Recent advances in the total syntheses of oxazole-containing natural products

Product branching in the photodissociation of oxazole detected by broadband rotational spectroscopy

The photodissociation of oxazole (c-C3H3NO) following excitation at 193 nm is studied using mm-Wave rotational spectroscopy in a uniform supersonic flow. Molecules entrained in the flow are excited to a ππ* state after which it is believed most relax back to the ground state via ring opening at the O-C[N] bond with subsequent fragmentation. From the line intensities of the probed products, we obtained the branching fractions for seven different products which are the result of five different dissociation pathways. The detected photoproducts and respective branching fractions (%) are the following: HCN (70.4), HCO (22.8), CH2CN (4.2), CH2CO (1.0), CH3CN (1.0), HNC (0.9), HNCO (0.08). We suspect much of the HCO may be formed in conjunction with the isocyanomethyl radical, CH2NC, which we did not probe. We discuss our results in relation to previous work, in particular our own study on the related isomer isoxazole, as well as direct dynamics theoretical simulations from the literature. We also studied the relaxation of a number of vibrationally excited levels of HCN produced at 20 K.

Metal free C-O bond cleavage: a new strategy for the synthesis of substituted oxazoles

A strategy for the efficient metal-free C-O bond cleavage of ester using amines for the synthesis of substituted oxazoles was developed for the first time. The synthesis proceeded smoothly under metal-free conditions, combining C-O bond cleavage as well as C-N and C-O bond formation in one pot to yield desired products in moderate to excellent yields, and accommodated a wide range of functional groups and substrates.

Natural and synthetic 5-(3'-indolyl)oxazoles: Biological activity, chemical synthesis and advanced molecules

5-(3'-Indolyl)oxazole moiety is a privileged heterocyclic scaffold, embedded in many biologically interesting natural products and potential therapeutic agents. Compounds containing this scaffold, whether from natural sources or synthesized, have demonstrated a wide array of biological activities. This has piqued the interest of synthetic chemists, leading to a large number of reported synthetic approaches to 5-(3'-indolyl)oxazole scaffold in recent years. In this review, we comprehensively overviewed the different biological activities and chemical synthetic methods for the 5-(3'-indolyl)oxazole scaffold reported in the literatures from 1963 to 2024. The focus of this study is to highlight the significance of 5-(3'-indolyl)oxazole derivatives as the lead compounds for the lead discovery of anticancer, pesticidal, antimicrobial, antiviral, antioxidant and anti-inflammatory agents, to summarize the synthetic methods for the 5-(3'-indolyl)oxazole scaffold. In addition, the reported mechanism of action of 5-(3'-indolyl)oxazoles and advanced molecules studied in animal models are also reviewed. Furthermore, this review offers perspectives on how 5-(3'-indolyl)oxazole scaffold as a privileged structure might be exploited in the future.

An Update on the Nitrogen Heterocycle Compositions and Properties of U.S. FDA-Approved Pharmaceuticals (2013-2023)

This Perspective is a continuation of our analysis of U.S. FDA-approved small-molecule drugs (1938-2012) containing nitrogen heterocycles. In this study we report drug structure and property analyses of 321 unique new small-molecule drugs approved from January 2013 to December 2023 as well as information about frequency of important heteroatoms such as sulfur and fluorine and key small nitrogen substituents (CN and NO2). The most notable change is an incredible increase in drugs containing at least one nitrogen heterocycle─82%, compared to 59% from preceding decades─as well as a significant increase in the number of nitrogen heterocycles per drug. Pyridine has claimed the #1 high-frequency nitrogen heterocycle occurrence spot from piperidine (#2), with pyrimidine (#5), pyrazole (#6), and morpholine (#9) being the big top 10 climbers. Also notable is high number of fused nitrogen heterocycles, apparently driven largely by newly approved cancer drugs.

