Catalytic Dual 1,1-H-Abstraction/Insertion for Domino Spirocyclizations

Formation of cyclopenta[c]quinolines through visible-light-induced photoredox cascade bis-annulations of 1,7-enynes with sulfoxonium ylides

A novel visible-light-driven photoredox-catalyzed cascade bicyclization of 1,7-enynes with aqueous sulfoxonium ylides is reported. The reaction is highly chemoselective with three new C-C bonds, two new rings, and an all-carbon quaternary stereocenter constructed in a one-pot fashion. This mild protocol features a remarkably broad substrate scope with good functional group tolerance, providing a general and practical approach to access various cyclopenta[c]quinolines.

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.

Stereoselective Three-Component Carbocyclization of 1,7-Enynes with Sulfoxonium Ylides and Water under Photoredox Catalysis

A visible-light-induced photoredox three-component carbocyclization reaction of 1,7-enynes with sulfoxonium ylides and water is reported. The protocol provides a facile entry to structurally valuable highly functionalized cyclopenta[c]quinoline scaffolds in a highly chemoselective and stereoselective manner. Salient features of this method include facile redox-neutral conditions, no requirement of base or other additive, and good functional-group tolerance.

Photochemical Divergent Ring-Closing Metathesis of 1,7-Enynes: Efficient Synthesis of Spirocyclic Quinolin-2-ones

A photocatalytic method for the ring-closing 1,7-enyne metathesis using the α-amino radical as an alkene deconstruction auxiliary is present. Preliminary mechanistic studies suggest that intramolecular 1,5-hydrogen atom transfer is the key to the generation and β-scission of the α-amino radical, while the dearomatization of arenes and ring opening of cyclopropanes are the key to construct spirocyclic quinolin-2-ones. This approach highlights the potential of ring-closing 1,7-enyne metathesis, providing a green, efficient, and step-economical way for the synthesis of spirocyclic quinolin-2-ones.

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Synthesis of Acyl Cyclopentaquinolinones through Simultaneous Construction of the Heterocyclic Scaffold and Introduction of the Acyl Group

Presented herein is an effective and concise synthesis of acyl cyclopentaquinolinone derivatives via the cascade reactions of N-(o-ethynylaryl)acrylamides with α-diazo carbonyl compounds. The formation of product involves a visible light-induced radical formation from α-diazo carbonyl compound followed by its addition onto the acrylamide moiety to trigger double radical annulation, single-electron oxidation, and β-elimination. To our knowledge, this is the first example in which the cyclopentaquinolinone scaffold was constructed along with the introduction of an acyl group under visible light irradiation conditions. Compared with literature methods for similar purpose, this newly developed protocol has advantages such as readily accessible substrates, mild reaction conditions, valuable products, concise synthetic procedure, and high sustainability. With all these merits, this method is expected to find wide applications in the construction of related acyl heterocyclic skeletons.

Metal-free radical bicyclization/chloroalkylarylation of 1,6-enynes with chloroalkanes

Free radical initiated bicyclization of 1,6-enynes with chloralkanes, is achieved via selective activation of the C(sp3)-H bond of the chloralkane, resulting in diverse polychlorinated/chlorinated polyheterocycles. Two kinds of transformations and a scaled-up experiment were performed to test the synthetic importance of the organic chlorides. Finally, a range of radical inhibition operations and radical clock tests were explored to support the reaction process.

Visible-Light-Initiated Catalyst-Free Radical Annulation Reactions of 1,6-Enynes and Aryl Sulfonyl Bromide to Assemble Sulfonation/Bromination Succinimide Derivatives

In the present study, the environment-friendly visible-light-promoted strategy is used to perform an efficient, simple, and straightforward photocatalytic succinimide derivative synthesis from the reaction of 1,6-enynes and aryl sulfonyl bromide at room temperature under air ambient conditions. This method features mild conditions, broad substrate scope, high yields, and excellent configurational selectivity. In addition, all the atoms of the substrates involved in the reaction converge in the product structures, showing a high atomic economy. Moreover, the most important characteristic of this study is that no photocatalyst and additives are used, while the key factor that triggers the reaction is visible light, indicating that this study has an important practical value.

Visible-Light-Mediated Radical Hydroalkylative Cyclization of 1,6-Enynes

A radical hydroalkylative cyclization approach accessing various alkenyl heterocyclic compounds was developed using dimethyl malonate and 1,6-enynes in the presence of visible-light photoredox catalysis. The use of Ir(dtbbpy)(ppy)2PF6 as a photosensitizer enables carbon atom radical formation and initiates the cascade cyclization reaction under mild conditions.

