Derivatives via Intramolecular Oxidative Cycloaddition of Aldoximes

The intramolecular oxidative cycloaddition reaction of alkyne- or alkene-tethered aldoximes was catalyzed efficiently by hypervalent iodine(III) species to afford the corresponding polycyclic isoxazole derivatives in up to a 94% yield. The structure of the prepared products was confirmed by various methods, including X-ray crystallography. Mechanistic study demonstrated the crucial role of hydroxy(aryl)iodonium tosylate as a precatalyst, which is generated from 2-iodobenzoic acid and m-chloroperoxybenzoic acid in the presence of a catalytic amount of p-toluenesulfonic acid.

A Study on Synthesis of Chalcone Derived -5- Membered Isoxazoline and Isoxazole Scaffolds

Chalcone-derived Isoxazole scaffolds remain the central focus because of their greater biological, clinical, and pharmacological properties. The present study reviews the synthesis of various chalcone derived - 5- Membered Isoxazoline and Isoxazole Scaffolds with the available literature until 2021. Keywords: Chalcones, Isoxazoles, Isoxazolines, Biological and Pharmacological properties.

Synthesis, Bioactivity, and QSAR Study of 3,4-Dichlorophenyl Isoxazole-Substituted Stilbene Derivatives against the Phytopathogenic Fungus Botrytis cinerea

Gray mold, caused by Botrytis cinerea, is one of the most destructive fungal diseases in crops, responsible for significant economic losses. In search of natural product-based fungicides, we designed and synthesized a series of novel 3,4-dichlorophenyl isoxazole-substituted stilbene derivatives, and their in vivo antifungal activities against B. cinerea were evaluated. The results indicated that some of the target molecules demonstrated remarkable efficiency for the control of tomato gray mold. In particular, compound 5r displayed the highest fungicidal potency with an inhibition rate of 56.11% comparable to that of positive control boscalid (66.96%). Moreover, a hologram quantitative structure-activity relationship (HQSAR) model with good predictive capability was developed to provide in-depth insight into the activity profiles of these compounds. Preliminary mechanism studies suggested that compound 5r might exert its antifungal effect by changing hyphal morphology and increasing the membrane permeability. The present study contributes to the development of natural stilbene derivatives as alternative bioactive agents against B. cinerea.

5-Nitroisoxazoles in SNAr reactions: access to polysubstituted isoxazole derivatives

An efficient protocol for the straightforward functionalization of the isoxazole ring via the reactions of aromatic nucleophilic substitution of the nitro group with various nucleophiles has been elaborated. The method features excellent chemical yields, easy operability of the reaction, mild reaction conditions and a broad scope of both 5-nitroisoxazoles and nucleophiles. A synthetic approach to 3,5- and 3,4,5-substituted isoxazoles via the sequential functionalization of the isoxazole ring has been developed based on the excellent regioselectivity of the reaction of 3,5-dinitroisoxazoles with nucleophiles.

1,3-Dipolar Cycloaddition Reactions of Nitrile Oxides under "Non-Conventional" Conditions: Green Solvents, Irradiation, and Continuous Flow

The 1,3-dipolar cycloaddition reactions (DCs) of nitrile oxides (NOs) to alkenes and alkynes are useful methods for the synthesis of 2-isoxazolines and isoxazoles respectively, which are important classes of heterocyclic compounds in organic and medicinal chemistry. Most of these reactions are carried out in organic solvents and under thermal activation. Nevertheless the use of supercritical carbon dioxide (scCO₂ ) and ionic liquids (Ils) as alternative solvents and the application of microwave (MW) and ultrasound (US) as alternative activation procedures have evident advantages from the "Green Chemistry" point of view. The critical discussion on the applications of these "unconventional" activation methods and reaction conditions in the 1,3-DCs of NOs is the objective of the present Review.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Synthesis of Isoxazole- and Oxazole-4-carboxylic Acids Derivatives by Controlled Isoxazole-Azirine-Isoxazole/Oxazole Isomerization

The first synthesis of isoxazole-4-carboxylic acid derivatives by domino isoxazole-isoxazole isomerization is reported. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxy-/5-aminoisoxazoles (dioxane, 105 °C) leads to the formation of isoxazole-4-carboxylic esters and amides in good yields. 4-Formyl-5-methoxyisoxazoles give methyl oxazole-4-carboxylates under the same reaction conditions. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxyisoxazoles under milder conditions (MeCN, 50 °C) allows the preparation of transient 2-acyl-2-(methoxycarbonyl)-2H-azirines. The azirines isomerize quantitatively either into isoxazoles under catalytic conditions (dioxane, 105 °C) or into oxazoles under noncatalytic thermolysis (o-dichlorobenzene, 170 °C). The mechanism of the isomerization and dependence of the reaction routes on substituents at starting isoxazole core and reaction conditions are discussed on the basis of DFT calculations.

