Preparation of 3-Acyl-4-arylcoumarins via Metal-Free Tandem Oxidative Acylation/Cyclization between Alkynoates with Aldehydes

n-Bu4NI/K2S2O8 Mediated Csp2-Csp2 Bond Cleavage - Transformylation from p-Anisaldehyde to Primary Amides

n-Bu4NI/K2S2O8 mediated transformylation from p-anisaldehyde to primary amides is reported. The mechanistic studies suggest the reaction occurs via a single electron transfer pathway. Based on the DFT electronic structure calculations of various reaction pathways, the most plausible mechanism involves the formation of a phenyl radical cation and an arenium ion as the key intermediates. It represents the first example where p-anisaldehyde is employed as a formyl source via a non-metal mediated Csp2-Csp2 bond cleavage.

Design of a new method for the synthesis of novel 2-aryl/alkyl-3H-indol-3-ones

In this research, a mild, efficient, and general method has been developed to synthesize the new derivatives of 2-aryl/alkyl-3H-indol-3-ones in moderate to excellent yields. This method allowed the syntheses of these compounds via the three-component reaction of anhydride compound, sodium cyanide, and aniline derivatives using acetic anhydride as an organic catalyst in one-pot reactions. The advantages of this method include mild reaction conditions, simple procedures, and easy workup.

Recent Advances in Molecule Synthesis Involving C-C Bond Cleavage of Ketoxime Esters

The synthetic strategies of oxime derivatives participating in radical-type reactions have been rapidly developed in the last few decades. Among them, the N–O bond cleavage of oxime esters leading to formation of nitrogen-centered radicals triggers adjacent C–C bond cleavage to produce carbon-centered free radicals, which has been virtually used in organic synthesis in recent years. Herein, we summarized the radical reactions involving oxime N–O bond and C–C bond cleavage through this special reaction form, including those from acyl oxime ester derivatives and cyclic ketoxime ester derivatives. These contents were systematically classified according to different reaction types. In this review, the free radical reactions involving acyl oxime esters and cyclic ketoxime esters after 2021 were included, with emphasis on the substrate scope and reaction mechanism.

Discovery of an Oxidative System for Radical Generation from Csp3-H Bonds: A Synthesis of Functionalized Oxindoles

A facile and versatile method for generating radicals from Csp³-H bonds under metal-free and organic-peroxide-free conditions was developed. By combining safe persulfate and low-toxic quaternary ammonium salt, a wide variety of Csp³-H compounds including ethers, (hetero)aromatic/aliphatic ketones, alkylbenzenes, alkylheterocycles, cycloalkanes, and haloalkanes were selectively activated to generate the corresponding C-centered radicals, which could be further captured by N-arylacrylamides to deliver the valuable functionalized oxindoles. Good functional group tolerance was demonstrated. The useful polycarbonyl compound and esters were also modified with the strategy. Moreover, the combination can also be applied to the practical coupling between simple haloalkanes and N-hydroxyphthalimide (NHPI).

The visible-light-induced acylation/cyclization of alkynoates with acyl oximes for the construction of 3-acylcoumarins

A nitrogen-centered radical-mediated carbon-carbon bond cleavage strategy is described to synthesize functionalized 3-acylcoumarins. The strategy is enabled by the visible-light-induced acylation/cyclization of alkynoates with various acyl oxime compounds in acetonitrile. The difunctionalization of carbon-carbon triple bonds precedes the generation of iminyl radicals, which is followed by the formation of acyl radicals. The acyl radicals then attack the carbon-carbon triple bonds, followed by 5-exo-trig cyclization and 1,2-ester migration. This strategy has wide substrate adaptability and good substituent tolerance.

Coumarins by Direct Annulation: β-Borylacrylates as Ambiphilic C3 -Synthons

Modular β-borylacrylates have been validated as programmable, ambiphilic C3 -synthons in the cascade annulation of 2-halo-phenol derivatives to generate structurally and electronically diverse coumarins. Key to this [3+3] disconnection is the BPin unit which serves a dual purpose as both a traceless linker for C(sp2 )-C(sp2 ) coupling, and as a chromophore extension to enable inversion of the alkene geometry via selective energy transfer catalysis. Mild isomerisation is a pre-condition to access 3-substituted coumarins and provides a handle for divergence. The method is showcased in the synthesis of representative natural products that contain this venerable chemotype. Facile entry into π-expanded estrone derivatives modified at the A-ring is disclosed to demonstrate the potential of the method in bioassay development or in drug repurposing.

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.

Visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates leading to coumarins via SO2 insertion

A visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates with cycloketone oxime compounds for the preparation of 3-cyanoalkylsulfonylcoumarins via SO₂ insertion is reported.

A computational study for the reaction mechanism of metal-free cyanomethylation of aryl alkynoates with acetonitrile

A computational study of metal-free cyanomethylation and cyclization of aryl alkynoates with acetonitrile is carried out employing density functional theory and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. Our results indicate that the reaction of aryl alkynoates with acetonitrile in the presence of tert-butyl peroxybenzoate (TBPB) under metal-free conditions tends to proceed through cyanomethylation, spirocyclization and ester migration of the kinetically favoured coumarin derivatives. 1,2-Ester migration in the spiro-radical intermediate 10 does not proceed via the formation of the carboxyl radical 11 suggested by Sun and co-workers. Our results also demonstrate that the t-butoxy radical is substantially responsible the formation of the cyanomethyl radical by the abstraction of a hydrogen atom from acetonitrile.

A computational study of metal-free cyanomethylation and cyclization of aryl alkynoates with acetonitrile is carried out employing density functional theory and high-level coupled-cluster methods, such as [CCSD(T)].