Synthesis of O-Heterocycle Spiro-Fused Cyclopentaquinolinones and Cyclopentaindenes through Visible Light-Induced Radical Cyclization Reactions

Presented herein is an effective and sustainable synthesis of O-heterocycle spiro-fused cyclopentaquinolinone and cyclopentaindene derivatives through light-driven cascade reactions of N-(o-ethynylaryl)acrylamides or 2-(2-(phenylethynyl)benzyl)acrylate with various O-heterocycles. Experimental mechanistic studies revealed that these reactions are initiated by visible light-induced radical formation from O-heterocycle and its regioselective addition onto the acrylamide or acrylate moiety followed by 6-exo-dig and 5-endo-trig cascade radical annulation, which is terminated by single electron oxidation and proton elimination. Compared with previously reported synthetic methods for similar purposes, this newly developed protocol has advantages such as a broad substrate scope, extremely mild reaction conditions, excellent atom-economy, high efficiency, and good compatibility with diverse functional groups. With all of these merits, this method is expected to find wide applications in the related research arena.

Palladium-Catalyzed Carbonylative Synthesis of Polycyclic 3,4-Dihydroquinolin-2(1H)-one Scaffolds Containing Perfluoroalkyl and Carbonyl Units

A palladium-catalyzed one-pot two-step radical mediated carbonylative cyclization of 1,7-enynes with perfluoroalkyl iodides and Mo(CO)₆ has been developed for the construction of polycyclic 3,4-dihydroquinolin-2(1H)-one scaffolds. This method realizes a facile synthesis of various polycyclic 3,4-dihydroquinolin-2(1H)-one derivatives containing perfluoroalkyl and carbonyl units in high yields. Moreover, modifications of several bioactive molecules were demonstrated using this protocol.

Visible-Light-Promoted Cyanoalkylation/Cyclization Cascade Reaction to Assemble Polyheterocycles in Continuous Flow

This study describes a visible-light-induced cascade reaction for preparing cyanoalkyl-containing polyheterocycles initiated by the photoinduced radical cascade addition of N-arylacrylamide derivatives using cyclic oxime esters as radical sources followed by cyanoalkyl-mediated cyclization. This protocol features outstanding functional group compatibility, providing a variety of desired phenanthridine derivatives in moderate to good yields. Moreover, the application of a microflow technique enhanced these reactions compared with the equivalent batch reaction, significantly reducing reaction times to 10 min.

Hypervalent iodine(iii) promoted C–H/C–H amination/annulation tandem reactions: synthesis of benzimidazoles from simple anilines and aldehydes

A novel hypervalent iodine mediated cascade transformation of anilines and aldehydes to benzimidazoles was developed.

Radical annulation using a radical reagent as a two-carbon unit

Radical annulation has emerged as one of the most efficient and straightforward methods for synthesizing cyclic/polycyclic compounds as the core structures can be constructed through a single procedure comprising multiple bond-forming steps. Particularly, radical annulation using a radical reagent as a cyclization partner and two-carbon unit greatly expands the diversity of cyclic skeletons and the functionality of radical reagents. We herein have highlighted the representative processes reported in the past decade for radical annulation using a radical reagent as a two-carbon unit, including [2 + 2 + 2], [3 + 2], [4 + 2], and [5 + 2] modes, with an emphasis on their reaction mechanisms. These studies not only pave the way toward annulation but also provide insight into the exploration of a new reaction mode for radical chemistry.

Iodo-sulphonylation of 1,6-enynones: a metal-free strategy to synthesize N-substituted succinimides

An iodine-mediated radical cyclization of 1,6-enynones with sulphonyl hydrazides using tert-butyl hydroperoxide (TBHP) as the oxidant has been developed for the synthesis of iodo-sulphonylated-succinimide derivatives. The notable advantages of the developed method are metal-free conditions, broad functional group tolerance, column chromatography-free purification, high stereoselectivity (E isomer), shorter reaction times, and the cascade construction of three new bonds (C-S, C-I, and C-C). The synthetic application of the iodo-functionality has been extended to the Heck coupling reaction with acrylonitrile and to the Suzuki coupling reaction with benzene boronic acid.

Visible-light photocatalytic radical addition-translocation-cyclization to construct sulfonyl-containing azacycles

Herein, a novel visible-light photocatalytic radical addition-translocation-cyclization (RATC) approach for the efficient synthesis of sulfonyl-containing azacycles is described. The reaction delivers a wide range of monocyclic, bicyclic and polycyclic azacycles by using easily prepared sodium sulfinates and N-homopropargylic amines as starting materials. Instead of the traditionally used toxic tin reagents and thermally hazardous azos in the RATC process, clean, renewable and sustainable visible light combined with a catalytic amount of photosensitizer is used in this process. Moreover, the successful transformation of some drug derivatives further highlights the potential application of this procedure.