3,3-Diazidoenones – new types of highly reactive bis-azides. Preparation and synthetic transformations

A new family of highly reactive 3,3-diazidoenones was obtained. Their chemistry was investigated by both experimental studies and DFT calculations.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

In situ generation of nitrile oxides from copper carbene and tert-butyl nitrite: synthesis of fully substituted isoxazoles

Herein, we present a novel [3 + 2] cycloaddition reaction of β-keto esters with nitrile oxides, which were generated in situ from copper carbene and tert-butyl nitrite. This three-component reaction provides new methodology for the direct synthesis of fully substituted isoxazole derivatives, featuring mild reaction conditions, readily accessible starting materials and simple operation. The experimental studies and DFT calculations suggest that the reaction starts with the generation of the key intermediate nitrile oxides, followed by a [3 + 2] cycloaddition reaction of β-keto esters to give the final isoxazole products.

Synthesis of Bis(Isoxazol-4-Ylmethylsulfanyl)Alkanes and Some Metal Complexes as a Hepatoprotective Agents

Purpose: This research is devoted to designing the synthesis of sulfanyl-substituted 3,5-dimethylisoxazoles, which contain structural analogues of the SAM drug in the molecule. SAM (S-adenosyl-L-methionine), formed in the biosynthetic process, is used as an effective hepatoprotective drug. Complexation and hepatoprotective properties of the combinatorial series of bis(isoxazolylsulfanyl)ethane have been studied. Methods: Bis(isoxazol-4-ylmethylsulfanyl)alkanes were synthesized using the one-pot method. The structures of compounds were established by one-dimensional (1H,13C) and two-dimensional (COSY, HCQS, HMBC) NMR spectroscopy, mass-spectrometry and X-ray diffraction. The biological activity of the combinatorial series of sulfanyl derivatives of diketones, azoles and their metal complexes has been studied by in vivo method. Simulation of the animal associated processes was carried out in accordance with the principles of bioethics. Screening studies of hepatoprotective activity were carried out in a model of acute CC14 intoxication after a single injection intraperitoneally as a 50% solution in olive oil. The pharmacologically known hepatoprotective drug SAM served as a control. Results: Two-step synthesis of novel α,ω-bis(isoxazol-4-ylmethylsulfanyl)alkanes was carried out via the multicomponent reaction between 2,4-pentandione, CH2O and α,ω-dithiols, then the resulting α,ω-bis(1,3-diketone-2-ylmethylsulfanyl)alkanes were transformed by hydroxyl amine to obtain bis-isoxasole derivatives. Promising precursor 1,2-bis(isoxazol-4-ylmethylsulfanyl)ethane was converted to metal complexes by interaction with PdCl2 or CuCl. The obtained compounds were found to be practically non-toxic compounds (1001 - 3000 mg/kg) according to the classification of K.K. Sidorov, but copper complex refers to low-toxic compounds substances (165 mg/kg). Compounds of sulfanyl ethane series demonstrate hepatoprotective activity. Conclusion: Palladium(II) complex being almost non-toxic possesses hepatoprotective activity comparable to the drug like SAM.

Metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides

In this review recent advances in the metal-catalyzed 1,3-dipolar cycloaddition reactions of nitrile oxides are highlighted, covering references from the period 2000 until August 2018.

Fe(II)/Au(I) Relay Catalyzed Propargylisoxazole to Pyridine Isomerization: Access to 6-Halonicotinates

An efficient synthesis of methyl nicotinates/6-halonicotinates by the domino isomerization of 4-propargyl/(3-halopropargyl)-5-methoxyisoxazoles under Fe(II)/Au(I) relay catalysis was developed. It was found that FeNTf₂ is an effective catalyst for first step of the domino isomerization, transformation of isoxazole to 2H-azirine, which is compatible with Ph₃PAuNTf₂, catalyzing the second step.