Regioselective 5-exo-dig (halo)cyclization of N-propargyloxycarbonyl guanidine derivatives under mild conditions

A highly regioselective 5-exo-dig cyclization of aromatic N-propargyloxycarbonyl guanidines was developed via an Ag(I)-catalyzed intramolecular hydroamination reaction. This method features a fast reaction rate and mild reaction conditions. Furthermore, it was extended to access halogenated analogues via a one-pot Ag(I)-catalyzed bromocyclization reaction or an I2-mediated iodocyclization reaction with high E/Z selectivity.

Continuous Flow Electroselenocyclization of Allylamides and Unsaturated Oximes to Selenofunctionalized Oxazolines and Isoxazolines

The synthesis of selenofunctionalized oxazolines and isoxazolines from N-allyl benzamides and unsaturated oximes with diselenides was studied by utilizing a continuous flow electrochemical approach. At mild reaction conditions and short reaction times of 10 min product yields of up to 90% were achieved including a scale-up reaction. A broad substrate scope was studied and the reaction was shown to have a wide functional group tolerance.

Palladium-Catalyzed Carbohalogenation of Olefins with Alkynyl Oxime Ethers: Rapid Access to Chlorine-Containing Isoxazoles

A palladium-catalyzed carbohalogenation of olefins with alkynyl oxime ethers has been described, which provides efficient and practical access to various chlorine-containing isoxazoles. This method exhibits excellent regioselectivity, good functional group compatibility, and mild reaction conditions. The mechanistic studies suggest that the reaction proceeds via a stabilized π-benzyl palladium intermediate, which is essential for the formation of C(sp3)-Cl bonds.

Electrochemical assembly of isoxazoles via a four-component domino reaction

Multicomponent domino reactions via electrochemical annulations have emerged as a robust strategy for the rapid assembly of heterocyclics. Herein, an electrochemical annulation via a [1 + 2 + 1 + 1] four-component domino reaction was accomplished in a user-friendly undivided cell setup to assemble valuable five-membered isoxazole motifs. Our approach is characterized by a high level functional group tolerance and operational simplicity, avoiding the tedious and time-consuming preparation of pre-functionalized substrates. Detailed mechanistic studies were conducted including isotopic labeling, kinetic studies, cyclic voltammetry (CV) analysis, and intermediate characterization, providing support for a radical pathway.

Palladium-Catalyzed Cyclization/Alkenylation of Ynone Oximes with Vinylsilanes for the Assembly of Isoxazolyl Vinylsilanes

A palladium-catalyzed cascade cyclization/alkenylation for the assembly of synthetically valuable isoxazolyl vinylsilane derivative has been accomplished. Easily accessible ynone oximes, and available vinylsilane agents were used as the reaction starting materials This protocol features broad substrate scope, good functional group tolerance, and good step- and atom-economy. Remarkably, this approach provides a new approach for the construction of structurally diverse isoxazolyl-containing vinylsilanes with high molecular complexity, showing a promising application in synthetic and pharmaceutical chemistry.

Iodobenzene-Catalyzed Synthesis of Fully Functionalized NH-Pyrazoles and Isoxazoles from α,β-Unsaturated Hydrazones and Oximes via 1,2-Aryl Shift

An iodine(III)-catalyzed general method for the synthesis of fully functionalized NH-pyrazoles and isoxazoles from α,β-unsaturated hydrazones and oximes, respectively, via cyclization/1,2-aryl shift/aromatization/detosylation, has been developed. The reaction progresses through an anti-Baldwin 5-endo-trig cyclization. It gives direct access to an advanced intermediate for the preparation of valdecoxib and parecoxib, drugs used for COX-inhibition. In addition, a method for N-alkynylation of pyrazoles has also been developed in the presence of TIPS-EBX.