Enhancing C-S and C-N bond formation with ultrasound assistance: lipase-catalyzed synthesis of 2,4-disubstituted thiazole derivatives from arylethanones and thioamides

The present study explores an innovative approach for the efficient synthesis of 2,4-disubstituted thiazole derivatives, a class of compounds with diverse biological and pharmaceutical significance. This research presents lipase as a highly effective and environmentally friendly catalyst for thiazole synthesis. Under mild circumstances, the condensation of aryl ethenone, KBrO3, and thioamide is aided by using ultrasonic energy. Moreover, we harness the power of ultrasound irradiation to accelerate the reaction, reducing reaction times and improving product yields. The lipase-catalyzed, ultrasound-assisted synthesis presented in this study represents a greener and more sustainable alternative to traditional synthetic pathways for these important compounds, offering promising potential for applications in medicinal chemistry and drug development. This approach holds the promise of advancing the field of thiazole synthesis, contributing to more sustainable and efficient chemical processes.

Design, Synthesis, Anticancer Evaluation, and Molecular Docking Studies of Oxazole-Incorporated Naphthyridine Derivatives

A novel series of oxazole incorporated naphthyridine (21 a-j) derivatives were designed and, synthesized followed by screening of their anticancer activity profiles against human breast cancer (MCF-7), human lung cancer (A549) and human prostate (PC3 & DU-145) cancer cell lines by employing MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide] assay using etoposide as the positive control. Of these compounds, N-(6-chloro-3-(4-(3,4,5-trimethoxyphenyl)oxazol-2-yl)-1,5-naphthyridin-4-yl)oxazol-2-amine with 3,4,5-trimethoxy substituent on the aryl moiety attached to oxazole ring showed potent anticancer activity against PC3, A549, MCF-7, and DU-145 cell lines with IC50 values of 0.13±0.095 μM; 0.10±0.084 μM; 0.18±0.087 μM and 0.15±0.076 μM respectively. Apart from this, compounds N-(6-chloro-3-(4-(3,5-dimethoxyphenyl)oxazol-2-yl)-1,5-naphthyridin-4-yl)oxazol-2-amine, N-(6-chloro-3-(4-(4-methoxyphenyl)oxazol-2-yl)-1,5-naphthyridin-4-yl)oxazol-2-amine, and N-(6-chloro-3-(4-(3,5-dimethylphenyl)oxazol-2-yl)-1,5-naphthyridin-4-yl)oxazol-2-amine also showed better anticancer activities against four cancer cell lines screened for. These activities were also validated through the molecular docking simulations, which further indicated demonstration of better interaction energy and profile by these compounds.

Dicationic copper(I) complexes bearing ENE (E = S, Se) pincer ligands; catalytic applications in regioselective cyclization of 1,6-diynes

Two novel dicationic binuclear Cu(I) complexes of the type [{(BPPP)E2}Cu]2[BF4]2 (E = S (3a); Se (3b)) bearing (BPPP)E2 (BPPP = bis(diphenylphosphino)pyridine) pincer systems were isolated, and structurally characterized. The solid-state structures of 3a/3b display the presence of intermolecular cuprophilic (Cu⋯Cu) interactions between the two monocationic species, and consist of weak Cu⋯S bonding between the two cations. Besides, complex 3a was introduced as a molecular copper(I) catalyst in cyclization reactions, and new protocols were developed for the synthesis of a series of new oxazole and triazole derivatives bearing alkyne-phenyl propargylic ether substituents. 3a was also found to be active in achieving these two classes of heterocyclic compounds by the mechanical grinding method. One of the key intermediate copper-azide species was detected by the high-resolution mass spectrometry technique, which supports the proposed catalytic pathway. All the reported transformations were accomplished sustainably by employing a well-defined, earth-abundant, and cheap copper(I) catalytic system.

TBAI-Mediated Cyclization and Methylsulfonylation of Propargylic Amides with Dimethyl Sulfite

A tetramethylammonium iodide (TBAI)-mediated cyclization and methylsulfonylation of propargylic amides enabled by dimethyl sulfite as a SO₂ surrogate and methyl source have been developed. The transition metal-free and oxidant-free reaction provides a practical and efficient approach for the selective synthesis of methylsulfonyl oxazoles in moderate to excellent yields with good functional group compatibility.

