[Au]-Catalyzed Cyclization of Propargyl-Tethered Ene-Amides: Substrate-Dependent Access to Tetrasubstituted Pyrroles, Aminophenols, and Dihydropyridines

[Au]-catalyzed and substrate-dependent intramolecular cyclization of sulfonyl ene-amides with a pendant propargyl group afford tetrasubstituted pyrroles, o-aminophenols, or 1,6-dihydropyridine carbaldehydes. While the pyrroles and aminophenols are formed when the propargylic alkyne is terminal, dihydropyridines are formed when internal alkyne is present. Internal alkyne substrates with 2-thienyl and 3-thienyl groups give different types of dihydropyridines. The dihydropyridines so obtained can be readily converted to nicotinaldehydes with concomitant sulfonyl migration upon heating in xylene.

Oxygen-Free Csp3-H Oxidation of Pyridin-2-yl-methanes to Pyridin-2-yl-methanones with Water by Copper Catalysis

Aromatic ketones are important pharmaceutical intermediates, especially the pyridin-2-yl-methanone motifs. Thus, synthetic methods for these compounds have gained extensive attention in the last few years. Transition metals catalyze the oxidation of Csp3-H for the synthesis of aromatic ketones, which is arresting. Here, we describe an efficient copper-catalyzed synthesis of pyridin-2-yl-methanones from pyridin-2-yl-methanes through a direct Csp3-H oxidation approach with water under mild conditions. Pyridin-2-yl-methanes with aromatic rings, such as substituted benzene, thiophene, thiazole, pyridine, and triazine, undergo the reaction well to obtain the corresponding products in moderate to good yields. Several controlled experiments are operated for the mechanism exploration, indicating that water participates in the oxidation process, and it is the single oxygen source in this transformation. The current work provides new insights for water-involving oxidation reactions.

Synthesis of substituted pyridines with diverse functional groups via the remodeling of (Aza)indole/Benzofuran skeletons

Substituted pyridines with diverse functional groups are important structural motifs found in numerous bioactive molecules. Several methodologies for the introduction of various bio-relevant functional groups to pyridine have been reported, but there is still a need for a single robust method allowing the selective introduction of multiple functional groups. This study reports a ring cleavage methodology reaction for the synthesis of 2-alkyl/aryl 3-electron-withdrawing groups (esters, sulfones, and phosphonates) 5-aminoaryl/phenol pyridines via the remodeling of 3-formyl (aza)indoles/benzofurans. Totally ninety-three 5-aminoaryl pyridines and thirty-three 5-phenol pyridines were synthesized showing the robustness of the developed methodology. The application of this methodology further provided a privileged pyridine scaffold containing biologically relevant molecules and direct drug/natural product conjugation with ethyl 2-methyl nicotinate.

Synthesis of Ag-Cu-Ni Nanoparticles Stabilized on Functionalized g-C3N4 and Investigation of Its Catalytic Activity in the A3-Coupling Reaction

In the present research, using ethylenediamine and hydrazine hydrate as the capping and reducing agents in this investigation, respectively, Ag-Cu-Ni NPs were immobilized on the functionalized g-C₃N₄ surface. This nanocatalyst was studied in terms of its catalytic activities for the A³-coupling reaction to synthesize propargylamine derivatives. According to the results, in the presence of 1 mL of toluene as the solvent and 20 mg of the g-C₃N₄-TCT-2AEDSEA-Ag-Cu-Ni nanocatalyst, the maximum efficiency of the nanocatalyst occurred at a temperature of 80 °C. Products were purified using thin-layer chromatography plates (silica gel) by employing n-hexane/ethyl acetate with a 90:10 ratio. In addition, the prominent benefits of the synthesized nanocatalyst include its high yields of the product, cost-effectiveness, recyclability, and easy separation. The novelty of the catalyst is due to the presence of Ag-Cu-Ni nanoparticles at the same time in the structure of the functionalized g-C₃N₄ substrate. So, Ag-Cu-Ni can be strongly connected to the substrate. The structure of the synthesized nanocatalyst was characterized using Fourier transformed infrared spectroscopy, X-ray powder diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, vibrating-sample magnetometry, and transmission electron microscopy.

