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.

Visible-Light-Mediated Vicinal Difunctionalization of Activated Alkynes with Boronic Acids: Substrate-Controlled Rapid Access to 3-Alkylated Coumarins and Unsaturated Spirocycles

A visible-light-mediated difunctionalization of activated alkynes with boronic acid is unveiled to synthesize 3-alkylated coumarins and unsaturated spiro-lactones. The substituent at the para-position of the aryl ring of aryl alkynoate plays a pivotal role in the selective formation of chain-alkylated coumarins or spirocyclic compounds under mild conditions. The reaction employs a hypervalent iodine reagent and ruthenium photocatalyst. The spiro-lactones thus obtained were subjected to another novel mode of visible-light-mediated radical addition cascade cyclization (RACC) to access various new fused spirocyclic compounds.

Copper-catalyzed cascade radical cyclization of alkynoates: construction of aryldifluoromethylated coumarins

A mild and simple method is reported for the construction of 3-difluoroarylmethylated coumarins using α,α-difluoroarylacetic acids as an easily handled difluoromethyl source in reaction with ester 3-arylpropiolates under the promotion of copper. The reaction involves a proposed radical-triggered domino decarboxylative aryldifluoromethylation/5-exo cyclization/ester migration process that directly forms Csp²-CF₂Ar and C-C bonds with good functional group tolerance.

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.

3-Nitro-coumarin synthesis via nitrative cyclization of aryl alkynoates using tert-butyl nitrite

We report a one-pot metal-free synthesis of 3-nitro-coumarins from aryl alkynoates using TBN (tert-butyl nitrite) as the sole reagent. The radical triggered cyclization and nitration of the aryl alkynoates proceeded in a cascaded manner via nitro radical addition to alkynoates, then 5-exo-trig spirocyclization and ester migration.

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)].

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.

Metal-free spirocyclization of N-arylpropiolamides with glyoxylic acids: access to complex azaspiro-fused tricycles

An efficient K₂S₂O₈-mediated oxidative cascade spirocyclization of N-arylpropiolamides with aryl glyoxylic acids was demonstrated for constructing azaspiro[4,5]-trienones and complex azaspiro-fused architectures.

Atom Transfer Oxidative Radical Cascade of Aryl Alkynoates towards 1,1-Dichalcogenide Olefins

An oxidative trifunctionalization of aryl alkynoates has been devised via the chalcogenide radical triggered intramolecular 1,4-aryl migration/decarboxylation cascade to prepare 1,1-dichalcogenide tetrasubstituted alkenes in high yields (up to 98 %). This operationally simple reaction proceeds under metal-free conditions, can be executed on gram scale, and highlights formal 1,1-difunctionalization of alkynes. Synthetic potential of this protocol was demonstrated through a twofold cascade rearrangement to access highly conjugated tetra-selenylated alkenes along with a cross-dehydrogenative annulation to prepare fluorene derivative.

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.

Synthesis of 3-Phenylsulfonylcoumarins by Cyclisation of Phenyl Propiolates with Sulfinic Acids or Sodium Sulfinates

Methods for the synthesis of 3-phenylsulfonylcoumarins by radical sulfonylation cyclisation of phenyl propiolates with sulfinic acids or sodium sulfinates as sulfonyl sources have been developed. A combination of potassium persulfate as oxidant and silver nitrate as catalyst, or tert-butyl hydroperoxide as oxidant and potassium iodide as catalyst were used. The products were formed by an ipso-cyclisation/1,2-ester migration mechanism. The toleration of functional groups is broad and the yield is up to 96%.

Regioselective 5-exo-dig oxy-cyclization of 2-alkynylbenzamide for the synthesis of isobenzofuran-1-imines and isobenzofuran

A TBAB-mediated regioselective radical cyclization of 2-alkynylbenzamide is described herein for the synthesis of isobenzofuran-1-imines. The reactions proceeded smoothly under mild conditions.

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.

Experimental and Theoretical Studies on Iron-Promoted Oxidative Annulation of Arylglyoxal with Alkyne: Unusual Addition and Migration on the Aryl Ring

An Fe(III)-promoted oxidative annulation reaction was developed for the synthesis of 1,2-naphthoquinones. A variety of substituted arylglyoxals and internal alkynes undergo the transformation in the presence of FeCl₃ at room temperature to afford the 1,2-naphthoquinone products in good yields in a short reaction time. Interestingly, the products show unusual pseudomigration of the substituent on the arene ring of arylglyoxals. A possible mechanism involving Fe(III)-promoted formation of a vinyl cation from arylglyoxal and alkyne, electrophilic addition of the vinyl cation to the ipso carbon of the aryl group to give a spiral intermediate, and then migration of the keto carbon to the ortho carbon was proposed as key steps and verified using quantum mechanics.

Iodine-catalyzed cascade annulation of alkynes with sodium arylsulfinates: assembly of 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives

An efficient and environmentally benign iodine-catalyzed cascade cyclization has been achieved to afford functionalized 3-sulfenylcoumarin and 3-sulfenylquinolinone derivatives.