Nickel/Copper Cooperative Catalysis Decarbonylative Heteroarylation of Aryl Anhydrides with Benzoxazoles via C-O/C-H Coupling

A new strategy for the synthesis of 2-arylbenzoxazole derivatives via nickel-/copper-catalyzed decarbonylative heteroarylation of aryl anhydrides via C-O/C-H coupling has been developed. The reaction is promoted by a user-friendly, inexpensive, and air- and moisture-stable Ni precatalyst. A variety of 2-arylbenzoxazole derivatives have been successfully synthesized and have good functional group tolerance in this process, which afforded products in moderate-to-excellent yields.

(Z)-5-Benzylidene-4-phenyl-2-(p-tolyl)-4,5-dihydrooxazole

By strategic use of the valence difference between hard gold(III) and soft gold(I) catalysts, one-pot synthesis of (Z)-5-benzylidene-4-phenyl-2-(p-tolyl)-4,5-dihydrooxazole (15) from propargylic alcohol (9) and p-toluamide (13) was achieved via gold(III)-catalyzed propargylic substitution followed by gold(I)-catalyzed cyclization. The structure of 15 was confirmed by X-ray crystallographic analysis.

Lewis Acid-Promoted Three-Component Cyclization for the Construction of Functionalized Oxazoles

A simple and efficient synthetic strategy from amides, ynals, and sodium sulfinates via a Lewis acid-promoted three-component reaction has been reported. Thus, a broad range of various aryl (not alkyl)-substituted oxazoles could be synthesized via the formation of C-N, C-O, and C-S bonds in a one-pot process. In addition, this reaction possesses other unique advantages, such as transition metal-free catalysis, high step economy, good functional group tolerance, and good regioselectivity.

Cascade Nucleophilic Attack/Addition Cyclization Reactions to Synthesize Oxazolidin-2-imines via (Z)-2-Bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and Diphenyl Carbodiimides

Two concise strategies to synthesize oxazolidin-2-imines by cascade nucleophilic attack/addition cyclization reactions of (Z)-2-bromo-3-phenylprop-2-en-1-ols/3-phenylprop-2-yn-1-ols and diphenyl carbodiimides without a transition-metal catalyst have been developed. The reactions exhibited good substrate applicability tolerance, and a variety of substituted (Z)-4-((Z)-benzylidene)-N,3-diphenyloxazolidin-2-imines were synthesized in moderate to excellent yields with good stereoselectivity. The reports also provided a convenient strategy to synthesize 3-phenylprop-2-yn-1-ols by (Z)-2-bromo-3-phenylprop-2-en-1-ols. The economic and practical methods provide a great advantage for potential industrial synthesis of oxazolidin-2-imines.

Synthesis of Tetrahydrofuro[2,3-d]oxazoles and Oxazoles by Hypervalent Iodine (III)-Promoted [2 + 2 + 1] Annulation

Reaction of a hypervalent iodine reagent with bistriflimide efficiently promotes three-component regioselective cyclization of tetrahydrofuro[2,3-d]oxazoles and oxazoles from homopropargyl alcohols bearing a phenyl group, with different substituents on the aryl alkyne compounds affecting the selectivity of the resulting product. Utilizing the hydroxyethyl oxazole derivatives obtained in this research could aid in the development of various peroxisome proliferator-activated receptor agonist derivatives.

Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art

Abstract:

Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limitations of the various developed arylation protocols, also comparing their results with those achieved in analogous Pd-catalysed arylation reactions involving the use of aryl halides as electrophiles. Mechanistic proposals have also been briefly summarised and discussed. However, data concerning Pd-catalysed direct C–H bond arylations involving the C–H bonds of aryl substituents of the examined heteroarene derivatives have not been taken into account.

An umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition of aryl aldehydes and nitriles: a facile access to 2,4,5-trisubstituted oxazoles

We have described the first example of an umpolung strategy for intermolecular [2 + 2 + 1] cycloaddition between two aryl aldehydes and a nitrile under the influence of TMSOTf that proceeds through the formation of N–C, O–C and C–C bonds providing a simple synthetic protocol for obtaining 2,4,5-trisubstituted oxazoles.

An unprecedented intermolecular [2 + 2 + 1] cycloaddition strategy between two aryl aldehydes and a nitrile, wherein one of the aryl aldehydes serves as a carbanion (or equivalent) in the presence of TMSOTf for obtaining oxazole framework is presented.