Primary nitro compounds: progress in the synthesis of isoxazoles by condensation with aldehydes or activated ketones

Among the known strategies directed towards the synthesis of isoxazole derivatives, the reactions of aldehydes with primary nitro compounds deserve a comprehensive treatment, including the historical development as well as the more recent applications. The reactions of aldehydes with primary nitro compounds in a 1 : 2 molar ratio have been shown to lead to isoxazoline-N-oxides or isoxazole derivatives, via β-dinitro derivatives. Several modifications of the process allowed the formation of products bearing substituents at various positions of the heterocyclic ring with control of regioselectivity. Ketones are reported to react with primary nitro compounds, only if activated (β-diketones, α-nitroketones, or strained ketones), to give isoxazole derivatives. Symmetric 2,4-dinitroglutarates formed from aromatic aldehydes and nitroacetate undergo ring closure to form isoxazole derivatives or, according to reaction conditions, 5-hydroxy-6-oxo-4-aryl-6H-1,2-oxazine-3-carboxylates ("oxazinones"), by loss of alcohol instead of water. Isoxazole-4-carbaldehydes are obtained by the reaction of 3-oxetanone with primary nitro compounds.

Synthesis of gem-Difluorinated Isoxazoles via Palladium-Catalyzed Oxylallylation of Alkynone Oxime Ethers

A novel and reliable palladium-catalyzed oxylallylation of alkynone oxime ethers with fluorine-containing alkenes was accomplished. Using the bulk industrial chemical 3-bromo-3,3-difluoroprop-1-ene as the coupling partner, this synthetic methodology offers the first example for the assembly of structurally diverse gem-difluorinated isoxazole derivatives in moderate to good yields with high atom- and step-economy and excellent functional group compatibility. More importantly, this strategy allows for the direct combination of the isoxazole motifs and gem-difluoroalkene unit, which is not easy to obtain through a general synthetic strategy.

Tunable Key [3 + 2] and [2 + 1] Cycloaddition of Enaminones and α-Diazo Compounds for the Synthesis of Isomeric Isoxazoles: Metal-Controlled Selectivity

The three-component reactions of enaminones, α-diazo esters/ketones, and t-butyl nitrite (TBN) for the switchable synthesis of isomeric isoxazoles have been realized. The catalysis with Cu(II) salt provides 3,4-disubsituted isoxazoles via [3 + 2] cycloaddition. On the other hand, the catalysis of Ag(I) with identical substrates leads to isomeric isoxazoles with reversed C3 and C4 substitution based on a key [2 + 1] cycloaddition.

Asymmetric Synthesis of Spiro-3,3'-cyclopropyl Oxindoles via Vinylogous Michael Initiated Ring Closure Reaction

A novel organocatalytic cascade approach for the synthesis of spiro-cyclopropyl oxindole derivatives has been developed. The methodology is based on asymmetric vinylogous Michael addition of 4-nitroisoxazole derivatives to N-Boc isatylidene malonates followed by intramolecular alkylation. Its remarkable stereocontrol, wide substrate scope, and scalability highlight this new developed strategy. Moreover, this work represents the first example of vinylogous Michael initiated ring closure (MIRC) reaction for the synthesis of chiral 3,3'-cyclopropyl oxindole.

Palladium-Catalyzed Cascade Cyclization/Alkylation of Oxime Ethers: Assembly of 4-Alkylisoxazoles by "Chain-Walking" Strategy

A reliable and efficient palladium-catalyzed cascade cyclization/alkylation of oxime ethers with unactivated alkenes is described, affording a whole variety of structurally diverse isoxazole derivatives in moderate to good yields with excellent functional group compatibility. Ionic liquid [Aeim]Br not only acts as an environmentally friendly solvent but also acts as an accelerating agent to provide excess bromine source to eliminate bromomethane from oxime ethers. More importantly, the use of "chain-walking" strategy provides a novel methodology in organic synthesis to rapid generation of molecular complexity from readily available starting materials.

Synthesis of Isoxazoles via One-Pot Oxidation/Cyclization Sequence from Propargylamines

A facile strategy for the synthesis of isoxazoles has been efficaciously developed, which involves oxidation of propargylamines to the corresponding oximes followed by CuCl-mediated intramolecular cyclization of the latter. This protocol shows a straightforward way to construct a series of isoxazole cores with a wide range of functional group compatibility. Meanwhile, a gram-scale experiment and synthetic applications can be successfully operated.

