Visible-Light-Mediated Decarboxylative Alkylation Cascade Cyano Insertion/Cyclization of N-Arylacrylamides under Transition-Metal-Free Conditions

Photocatalytic radical cyclization of N-(o-cyanobiaryl)acrylamides with oxime esters

A visible-light-mediated radical cascade cyclization of N-(o-cyanobiaryl)acrylamides with oxime esters for the assembly of acyl-containing pyrido[4,3,2-gh]phenanthridines has been developed. The present protocol tolerates a wide range of oxime esters through a single reaction via fragmentation, radical addition, nitrile insertion, and cyclization under mild conditions.

Photocatalytic decarboxylation of free carboxylic acids and their functionalization

Visible light mediated decarboxylative functionalization of carboxylic acids and their derivatives has recently emerged as a novel and powerful toolkit for small molecule activation in diverse carbon-carbon and carbon-hetero bond forming reactions. Naturally abundant highly functionalized bench-stable carboxylic acid analogs have been employed as promising alternatives to non-trivial organometallic reagents for mild and eco-benign synthetic transformation with traceless CO2 by-products. In this highlight article, we focus on the development of various photodecarboxylative functionalization strategies along with intra/inter-molecular cyclization via concerted single electron transfer (SET) or energy transfer (ET) pathways. Moreover, widely explored carboxylic acids are systematically classified here into four categories; i.e., α-keto, aliphatic, α,β-unsaturated, and aromatic analogs for a concise overview to the readership. The association of decarboxylative radical species with coupling partners to construct C-C and C-N/O/S/P/X bonds for each analogous acid has been presented in brief.

Oxidative cascade cyclization of N-arylacrylamides with TMSN3

An azido-radical-triggered cyclization of N-(o-cyanobiaryl)acrylamides with TMSN3via a C(sp3)-N/C(sp2)-C(sp3)/C(sp2)-N bond formation cascade is described. This reaction features mild conditions and high bond-forming efficiency, making it an efficient method for the construction of azide-substituted pyridophenanthridines.

Visible-Light-Promoted Cascade Cyclization of 3-Ethynyl-[1,1'-biphenyl]-2-Carbonitriles with Unsaturated α-Bromocarbonyls

A visible-light-promoted cascade cyclization of 3-ethynyl-[1,1'-biphenyl]-2-carbonitriles with unsaturated α-bromocarbonyls for the synthesis of tetrahydrobenzo[mn]cyclopenta[b]acridines is described. Three C(sp3)-C(sp2) bonds, one C(sp2)-N bond, and three cycles can be formed in a single reaction through the addition of a C-centered radical to the carbon-carbon triple bond and three radical cyclizations. This reaction features mild conditions, wide substrate scope, and high bond-forming efficiency.

Divergent Construction of Azaheterocycles via Alkoxyl Radical-Triggered C-C Bond Cleavage/Cyclization of N-Functionalized Acrylamides

An array of redox-neutral alkylation/cyclization cascade reactions of N-functionalized acrylamides with cycloalkyl hydroperoxides were achieved via the alkoxyl radical-triggered C-C bond cleavage. Through adjusting the radical acceptors on the N atom, a variety of keto-alkylated chain-containing azaheterocycles, including indolo[2,1-a]isoquinolin-6(5H)-ones, quinoline-2,4-diones, and pyrido[4,3,2-gh]phenanthridines were constructed by a one-pot procedure with good yields and excellent functional group tolerance.

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.

Metal- and Base-Free Radical Cascade Cyclization/Hydrolysis of CN-Containing 1,6-Enynes with Ethers to Access Polyheterocycles

A convenient and straightforward approach for the radical cascade cyclization/hydrolysis of CN-containing 1,6-enynes with simple ethers under metal- and base-free conditions is described. This strategy provides a variety of valuable ethers-substituted polyheterocycles via the construction of three C-C bonds, one C=O bond, and two new six-membered rings within a single procedure. The resulting products can smoothly undergo follow-up conversions to various useful scaffolds. The methodology shows excellent functional group tolerance, high step- and atom- economy, and mild reaction conditions, which can be further scaled up to gram quantity in a satisfactory yield.

