Recent advances of 5-endo-trig radical cyclization: promoting strategies and applications

5-endo-trig radical cyclization has long been recognized as one of the most straightforward ways for the construction of densely functionalized five-membered rings. Nevertheless, according to Baldwin's rules, the 5-endo-trig radical cyclization is kinetically disfavored due to stereoelectronic effects and thus usually proceeds via a slow rate, which renders its application a challenging task. In recent years, with the emergence of efficient radical generation methods and effective cyclization strategies, 5-endo-trig radical cyclization has been successfully accelerated to a synthetically useful rate and has been utilized in the access of diverse five-membered carbo- and heterocyclic compounds. This review comprehensively summarizes the methodologies involving the 5-endo-trig radical cyclization process, with particular emphasis on the elucidation of the promoting strategies, which include the polar effect, geometrical constraints, spin delocalization effect, and persistent radical effect. Each of these strategies is discussed in detail, with illustrative examples from recent literature studies to highlight their practical applications and effectiveness. It is anticipated that the in-depth understanding of the 5-endo-trig radical cyclization provided by this review would inspire further innovation of this privileged reaction mode and expand its applications. Moreover, the potent ring-closure-promoting strategies revealed herein would also contribute to achieving other challenges of cyclizations with particular significance for organic synthesis.

Copper Loading-Controlled Selective Synthesis of 2,5-Bis(trifluoromethyl) and Monotrifluoromethyl-Substituted Oxazoles

A general domino annulation reaction of sulfonylmethyl isocyanide with trifluoroacetic anhydride in the presence of copper chloride as an additive is developed. The reaction affords 2,5-bis(trifluoromethyl)oxazoles in modest to good yields under mild conditions. A wide variety of sulfonylmethyl isocyanide and perfluorocarboxylic anhydride substrates are amenable to this transformation. Under a higher copper salt loading conditions, the reaction led to the formation of monotrifluoromethyl-substituted oxazole product.

Synthesis of 2-Fluoroalkylated Oxazoles from β-Monosubstituted Enamines via Fluoroacyloxylation and Cyclization Mediated by Fluoroalkyl-Containing Hypervalent Iodine(III) Species Generated In Situ

A metal-free synthesis of a series of fluoroalkyl-containing oxazoles from β-monosubstituted enamines was developed. This fluoroacyloxylation/cyclization cascade process was mediated by fluoroalkyl-containing hypervalent iodine(III) species formed in situ from the reaction of phenyliodine(III) diacetate (PIDA) and RCF₂CO₂H (R = H, Cl, Br, F, CF₃, CH₃, Ph, SAr, OAr).

Iodine-Promoted Oxidative Cyclization of Methyl Azaarenes and α-Amino Ketones for One-Pot Synthesis of 2-Azaaryl-5-aryl Oxazoles

A high efficiency protocol was developed for the synthesis of 2,5-disubstituted oxazoles via iodine-promoted oxidative domino cyclization. These reactions were performed with readily available methyl azaarenes and α-amino ketones under metal-free conditions. This protocol is a simple method with high functional group compatibility, a wide range of substrates, and excellent yield, providing a new way to synthesize azaarene-attached oxazoles.

Copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate

A novel route for tandem C-N/C-O formation via copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate to synthesize valuable trifluoromethyl-containing 2H-1,3-oxazines in moderate to good yields is developed. This procedure represents the first [4+2] oxidative annulation of oxime derivatives with activated CO bonds and provides an alternative route towards functionalized 2H-1,3-oxazines.

Tellurium-induced cyclization of olefinic compounds

In this article, we discuss about the importance of Tellurium (Te) in organic synthesis. Tellurium-induced cyclization of alkenyl compounds, as well as alkynyl compounds, are considered for the study. The developments in this area are incorporated in great detail. The mechanism of the reactions does not follow any straightforward process. This study opens up the possibility of stereocontrolled synthesis of complex natural products.

One-pot synthesis of 4-substituted 2-fluoroalkyloxazoles from NH-1,2,3-triazoles and fluoroalkylated acid anhydrides

An efficient one-pot method for the synthesis of 2-fluoroalkyloxazoles from 4-substituted NH-1,2,3-triazoles, fluorinated anhydrides and triethylamine was developed.