Aldehydes: magnificent acyl equivalents for direct acylation

From the viewpoint of meeting the current green chemistry challenges in chemical synthesis, there is a need to disseminate how the cocktail of acylation and activation can play a pivotal role in affording bioactive acylated products comprising substituted ketone motifs in fewer reaction steps, with higher atom-economy and improved selectivity. In recent years, a significant number of articles employing the title compounds "aldehydes" as magnificent acylation surrogates which are less toxic and widely applicable have been published. This review sheds light on the compounds use for selective acylation of arene, heteroarene and alkyl (sp3, sp2 and sp) C-H bonds by proficient utilization of the C-H activation strategy. Critical insights into selective acylation of diverse moieties for the synthesis of bioactive compounds are presented in this review that will enable academic and industrial researchers to understand the mechanistic aspects involved and fruitfully employ these strategies in designing novel molecules.

Recent Advances in the Synthesis of Coumarin Derivatives from Different Starting Materials

The study of coumarin dates back to 1820 when coumarin was first extracted from tonka bean by Vogel. Compounds containing coumarin backbone are a very important group of compounds due to their usage in pharmacy and medicine. Properties and biological activities of coumarin derivatives have a significant role in the development of new drugs. Therefore, many different methods and techniques are developed in order to synthesize coumarin derivatives. Coumarin derivatives could be obtained from different starting materials with various methods but with big differences in yield. This review summarized various methods, techniques and reaction conditions for synthesis of coumarins from different compounds such as aldehydes, phenols, ketones and carboxylic acids.

New frontiers in the transition-metal-free synthesis of heterocycles from alkynoates: an overview and current status

This review highlights the successful utilization of transition-metal-free approaches for the modular assembly of various heterocycles from alkynoates.

ZnBr2/Oxone-mediated ipso-cyclization of N-(3-phenylprop-2-yn-1-yl)aniline

In this work, a selective synthetic strategy towards 1-azaspiro[4.5]deca-3,6,9-trien-8-ones from N-tosyl-N-(prop-2-yn-1-yl)aniline is developed. The transformation proceeds smoothly in a mixed solvent including acetonitrile and water when ZnBr2 and Oxone are employed. Mechanism studies show that the reaction proceeds in a regioselective manner via a radical ipso-cyclization pathway.

Highly efficient copper-catalyzed direct C–H amidation of quinoxalin-2(1H)-ones with amidates under microwave irradiation

A novel and convenient protocol for the construction of 3-amidated quinoxalin-2(1H)-ones was developed via copper-catalyzed direct oxidative amidation of quinoxalin-2(1H)-ones with amidates using K₂S₂O₈ as an oxidant, and under microwave irradiation.

An efficient light on–off one-pot method for the synthesis of 3-styryl coumarins from aryl alkynoates

An efficient one-pot stepwise method to synthesize 3-styryl-4-arylcoumarins from simple alkynoates is demonstrated.

Regioselective radical arylation: silver-mediated synthesis of 3-phosphorylated coumarins, quinolin-2(1H)-one and benzophosphole oxides

A silver-mediated domino radical addition/cyclization reaction of diaryl- or dialkyl-phosphine oxides with propynoic acid derivatives delivering 3-phosphorylated coumarins, quinolin-2(1H)-one and benzophosphole oxides is presented.

Acyl Radical Chemistry via Visible-Light Photoredox Catalysis

Visible light photoredox catalysis has enabled easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-ketoacids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes such recent progress of visible-light-driven acyl radical generation using transition metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts.

One-pot preparation of 4-aryl-3-bromocoumarins from 4-aryl-2-propynoic acids with diaryliodonium salts, TBAB, and Na2S2O8

Various 4-aryl-3-bromocoumarins were smoothly obtained in moderate yields in one pot by treating 3-aryl-2-propynoic acids with diaryliodonium triflates and K₂CO₃ in the presence of CuCl, followed by the reaction with tetrabutylammonium bromide (TBAB) and Na₂S₂O₈. The obtained 3-bromo-4-phenylcoumarin was transformed into 4-phenylcoumarin derivatives bearing C-H, C-S, C-N, and C-C bonds at 3-position.

Synthesis of 3-cyanomethylated coumarins by a visible-light-mediated direct cyanomethylation of aryl alkynoates

A visible-light-promoted, mild, and direct cyanomethylation of aryl alkynoates has been developed.

Visible-light-promoted radical acylation/cyclization of alkynoates with aldehydes for the synthesis of 3-acylcoumarins

A new, efficient, and atom-economic visible-light-promoted radical acylation/cyclization of alkynoates with aldehydes was developed for the synthesis of 3-acylcoumarins.

Recent advances in radical transformations of internal alkynes

This review highlights the recent progress in the radical transformation of internal alkynes and focuses on the reaction mechanisms by carbon/heteroatom-centered triggered additions, and offers a comprehensive overview on the existing procedures and employed methodologies.

Catalyst-free synthesis of α-thioacrylic acids via cascade thiolation and 1,4-aryl migration of aryl alkynoates at room temperature

A simple and facile catalyst-free strategy has been developed for the synthesis of α-thioacrylic acids from readily-available aryl alkynoates and thiols at room temperature under air.

Synthesis of 2H-Chromenones from Salicylaldehydes and Arylacetonitriles

An efficient and convenient protocol for the synthesis of 2H-chromenones has been developed. In the presence of ^tBuOK in DMF, good to excellent yields of various chromenones were obtained from the corresponding salicylaldehydes and arylacetonitriles. No protection of inert gas atmosphere is required here.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).