Solvent-free synthesis of 3,5-isoxazoles via 1,3-dipolar cycloaddition of terminal alkynes and hydroxyimidoyl chlorides over Cu/Al2O3 surface under ball-milling conditions

Scalable, solvent-free synthesis of 3,5-isoxazoles under ball-milling conditions has been developed. The proposed methodology allows the synthesis of 3,5-isoxazoles in moderate to excellent yields from terminal alkynes and hydroxyimidoyl chlorides, using a recyclable Cu/Al₂O₃ nanocomposite catalyst. Furthermore, the proposed conditions are reproducible to a 1.0-gram scale without further milling time variations.

A practical and scalable mechanochemical 1,3-dipolar cycloaddition between hydroxyimidoyl chlorides and terminal alkynes catalyzed by Cu/Al₂O₃ allows a quick access to 3,5-isoxazole derivatives.

Regioselective synthesis of methyl 5-(N-Boc-cycloaminyl)-1,2-oxazole-4-carboxylates as new amino acid-like building blocks

A convenient and efficient synthesis of novel achiral and chiral heterocyclic amino acid-like building blocks was developed. Regioisomeric methyl 5-(N-Boc-cycloaminyl)-1,2-oxazole-4-carboxylates were prepared by the reaction of β-enamino ketoesters (including azetidine, pyrrolidine or piperidine enamines) with hydroxylamine hydrochloride. Unambiguous structural assignments were based on chiral HPLC analysis, 1H, 13C, and 15N NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.

Palladium-catalyzed Lewis acid-regulated cascade annulation of alkynes with unactivated alkenes to access diverse α-methylene-γ-lactones

A palladium-catalyzed Lewis acid-regulated cascade annulation of alkynes with unactivated alkenes for the preparation of alkyl substituted α-methylene-γ-lactones with excellent Z/E selectivities was accomplished.

Metal-/solvent-free oxidative [4+2]/[3+2] annulation of 2-ethynylbenzaldehydes with arylalkenes: Facile synthesis of benzo[a]fluoren-5-ones

We report here a DTBP-mediated oxidative [4+2]/[3+2] annulation between 2-alkynylbenzaldehydes and terminal arylalkenes enabled by aldehyde C(sp2)-H functionalization for assembling a range of benzo[a]fluoren-5-ones with high step-economy, simple operations under...

Photocatalytic functionalizations of alkynes

Visible light mediated functionalizations have significantly expanded the scope of alkynes by unraveling new mechanistic pathways and enabling their transformation to diverse structural entities. The photoredox reactions on alkynes rely on their innate capability to generate myriad carbon-centred radicals via single electron transfer (SET), thereby, allowing the introduction of new radical precursors. Moreover, an array of methods have been developed facilitating transformations such as vicinal or gem-difunctionalization, annulation, cycloaddition and oxidative reactions to construct numerous key building blocks of natural and pharmaceutically important molecules. In addition, the introduction of photoredox chemistry has successfully been used to deal with the challenges associated with alkyne functionalization such as stereoselective and regioselective control. This article accounts for several visible light mediated functionalization reactions of alkynes, wherein they have been transformed into α-oxo compounds, β-keto sulfoxides, substituted olefins, N-heterocycles, internal alkynes and sulfur containing compounds. The article has been primarily categorized into various sections based on the reaction type with particular attention being paid to mechanistic details, advancement and future applications.