Copper-mediated three-component synthesis of 2-trifluoromethyl benzimidazoles

A copper-mediated three-component reaction of o-iodoanilines, anilines, and ethyl trifluoropyruvate is reported. The transformation allows diverse scopes on both o-iodoanilines and anilines delivering various 2-trifluoromethylbenzimidazole products in moderate to good...

Synthesis of 2-trifluoromethyl chromenes via double carbonyl–ene reaction

A synthesis of 2-trifluoromethyl chromenes through double carbonyl–ene reaction of o-isopropenylphenols with trifluoroacetic anhydride is reported.

Chemoselective [3 + 2] annulation of oxime acetate with 2-aryl-3-ethoxycarbonyl-pyrroline-4,5-dione: an entry to pyrrolo[2,3-b]pyrrole derivatives

A novel chemoselective [3 + 2] annulation reaction of easily accessible ketoxime acetate with 2-aryl-3-ethoxycarbonyl pyrroline-4,5-dione has been developed for the synthesis of unknown pyrrolo[2,3-b]pyrrole frameworks. This method involves copper-mediated N-O bond cleavage followed by the formation of carbon-carbon and carbon-nitrogen bonds. This operationally simple protocol provides broader functional group compatibility and good yields.

Recent Developments in Oxazole Derivatives as Anticancer Agents: Review on Synthetic Strategies, Mechanism of Action and SAR studies

BACKGROUND

Cancer is the world's third deadliest disease. Despite the availability of numerous treatments, researchers are focusing on the development of new drugs lacking resistance and toxicity issues. Many newly synthesized drugs fail to reach clinical trials due to poor pharmacokinetic properties. Therefore, there is an imperative requisite to expand novel anticancer agents with in vivo efficacy.

OBJECTIVE

This review emphasizes synthetic methods, contemporary strategies used for the inclusion of oxazole moiety, mechanistic targets along with comprehensive structure-activity relationship studies to provide perspective into the rational design of highly efficient oxazole-based anticancer drugs.

METHODS

Literature related to oxazole derivatives engaged in cancer research is reviewed. This article gives a detailed account of synthetic strategies, targets of oxazole in cancer, including STAT3, Microtubules, G-quadruplex, DNA topoisomerases, DNA damage, Protein kinases, miscellaneous targets, in vitro studies, and some SAR studies.

RESULTS

Oxazole derivatives possess potent anticancer activity by inhibiting novel targets such as STAT3 and G-quadruplex. Oxazoles also inhibit tubulin protein to induce apoptosis in cancer cells. Some other targets such as DNA topoisomerase enzyme, protein kinases, and miscellaneous targets including Cdc25, mitochondrial enzymes, HDAC, LSD1, HPV E2 TAD, NQO1, Aromatase, BCl-6, Estrogen receptor, GRP-78, and Keap-Nrf2 pathway are inhibited by oxazole derivatives Many derivatives showed excellent potencies on various cancer cell lines with IC50 values in nanomolar concentrations.

CONCLUSION

Oxazole is a five-membered heterocycle, with oxygen and nitrogen at 1 and 3 positions respectively. It is often combined with other pharmacophores in the expansion of novel anticancer drugs. In summary, oxazole is a promising entity to develop new anticancer drugs.

A bench-stable N-trifluoroacetyl nitrene equivalent for a simple synthesis of 2-trifluoromethyl oxazoles

ortho-Nitro-substituted N-trifluoroacetyl imino-λ³-iodane is a bench-stable trifluoroacetyl nitrene precursor, in which intra- and intermolecular halogen bonding (XB) plays an important role. Potential synthetic applications of this novel precursor were explored.

Copper-Catalyzed Cascade Annulation of Malonate-Tethered O-Acyl Oximes with Cyclic 1,3-Dicarbonyl Compounds for the Synthesis of Spiro-Pentacyclic Derivatives

A copper-catalyzed cascade annulation of malonate-tethered O-acyl oximes with cyclic 1,3-dicarbonyl compounds has been developed for the rapid synthesis of spiro-pentacyclic derivatives. This reaction allows the one-step formation of five C-C/N/O bonds and an angular tricyclic core under very mild conditions and shows excellent regioselectivity and stereoselectivity.