Gold-catalyzed synthesis of oxazoles from alkynyl triazenes and dioxazoles

A gold-catalyzed regioselective [3 + 2] cycloaddition of alkynyl triazenes with 1,2,4-dioxazoles was developed.

K2S2O8-promoted rearrangement of nitrones for the synthesis of benzo[d]oxazoles

An efficient route for the synthesis of valuable benzoxazoles has been developed through self-oxidative cyclization with N–O bond cleavage.

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

The preparation of aromatic hydride-osmaoxazolium and hydride-oxazole compounds is reported and their reactivity toward phenylacetylene investigated. Complex [OsH(OH)(≡CPh)(IPr)(PiPr3)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF3SO3) reacts with acetonitrile and benzonitrile to give [OsH{κ2-C,O-[C(Ph)NHC(R)O]}(NCR)(IPr)(PiPr3)]OTf (R = Me (2), Ph (3)) via amidate intermediates, which are generated by addition of the hydroxide ligand to the nitrile. In agreement with this, the addition of 2-phenylacetamide to acetonitrile solutions of 1 gives [OsH{κ2-C,O-[C(Ph)NHC(CH2Ph)O]}(NCCH3)(IPr)(PiPr3)]OTf (4). The deprotonation of the osmaoxazolium ring of 2 and 4 leads to the oxazole derivatives OsH{κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (5), CH2Ph (6)). Complexes 2 and 4 add their Os-H and Os-C bonds to the C-C triple bond of phenylacetylene to afford [Os{η3-C 3 ,κ1-O-[CH2C(Ph)C(Ph)NHC(R)O]}(NCCH3)2(IPr)]OTf (R = Me (7), CH2Ph (8)), bearing a tridentate amide-N-functionalized allyl ligand, while complexes 5 and 6 undergo a vicarious nucleophilic substitution of the hydride at the metal center with the alkyne, via the compressed dihydride adduct intermediates OsH2(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (9), CH2Ph (10)), which reductively eliminate H2 to yield the acetylide-osmaoxazoles Os(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (11), CH2Ph (12)).

Delivering 2-Aryl Benzoxazoles through Metal-Free and Redox-Neutral De-CF3 Process

An unexpected cleavage of the C_sp3-CF₃ bond of CF₃-hydrobenzoxazoles has been disclosed, affording a range of 2-aryl benzoxazoles under metal-free and redox-neutral conditions. This transformation has demonstrated broad substrate scope and good compatibility of functional groups. 2-Aryl benzothiazole and 2-aryl benzoimidazole could be smoothly assembled in the same manner. On the basis of preliminary mechanistic studies, base initiated and aromatization driven β-carbon elimination was considered to be the key step for the formation of 2. This reaction offers an alternative, facile, and sustainable route to access important 2-aryl benzoxazole motifs.

Enantiomeric Isopulegol as the Chiral Pool in the Total Synthesis of Bioactive Agents

Isopulegol, a pool of abundant chiral terpene, has long served as the starting material for the total synthesis of isopulegol-based drugs. As an inexpensive and versatile starting material, this compound continues to serve modern synthetic chemistry. This review highlights the total syntheses of terpenoids in the period from 1980 to 2020 in which with isopulegol applied as a building block.

Consecutive O-S/N-S Bond Cleavage in Gold-Catalyzed Rearrangement Reactions of Alkynyl N-Sulfinylimines

Gold-catalyzed reactions of alkynyl N-sulfinylimines were used to produce the corresponding 2H-azirines possessing sulfenyl and acyl groups at the 3-position of the azirine ring in good to excellent yields. These reactions involved internal transfer of the sulfinyl oxygen atom to form a thiooxime intermediate tethered to an α-oxo gold carbene moiety. Subsequent insertion of the carbene into the N-S bond resulted in ring construction.

Direct Arylation of Azoles Enabled by Pd/Cu Dual Catalysis

A practical approach toward the synthesis of 2-arylazoles via direct arylation is described. The transformation relies on a Pd/Cu cocatalyst system that operates with low catalyst loadings. The reaction conditions were found to be tolerant of a wide range of functional groups and nitrogen-containing heterocycles commonly employed in a drug discovery setting.