Medicinal Importance and Chemosensing Applications of Pyridine Derivatives: A Review

Pyridine derivatives are the most common and significant heterocyclic compounds, which play an important role in various fields ranging from medicinal to chemosensing applications. Pyridine derivatives possess different biological activities such as antifungal, antibacterial, antioxidant, antiglycation, analgesic, antiparkinsonian, anticonvulsant, anti-inflammatory, ulcerogenic, antiviral, and anticancer activity. Furthermore, these derivatives have a high affinity for various ions and neutral species and can be used as a highly effective chemosensor for the determination of different species. In this review article, generally used synthetic routes of pyridine, structural characterization, medicinal applications, and potential of pyridine derivatives in analytical chemistry as chemosensors have been discussed. We hope this study will support the new thoughts to design biological active compounds and highly selective and effective chemosensors for the detection of various species (anions, cations, and neutral species) in various samples (environmental, agricultural, and biological). [Figure: see text].

α-Tertiary Propargylamine Synthesis via KA2-Type Coupling Reactions under Solvent-Free CuI-Zeolite Catalysis

The potential of copper(I)-zeolite catalysis was evaluated in the three-component KA²-coupling mediated synthesis of α-tertiary propargylamines. Our archetypal copper(I)-doped zeolite Cu^I-USY proved to be efficient under ligand- and solvent-free conditions at 80 °C. Usable up to four times, this catalytic material enables the coupling of diverse ketones, alkynes, and amines with a broad functional group tolerance. A decarboxylative and a desilylative version, respectively, involving an alkynoic acid and trimethylsilylacetylene as alkyne surrogates, was also set up to bypass selectivity issues and/or to access α-tertiary propargylamines that are unattainable under standard KA² conditions. Interestingly, the KA²-type coupling reactions were successfully linked to other Cu^I-catalyzed reactions, thus resulting in sequential one-pot processes under full Cu^I-USY catalysis.

Base-Promoted One-Pot Synthesis of Pyridine Derivatives via Aromatic Alkyne Annulation Using Benzamides as Nitrogen Source

In the presence of Cs₂CO₃, the first simple, efficient, and one-pot procedure for the synthesis of 3,5-diaryl pyridines via a variety of aromatic terminal alkynes with benzamides as the nitrogen source in sulfolane is described. The formation of pyridine derivatives accompanies the outcome of 1,3-diaryl propenes, which are also useful intermediates in organic synthesis. Thus, pyridine ring results from a formal [2+2+1+1] cyclocondensation of three alkynes with benzamides, and one of the alkynes provides one carbon, whilst benzamides provide a nitrogen source only. A new transformation of alkynes as well as new utility of benzamide are found in this work.

Microwave-Assisted Cu(I)-Catalyzed Synthesis of Unsymmetrical 1,4-Diamino-2-butynes via Cross-A3-Coupling/Decarboxylative A3-Coupling

1,4-Diamino-2-butynes display both chemical and physiological properties. Here a highly efficient synthesis avenue to generate unsymmetric 1,4-diamino-2-butynes has been developed by microwave-assisted Cu(I)-catalyzed cross-A³-coupling/decarboxylative coupling of two different amines, formaldehyde, and propiolic acid through a domino process. This multicomponent reaction provides a series of target products in moderate to good yields with high chemoselectivity.

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

One-Pot Synthesis of 2-Acetyl-1H-pyrroles from N-Propargylic β-Enaminones via Intermediacy of 1,4-Oxazepines

A one-pot two-step protocol for the synthesis of 2-acetyl-1H-pyrroles from N-propargylic β-enaminones was described. When treated with zinc chloride in refluxing chloroform, N-propargylic β-enaminones produced in situ 2-methylene-2,3-dihydro-1,4-oxazepines, which, upon further refluxing in methanol with zinc chloride, afforded 2-acetyl-1H-pyrroles. The process was found to be general for a wide variety of N-propargylic β-enaminones and yielded a diverse range of 2-acetyl-1H-pyrroles in good to high yields with large substrate scope and good functional group tolerance. This operationally easy method may provide a rapid access to functionalized 2-acetyl-1H-pyrroles of pharmacological interest.