Copper catalyzed five-component domino strategy for the synthesis of nicotinimidamides

A library of medicinally and synthetically important nicotinimidamides was synthesized by a copper-catalyzed multicomponent domino reaction of oxime esters, terminal ynones, sulfonyl azides, aryl aldehydes and acetic ammonium. Its synthetic pathway involves the formation of a highly reactive N-sulfonyl acetylketenimine, characterized by high selectivity, combinations of potential nucleophiles and electrophiles, mild reaction conditions and a wide substrate scope, and is a rare five-component example of a CuAAC/ring-opening reaction.

Copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins

A copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins was developed, affording valuable 2,3-dihydrooxazole-spirooxindoles in moderate to good yields with excellent diastereoselectivity.

Switching the Regioselectivity Access to Pyrroles and Isoquinolines from Ketoxime Acetates and Ynals

A novel formal [3+2] and [4+2] annulation of ketoxime acetates and ynals for the synthesis of pyrroles and isoquinolines has been developed. By simply switching the catalyst and solvent, the reaction proceeds via two pathways. The reactions are achieved under mild conditions with broad substrate scope and excellent regioselectivity.

BF3·OEt2-TFAA Mediated Tetra-Functionalization of Amino Acids - Synthesis of Di- and Tri-Substituted 2-Trifluoromethyl Oxazoles in One Pot

A highly efficient, TFAA-BF₃·OEt₂ mediated multicomponent coupling of amino acid, TFAA, and aromatics provides a broad library of 2-trifluoromethyl equipped 2,5-disubstituted/2,4,5-trisubstituted oxazoles or N-(trifluoroacetyl)-β-aminoalkyl ketones. This amino acid tetra-functionalization approach involves amidation (C-N), anhydride (C-O), Friedel-Crafts acylation (C-C), and Robinson-Gabriel annulation (C-O) followed by dehydrative aromatization. This reaction takes place under operationally simple, mild, and metal-free conditions using readily available amino acids and aromatic compounds.

KOt-Bu-promoted selective ring-opening N-alkylation of 2-oxazolines to access 2-aminoethyl acetates and N-substituted thiazolidinones

An efficient and simple KOt-Bu-promoted selective ring-opening N-alkylation of 2-methyl-2-oxazoline or 2-(methylthio)-4,5-dihydrothiazole with benzyl halides under basic conditions is described for the first time. The method provides a convenient and practical pathway for the synthesis of versatile 2-aminoethyl acetates and N-substituted thiazolidinones with good functional group tolerance and selectivity. KOt-Bu not only plays an important role to promote this ring-opening N-alkylation, but also acts as an oxygen donor.

Copper-catalyzed synthesis of trifluoromethylated bis(indolyl)arylmethanes from 2-arylindoles and 2,2,2-trifluoroacetohydrazide

A copper-catalyzed synthesis of trifluoromethylated bis(indolyl)arylmethanes via reaction of 2-arylindoles with 2,2,2-trifluoroacetohydrazide was developed.

Synthesis of polysubstituted 5-trifluoromethyl isoxazoles via denitrogenative cyclization of vinyl azides with trifluoroacetic anhydride

A general method for the synthesis of 5-(trifluoromethyl)isoxazoles by denitrogenative cyclization of vinyl azides with trifluoroacetic anhydride has been developed.

Synthesis of pertrifluoromethyl pyridazine derivatives via a tandem reaction of aryldiazonium salts with hexafluoroacetylacetone

A method for the synthesis of pertrifluoromethyl pyridazine derivatives by the condensation of arenediazonium salts with hexafluoroacetylacetone has been developed.

Base-Mediated Tunable Synthesis of 2-Trifluoromethylated Furans and Dihydrofuranols: Extraordinary Stable in Sulfuric Acid

2-Trifluoromethylated furans and dihydrofuranols were tunably synthesized from the cyclization of β-ketonitriles with 3-bromo-1,1,1-trifluoroacetone mediated by bases. In addition, dehydration of dihydrofuranol compounds with concentrated sulfuric acid gave another 2-(trifluoromethyl)furans isomer. The developed methodology exhibits an excellent functional group tolerance for both aromatic and aliphatic β-ketonitriles.