[3+2] Cycloaddition of alkyl aldehydes and alkynes enabled by photoinduced hydrogen atom transfer

[3+2] Cycloaddition is a step- and atom-economic method for the synthesis of five-membered rings. Despite the great success of 1,3-dipolar cycloadditions, the radical [3+2] annulation of alkynes remains a formidable challenge. Herein, a photoinduced decatungstate-catalyzed [3+2] cycloaddition of various internal alkynes using abundant aliphatic aldehydes as a three-carbon synthon is developed, producing elaborate cyclopentanones in 100% atom economy with excellent site-, regio-, and diastereoselectivity under mild conditions. The catalytic cycle consists of hydrogen atom abstraction from aldehydes, radical addition, 1,5-hydrogen atom transfer, anti-Baldwin 5-endo-trig cyclization, and back hydrogen abstraction. The power of this method is showcased by the late-stage elaboration of medicinally relevant molecules and total or formal synthesis of (±)-β-cuparenone, (±)-laurokamurene B, and (±)-cuparene.

In contrast to the prevalence of 1,3-dipolar cycloadditions, radical [3+2] annulations of alkynes are underexplored. Here, the authors describe [3+2] cycloadditions of various internal alkynes with readily accessible aliphatic aldehydes via photoinduced decatungstate catalysis.

Copper-assisted trifluoromethylthiolation/radical cascade cyclization of alkynes to construct SCF3-containing dioxodibenzothiazepines

A mild and efficient Cu-assisted trifluoromethylthiolation/radical cascade cyclization of alkynes with readily available and stable AgSCF₃ as the trifluoromethylthiolating reagent has been disclosed. This transformation provides an opportunity to construct a series of potential medicinally valuable trifluoromethylthio-substituted dioxodibenzothiazepines with wide functional group compatibility. This protocol opens up a new avenue for the construction of useful trifluoromethylthiolated seven-membered N-heterocycles.

Access to cyclopentenones via copper-catalyzed 5-endo trifluoromethylcarbocyclization of ynones

A copper-catalyzed 5-endo-selective trifluoromethylcarbocyclization of ynones is realized for the direct construction of trifluoromethylated cyclopentenones.

Nickel-catalyzed cascade hydrosilylation/cyclization of 1,7-enynes leading to silyl-containing quinolinones

In this paper, we disclose a nickel-catalyzed cascade hydrosilylation/cyclization reaction of 1,7-enynes with bulky silanes. The reaction features excellent chemoselectivity, broad functional group tolerance, and mild reaction conditions.

Providing direction for mechanistic inferences in radical cascade cyclization using Transformer model

Even in modern organic chemistry, predicting or proposing a reaction mechanism and speculating on reaction intermediates remains challenging. For example, it is challenging to predict the regioselectivity of radical attraction...

C-centered radical-initiated cyclization by directed C(sp3)–H oxidative functionalization

C(sp3)–H functionalization is attracting constant attention. This review emphasizes C-centered radicals initiated cyclization strategies by directed C(sp3)–H oxidative functionalization since 2012.

Nickel-catalyzed cyclization of 1,7-enynes for the selective synthesis of dihydrocyclobuta[c]quinolin-3-ones and benzo[b]azocin-2-ones

We herein described a nickel-catalyzed cyclization of N-(o-ethynylaryl)acrylamides for the selective synthesis of dihydrocyclobuta[c]quinolin-3-ones and benzo[b]azocin-2-ones. The two varied products could be easily obtained by tuning the reaction temperature. This reaction features easy temperature-control, high efficiency, and gram-scale synthesis.

The construction of benzimidazo[2,1-a]isoquinolin-6(5H)-ones from N-methacryloyl-2-phenylbenzoimidazoles through radical strategies

Benzimidazo[2,1-a]isoquinolin-6(5H)-one constitutes a structurally unique class of tetracyclic N-heterocycles that are found throughout a myriad of biologically active natural products, pharmaceutical compounds, and functional materials. Various synthetic routes for the preparation of benzimidazo[2,1-a]isoquinolin-6(5H)-ones have been reported. In particular, the use of N-methacryloyl-2-phenylbenzoimidazoles to construct benzimidazo[2,1-a]isoquinolin-6(5H)-ones through various radical strategies have attracted widespread attention due to the versatility and simple preparation of raw materials, as well as the step-economy and mild reaction conditions. Using representative examples, we highlight significant progress in the synthesis of benzimidazo[2,1-a]isoquinolin-6(5H)-ones, including the selection of the catalytic system, substrate scope, mechanistic understanding, and applications. The contents of this review focus on the development of C-, S-, P-, and Si-centered radical addition-intramolecular cyclization strategies.