Regiocontrolled synthesis of 2,4,6-triarylpyridines from methyl ketones, electron-deficient acetylenes and ammonium acetate

A novel one-pot two-step approach for the synthesis of 2,4,6-triarylpyridines via t-BuOK/DMSO-promoted C-vinylation of a variety of methyl ketones with electron-deficient acetylenes (alkynones) followed by a cyclization of the in situ generated unsaturated 1,5-dicarbonyl species with ammonium acetate has been developed. This approach possesses competitive advantages such as high regioselectivity, available starting materials and the absence of transition-metal catalysts, oxidants and undesirable byproducts. A wide synthetic utility of the developed approach was demonstrated by the synthesis of trisubstituted, tetrasubstituted and fused pyridines.

Preliminary Study on Novel Expedient Synthesis of 5-Azaisocoumarins by Transition Metal-Catalyzed Cycloisomerization

A preliminary study to develop a novel synthetic method for 3-aryl-5-azaisocoumarins was performed herein. The cycloisomerization of N-pyranonyl propargylamines in the AgOTf-catalyzed system efficiently afforded the desired 3-aryl-5-azaisocoumarins in a highly regioselective manner. This unprecedented method is expected as an expedient alternative synthetic route to 5-azaisocoumarins because the regioselectivity problem is circumvented, and it is easier to introduce substituents on the pyridine ring compared to previously reported intramolecular lactonization approaches.

Crystal structure of 5-methyl-3-phenyl-1-tosyl-1,2,3,4-tetrahydropyridine, C19H21NO2S

C19H21NO2S, orthorhombic, Pbca (no. 61), a = 16.792(5) Å, b = 9.153(3) Å, c = 22.809(6) Å, V = 3505.8(16) Å3, Z = 8, R gt(F) = 0.0622, wR ref(F 2) = 0.1494, T = 296(2) K.

Synthesis of 3-[(4-Nitrophenyl)thio]-Substituted 4-Methylene-1-pyrrolines from N-Propargylic β-Enaminones

A facile and efficient method for the synthesis of 3-[(4-nitrophenyl)thio]-substituted 4-methylene-1-pyrrolines is described. When treated with 4-nitrobenzenesulfenyl chloride in refluxing acetonitrile, N-propargylic β-enaminones produced α-sulfenylated N-propargylic β-enaminones, which, in the presence of sodium hydride or cesium carbonate, underwent nucleophilic cyclization to afford 4-methylene-3-[(4-nitrophenyl)thio]-1-pyrrolines in good to high yields. It was shown for the first time that on N-propargylic β-enaminone systems, α-sulfenylation dominates over the formation of thiirenium ion. This one-pot two-step process was found to be general for a variety of N-propargylic β-enaminones and demonstrated good tolerance to a diversity of aromatic and heteroaromatic groups with electron-withdrawing and electron-donating substituents. This process is also applicable to the cyclization of internal alkyne-tethered N-propargylic β-enaminones. The enrichment of 1-pyrroline core with an aryl sulfide moiety might exhibit potential for the synthesis of molecules of pharmacological interest.

Recent Advances in the Application of Nanometal Catalysts for Glaser Coupling

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Synthesis of symmetrical 1,3-diynes from terminal alkynes through an oxidative process is generally called Glaser coupling. The classic Glaser coupling is catalyzed by copper salts under an atmosphere of molecular oxygen as an oxidant. Over the past years, different metal catalysts and oxidants were successfully used in this atom economical C-C coupling reaction. Moreover, several procedures for the preparation of unsymmetrical 1,3-diynes by coupling two different alkyne substrates have been developed. In this review, we will highlight the usefulness of transition metal nanoparticles as efficient catalysts in homo- and hetero-coupling of alkynes by hoping that it will be beneficial to the development of novel and extremely efficient catalytic systems for this fast-growing and important reaction.

Nucleophile-assisted cyclization of β-propargylamino acrylic compounds catalyzed by gold(i): a rapid construction of multisubstituted tetrahydropyridines and their fused derivatives

A variety of medicinally important multisubstituted tetrahydropyridines, 4-aryl piperidines, isoquinolines and fused pyridines were prepared in 1–3 step sequences via nucleophile-assisted Au(i)-catalyzed cyclizations of easily available azaenynes.