Perfluorocarboxylic Anhydrides Triggered Cyclization: Access to 4-Perfluoroalkylpyridines

A double nucleophilic addition-cyclization-elimination cascade is developed, that allows various 2,6-diaryl-4-perfluoroalkylpyridines to be synthesized in one step from easily available enamides and perfluorocarboxylic anhydrides. The procedure is also operationally simple and scalable and provides access to the facial construction of 4-fluoroalkylpyridines, which are of great interest in medicinal chemistry.

Synthesis of 3-(tri/difluoromethyl)-1H-1,2,4-triazol-5(4H)-ones via the cyclization of hydrazinecarboxamides with tri/difluoroacetic anhydride

An efficient method for the synthesis of structurally diverse 4-aryl-3-(tri/difluoromethyl)-1H-1,2,4-triazol-5(4H)-ones through the cyclization of hydrazinecarboxamides with tri/difluoroacetic anhydride is presented. The method is simple and environmentally benign, providing tri/difluoromethylated 1,2,4-triazol-5(4H)-ones in moderate-to-good yields. A mechanism is proposed to proceed via a tandem reaction of tri/difluoroacetylation, nucleophilic addition and water elimination. Some of these compounds exhibit promising insecticidal activities.

Copper-Catalyzed Three-Component Domino Cyclization for the Synthesis of 4-Aryl-5-(arythio)-2-(trifluoromethyl)oxazoles

A copper-catalyzed oxidative cyclization of oxime, arylthiol, and trifluoroacetic anhydride for the construction of trisubstituted oxazoles has been developed. This transformation combines N-O bond cleavage, C-H functionalization, and intramolecular annulation, providing a practical protocol for the introduction of a trifluoromethyl (-CF₃) group at oxazole rings.

Ruthenium(II) Acetate Catalyzed Synthesis of Silylated Oxazoles via C-H Silylation and Dehalogenation

An efficient ruthenium(II)-catalyzed intermolecular selective ortho C-H silylation of 2-aryloxazoles has been described for the first time, which provides a convenient and practical pathway for the synthesis of versatile organosilane compounds with good functional group tolerance and regioselectivity. This catalytic system could be also applied to the dehalogenation of Cl or Br group.

Direct access to bis-S-heterocyclesviacopper-catalyzed three component tandem cyclization using S8as a sulfur source

An elemental sulfur atom donor strategy for constructing a thiophene-fused thiazole bis-S-heterocyclic skeletonviaCu-catalyzed three-component tandem cyclization has been developed.

Syntheses of 2-(2,2,2-trifluoroethylidene)/(2,2-difluoroethyl)-1,3-dicarbonyl compounds and their fungicidal activities

A Knoevenagel-type reaction of 1,3-diketone substrates with trifluoroacetic/difluoroacetic anhydride for the synthesis of 2-(2,2,2-trifluoroethylidene)/(2,2-difluoroethyl)-1,3-dicarbonyl compounds has been developed.

General approach to 2-fluoroalkyl 1,3-azolesviathe tandem ring opening and defluorinative annulation ofN-fluoroalkyl-1,2,3-triazoles

Synthesis of 2-fluoroalkyl imidazoles, oxazoles and thiazoles fromN-fluoroalkyl-1,2,3-triazoles was developed.

Elemental sulfur-promoted one-pot synthesis of 2-(2,2,2-trifluoroethyl)benzoxazoles and their derivatives

An elemental sulfur promoted one-pot synthesis of 2-(2,2,2-trifluoroethyl)benzoxazole derivatives from the reaction of o-aminophenols, thiols, and anilines with 2-bromo-3,3,3-trifluoropropene is reported.

Synthesis of 4-trifluoromethyl 2-pyrones and pyridones through the Brønsted base-catalyzed Pechmann-type reaction with cyclic 1,3-diones

An efficient method for the synthesis of 4-trifluoromethyl 2-pyrones through the Brønsted base-catalyzed Pechmann-type reaction of cyclic 1,3-diones with ethyl 4,4,4-trifluoroacetoacetate is described. In the presence of 2-dimethylamino pyridine (2-DMAP) as a catalyst, the resulting 4-trifluoromethyl 2-pyrones are formed in good to excellent yields. Additionally, the reaction also provides 4-trifluoromethyl 2-pyridones by using the easily available NH4OAc as a source of NH3.