K2S2O8-Mediated Synthesis of Highly Functionalized Pyrroles via Oxidative Self-Dimerization of N-Propargylamines

An efficient methodology has been developed for the synthesis of tetra- and pentasubstituted pyrroles via oxidative self-dimerization of N-propargylamines catalyzed by silver benzoate in the presence of K₂S₂O₈ in good yields. The protocol provides a simple route for the synthesis of both tetra- and pentasubstituted pyrroles with two carbonyl groups in the side chain. The methodology can be extended toward the synthesis of pyrrolo[3,4-d]pyridazine.

Copper-catalyzed aerobic double functionalization of benzylic C(sp3)-H bonds for the synthesis of 3-hydroxyisoindolinones

A copper-catalyzed aerobic 3-hydroxyisoindolinone synthesis was developed via the benzylic double C(sp³)-H functionalization of 2-alkylbenzamides. In this reaction, molecular oxygen was used as both an oxidant for C(sp³)-H functionalization and an oxygen source. Our method can be extended to diverse benzylic C(sp³)-H bonds and shows excellent functional group tolerance.

Diacyl peroxides: practical reagents as aryl and alkyl radical sources

Diacyl peroxides, which can be easily synthesized from corresponding carboxylic acids, are commonly utilized as radical initiators and one electron oxidants. Under thermal, transition-metal catalysis or irradiation conditions the cleavage of relatively weak O-O bonds would occur followed by CO₂ extrusion to generate the corresponding aryl or alkyl radicals. Thus, diacyl peroxides can be employed as ideal arylating and alkylating reagents in organic synthesis, including C-H/N-H arylation/alkylation, aryl/alkyl radical addition to unsaturated bonds, hetero arylation/alkylation, eliminative arylation/alkylation, perfluoroalkylation etc. Moreover, these arylation/alkylation protocols have been successfully utilized in the synthesis and late-stage functionalization of natural products as well as bioactive molecules. In this review, recent advances on arylation and alkylation using diacyl peroxides as aryl and alkyl radical sources are summarized and discussed.

PhI(OAc)2-Mediated Dihalogenative Cyclization of 1,6-Enyne with Lithium Halide

The dihalogenative cyclization of 1,6-enyne with the assistance of PhI(OAc)₂ and lithium halide is presented. A plausible radical mechanism is proposed, which consists of addition of halogen radical to alkene, 5-exo-dig radical cyclization of enyne and halogenation via radical coupling. The alkenyl- and alkyl-halide groups in the resulted pyrrolidine products have been demonstrated to be facile handles for further transformations.

Silver-Catalyzed Radical Cascade Sulfonation/Cycloaddition for the Construction of Multifunctional Succimides Containing Separable Z/E-Isomers

A silver-catalyzed cascade cycloaddition of aza-1,6-enynes, affording multifunctional succimide frameworks initiated by the arylsulfonyl radical addition, has been developed. This process shows mild reaction conditions, excellent structural selectivity, and broad functional group tolerance. In addition, the Z/E-isomers can be easily separated, which provides an efficient method for obtaining pure Z/E-configuration products.

Metal-Free Radical Annulation of Oxygen-Containing 1,7-Enynes: Configuration-Selective Synthesis of (E)-3-((Arylsulfonyl)methyl)-4-Substituted Arylidenechromene Derivatives

A novel strategy for the synthesis of (E)-3-((arylsulfonyl)methyl)-4-substituted benzylidenechromene derivatives via a metal-free radical annulation reaction of oxygen-containing 1,7-enynes with thiosulfonates has been developed. The reaction shows broad substrate scope, wide functional group tolerance, and moderate to excellent yields. Moreover, thiosulfonates were well driven to achieve the bifunctionalization reaction of oxo-1,7-enynes which derived from aliphatic alkynes. In addition, the (E)-configuration of the products was highly controlled by the structure of 1,7-enyne.

Recent advances in cascade radical cyclization of radical acceptors for the synthesis of carbo- and heterocycles

This review is devoted to highlighting main achievements in the development of cascade radical cyclization of radical acceptors for the synthesis of carbo- and heterocycles.

Radical Tandem Bicyclization Triggered by the α-Position of α,β-Unsaturated Ketones Bearing Nonterminal 1,6-Enynes: Synthesis of the 5H-Benzo[a]fluoren-5-one Skeleton

A novel method for the synthesis of 5H-benzo[a]fluoren-5-one from inactive nonterminal 1,6-alkynes through a free radical tandem bicyclization process has been established. This process achieved a continuous cyclization triggered rarely by the α-position of α,β-unsaturated ketones. The reaction constructed three C-C bonds in one step with the readily accessible substrates and excellent substrate compatibility.