Cycloaddition of atmospheric CO2 to epoxides under solvent-free conditions: a straightforward route to carbonates by green chemistry metrics

The conversion of carbon dioxide (CO2) into value-added organic compounds has received more and more attention over recent years, not only because this gas is one of the major anthropogenic greenhouse gases, but also because it has been regarded as an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon (C1) resource. Along these lines, the synthesis of five-membered cyclic carbonates employing CO2 as a safe alternative to toxic reagents such as phosgene or its derivatives is of great interest because of their wide range of applications in organic synthesis. However, most of CO2 incorporation reactions into carbonates are carried out in toxic and non-recyclable organic solvents. Furthermore, these transformations usually proceed at elevated pressures or supercritical CO2 conditions. Recently, several catalytic systems have been developed that allow the synthesis of functionalized carbonates from the reaction of atmospheric CO2 with corresponding epoxides under solvent-free conditions. This review is an attempt to summarize the most important advances and discoveries in this interesting research arena. The review is divided into three major sections. The first section will discuss ionic liquid catalyzed coupling reactions. The second will cover organocatalyzed reactions. The third focuses exclusively on metal-catalyzed fixations. Notably, the third section has been classified based on the metal element that carries out the catalysis (i.e. copper, palladium, zinc).

A new strategy for the synthesis of pyridines from N-propargylic β-enaminothiones

A new robust method for the synthesis of 2,4,5-trisubstituted pyridines with high efficiency and broad functional group tolerance is described.

Recent developments in decarboxylative cross-coupling reactions between carboxylic acids and N–H compounds

Carboxylic acids and their derivatives are ubiquitous compounds in organic chemistry, and are widely commercially available in a large structural variety. Recently, carboxylic acids have been frequently used as non-toxic and environmentally benign alternatives to traditional organohalide coupling partners in various carbon–carbon and carbon–heteroatom cross-coupling reactions. Along this line, several methods have been reported for the synthesis of nitrogen-containing organic compounds through decarboxylative cross-coupling reactions between carboxylic acids and N–H compounds. This review focuses on recent advances and discoveries on these reactions with special attention on the mechanistic aspects of the reactions.

Carboxylic acids and their derivatives are ubiquitous compounds in organic chemistry, and are widely commercially available in a large structural variety.

Synthesis of 4-styrylcoumarins via FeCl3-promoted cascade reactions of propargylamines with β-keto esters

A versatile and highly regioselective FeCl₃-promoted tandem cyclization reaction of in situ generated alkynyl o-quinone methides (o-AQMs) with β-keto esters has been developed.

Recent advances in the application of nano-catalysts for Hiyama cross-coupling reactions

This mini-review highlights the recent developments in the field of metal nanoparticle (NP) catalyzed Hiyama cross-coupling reactions.

Solvent-free incorporation of CO2 into 2-oxazolidinones: a review

This review is an attempt to give an overview on the recent advances and developments in the synthesis of 2-oxazolidinone frameworks through carbon dioxide (CO₂) fixation reactions under solvent-free conditions. The cycloaddition of CO₂ to aziridine derivatives is discussed first. This is followed by carboxylative cyclization of N-propargylamines with CO₂ and three-component coupling of epoxides, amines, and CO₂. Finally, cycloaddition of CO₂ to propargylic alcohols and amines will be covered at the end of the review. The literature has been surveyed up until the end of 2018.

This review is an overview on the recent advances in the synthesis of 2-oxazolidinones through CO₂ fixation reactions under solvent-free conditions.

Facile Synthesis of Pyridines from Propargyl Amines: Concise Total Synthesis of Suaveoline Alkaloids

A general and efficient protocol was developed for the synthesis of polysubstituted pyridines from propargyl amines and unsaturated carbonyl compounds through a tandem condensation/alkyne isomerization/6π 3-azatriene electrocyclization sequence. This process was found to be applicable to a wide range of readily available substrates (30 examples, up to 95 % yield) and could be readily performed on a preparative (20 g) scale. By taking advantage of this method for late-stage pyridine incorporation, we successfully completed the collective total synthesis of suveoline, norsuveoline, and macrophylline.

Nanocatalysts for C–Se cross-coupling reactions

This mini review is an attempt to highlight the most important contributions toward the applications of nanocatalysts in carbon–selenium cross-coupling reactions with the emphasis on the mechanistic aspects of the reactions.