1,2-Aminofunctionalization Reactions of Pyridino-Alkynes via Carbophilic Activation

Transition metal-catalyzed 1,2-difunctionalization reactions of alkynes have emerged as a powerful tool to forge carbon-carbon and carbon-heteroatom bonds for the rapid synthesis of polyfunctionalized molecular scaffolds. In this regard, our group has persistently been developing transition metal-mediated 1,2-aminofunctionalization reactions of alkynes through a rationally designed pyridino-alkyne core by utilizing the carbophilic activation strategy. In this account, we present an array of such 1,2-aminofunctionalization reactions which have been successfully executed on this core to afford important polycyclic frameworks such as functionalized quinalizinones, pyridinium oxazole dyads (PODs), N-doped polycyclic aromatic hydrocarbons (PAHs), N-doped spiro-PAHs. Additionally, the synthesis of phosphine ligated gold complexes bearing pyrido-isoquinoline scaffold from the pyridino-alkynes will be discussed briefly.

An overview of metal-free synthetic routes to isoxazoles: the privileged scaffold

In the field of drug discovery, isoxazole is a five-membered heterocyclic moiety commonly found in many commercially available drugs. In view of their enormous significance, it is always imperative to unleash new eco-friendly synthetic strategies. Among various novel synthetic techniques in use for isoxazole synthesis, most synthetic methods employ Cu(i) or Ru(ii) as catalysts for (3 + 2) cycloaddition reaction. The particular disadvantages associated with metal-catalyzed reactions are high costs, low abundance, toxicity, a significant generation of waste, and difficulty to separate from the reaction mixtures. In view of these drawbacks, it is always imperative to develop alternate metal-free synthetic routes. This review article highlights a comprehensive overview on the potential application of metal-free synthetic routes for the synthesis of isoxazoles with significant biological interests.

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

Diorganyl Dichalcogenides and Copper/Iron Salts: Versatile Cyclization System To Achieve Carbo- and Heterocycles from Alkynes

Organochalcogen-containing cyclic molecules have shown several promising pharmacological properties. Consequently, different strategies have been developed for their synthesis in the past few years. Particularly due to the low cost and environmental aspects, copper- and iron-promoted cyclization reactions of alkynyl substrates have been broadly and efficiently applied for this purpose. This short review presents an overview of the most recent advances in the synthesis of organochalcogen-containing carbo- and heterocycles by reacting diorganyl disulfides, diselenides, and ditellurides with alkyne derivatives in the presence of copper and iron salts to promote cyclization reactions.

1 Introduction

2 Synthesis of Carbo- and Heterocycles via Reactions of Alkynes with Diorganyl Dichalcogenides and Copper Salts

3 Synthesis of Carbo- and Heterocycles via Reactions of Alkynes with Diorganyl Dichalcogenides and Iron Salts

4 Conclusions

Transition Metal-Free Synthesis of Carbo- and Heterocycles via Reaction of Alkynes with Organylchalcogenides

This manuscript intends to overview the most recent advances in the synthesis of carbo- and heterocycles through reactions of alkynes with organyl chalcogenides (S, Se, Te) under metal-free conditions. Firstly, the use of electrophilic chalcogenyl halides as a selective reagent for alkyne carbon-carbon triple bond activation will be presented. After that, radical cyclization protocols employing electrochemical oxidative conditions, light-induced photoredox catalysis, or mild oxidants with direct chalcogenyl group installation will be discussed accompanied by the proposed mechanisms.

Recent advances in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications

This review summarizes the recent developments in NHC–palladium catalysis for alkyne chemistry: versatile synthesis and applications.

Palladium-catalyzed aerobic oxyarylthiolation of alkynone O-methyloximes with arylhydrazines and elemental sulfur

A novel and practical palladium-catalyzed aerobic oxyarylthiolation of alkynone O-methyloximes for the assembly of 4-sulfenylisoxazole derivatives using S8 and arylhydrazines as the S-aryl sources is accomplished. In the presence of 0.1 mol% of IPr-Pd-allyl-Cl as the catalyst and O2 (1 atm) as the sole oxidant, both alkynone O-methyloximes and arylhydrazines are suitable substrates, delivering diverse 4-sulfenyl isoxazoles in moderate to good yields with good functional group tolerance. Notably, the phenyl diazonium salt and sodium phenyl sulfinate are also suitable arylation reagents, providing an alternative synthetic strategy to access structurally diverse 4-sulfenyl isoxazoles.