Atmospheric chemistry of oxazole: the mechanism and kinetic studies of the oxidation reaction initiated by OH radicals

Oxidation of oxazole by OH˙ radicals studied by DFT methods coupled with reaction kinetics calculations using TST and RRKM theories.

2-Aminooxazole as a Novel Privileged Scaffold in Antitubercular Medicinal Chemistry

To obtain effective eradication of numerous infectious diseases such as tuberculosis, it is important to supply the medicinal chemistry arsenal with novel chemical agents. Isosterism and bioisosterism are widely known concepts in the field of early drug discovery, and in several cases, rational isosteric replacements have contributed to improved efficacy and physicochemical characteristics throughout the hit-to-lead optimization process. However, sometimes the synthesis of isosteres might not be as straightforward as that of the parent compounds, and therefore, novel synthetic strategies must be elaborated. In this regard, we herein report the evaluation of a series of N-substituted 4-phenyl-2-aminooxazoles that, despite being isosteres of a widely used nucleus such as the 2-aminothiazole, have been only seldom explored. After elaboration of a convenient synthetic strategy, a small set of 2-aminothiazoles and their 2-aminooxazole counterparts were compared with regard to antitubercular activity and physicochemical characteristics.

Synthesis of monofluorooxazoles with quaternary C-F centers through photoredox-catalyzed radical addition of methylene-2-oxazolines

A novel photoredox-catalyzed radical addition of methylene-2-oxazolines has been developed under visible light irradiation to synthesize monofluorooxazoles with a quaternary carbon center using 2-bromo-2-fluoro-3-oxo-3-phenylpropionates as radical source. This method with a simple protocol, scalability and high yield offers a facile path to get diverse monofluorinated oxazoles with quaternary C-F centers, which are a class of highly valuable motifs and synthons.

Synthesis and in vitro evaluation of antimycobacterial and cytotoxic activity of new α,β-unsaturated amide, oxazoline and oxazole derivatives from l-serine

The synthesis of 19 compounds derived from l-serine and analogs of p-substituted cinnamic acid is reported. Oxazolines 9 and oxazoles 10 have high antitubercular activity with Minimum Inhibitory Concentration (MIC) of 0.7812-25.0 µg/mL (3.21-100.3 µM), against two strains of Mycobacterium tuberculosis sensitive to first-line drugs Isoniazid (INH), Rifampicin (RIF), Ethambutol (EMB), Pyrazinamide (PZE) (H37Rv) and a clinical isolate resistant to INH, RIF and EMB (G122). The cytotoxic evaluation shows that oxazoles have low activity, finding viability>96% against the VERO cell line. The results show these compounds could be considered as future alternatives for antitubercular treatment.

Iodine-Catalyzed Functionalization of Primary Aliphatic Amines to Oxazoles, 1,4-Oxazines, and Oxazinones

Unprecedented I₂-catalyzed α,α-C(sp³)-H, decarboxylative α-C(sp³)-H, lactonized α-C(sp³)-H, and α,β-C(sp³)-H functionalized 5- and 6-annulation as well as α-C(sp³)-H activated 6-lactonization of primary aliphatic amines are devised under aerobic conditions. The metal-free sustainable strategy was employed for the diverse construction of valuable five-and six-membered polycyclic N,O-heteroaromatics such as oxazoles, 1,4-oxazines, and oxazin-2-one with a rapid reaction rate and high yield. The viability of this mild nonmetallic catalysis is successfully verified through syntheses of labile chiral heterocyclic analogues. In contrast to the common practice, this method is not limited to use of prefunctionalized amines, directing groups (DGs) and/or transient DGs, metal catalysts, and traditional oxidants. The possible mechanistic pathway of the annulation reaction is investigated by control experiments and ESI-MS data collected for a reaction mixture of the ongoing reaction. The synthesized new compounds are potent organic nanobuilding blocks to achieve valuable organic nanomaterials of different sizes, shapes, and dimensions, which are under investigation for the discovery of high-tech devices of innovative organic nanoelectronics and photophysical properties.