The Renaissance of Organo Nitriles in Organic Synthesis

In the arena of functional group-oriented organic synthesis, the nitrile or cyano functionality is of immense importance. The presence of nucleophilic N -atom, π-coordinating ability of the triple bond, and electrophilic C-center imparts unique and interesting reactivity. Owing to the ability of the nitrile to transform into various other functional groups or intermediates, the chemistry is very rich and diverse. In particular, the involvement of nitrile in numerous organic reactions such as inter- or intramolecular alkyne insertion, [2 + 2 + 2] cycloaddition with alkynes, [3 + 2] cycloaddition with azides, [4 + 2] cycloaddition with diene allow the synthesis of many important carbocycles and heterocycles. Furthermore, the nitrile serves as a directing group in many C-H bond functionalization reactions to introduce diverse functionality and participate as a radical acceptor in radical cascade strategies to obtain a large variety of functional molecules. This review mainly focuses on the reactivity and diverse synthetic application of the nitrile including C-H bond functionalization, alkyne insertion, cycloaddition, and thermal or photochemical cascade strategy. The objective of the current review aims at bringing out the striking collection of various nitrile-triggered organic transformations.

A Review on Synthetic Approaches of Phenanthridine

Abstract:

The phenanthridine family is widely found in medicinal chemistry and material science because of the biological activity and its presence in a variety of significant natural products and synthetic dye stuffs. The phenanthridine has many clinical applications, for e.g., being used as an anticancer agent, possessing antibacterial, antiprotozoal, pharmaceutical, and optoelectronic properties. Many methods have been reported for the synthesis of phenanthridine and phenanthridine alkaloids, such as Pd catalyzed C-C bond formation, a reaction involving C-H activation, radical, microwave-assisted, transition metal-catalyzed, one-pot cascade, benzyne mediated, photochemical, hypervalent iodine promoted methods, etc. Here, we have summarized the literature data from 2014 to the present concerning novel or improved synthetic approaches.

Direct decarboxylative Giese reactions

The quest to find milder and more sustainable methods to generate highly reactive, carbon-centred intermediates has led to a resurgence of interest in radical chemistry. In particular, carboxylic acids are seen as attractive radical precursors due their availability, low cost, diversity, and sustainability. Moreover, the corresponding nucleophilic carbon-radical can be easily accessed through a favourable radical decarboxylation process, extruding CO₂ as a traceless by-product. This review summarizes the recent progress on using carboxylic acids directly as convenient radical precursors for the formation of carbon-carbon bonds via the 1,4-radical conjugate addition (Giese) reaction.

Visible-Light-Induced C-H Bond Aminoalkylation of Heterocycles by the Decarboxylation Coupling of Amino Acids

An efficient visible-light-induced decarboxylative coupling reaction of N-protecting α-amino acids with heterocycles for the generation of aminoalkylated heterocycles is presented. A series of aminoalkylated heterocycles were obtained in moderate to good yields. Attractive features of this process include the generation of aminomethyl radical by an inexpensive organic photocatalyst under transition-metal-free conditions.

A Photochemical Intramolecular C-N Coupling Toward the Synthesis of Benzimidazole-Fused Phenanthridines

Herein, we report a direct photochemical dehydrogenative C-N coupling of unactivated C(sp²)-H and N(sp²)-H bonds. The catalysts or additive-free transformation of 2-([1,1'-biphenyl]-2-yl)-1H-benzo[d]imidazole to benzo[4,5]imidazo[1,2-f]phenanthridine was achieved at ∼350 nm of irradiation via ε-hydrogen abstraction. DFT calculations helped to understand that the N-H···π interaction was essential for the reaction to proceed at a lower energy than expected.

Nitriles as radical acceptors in radical cascade reactions

The application of the cyano group as a radical acceptor in the cascade reactions for the construction of various important heterocycles and carbocycles was summarized.

Photocatalyzed syntheses of phenanthrenes and their aza-analogues. A review

Phenanthrenes and their aza-analogues have important applications in materials science and in medicine. Aim of this review is to collect recent reports describing their synthesis, which make use of radical cyclizations promoted by a visible light-triggered photocatalytic process.

Visible-light-promoted acyl radical cascade reaction for accessing acylated isoquinoline-1,3(2H,4H)-dione derivatives

A visible-light-promoted radical acylation/cyclization cascade reaction of N-methacryloylbenzamides with α-keto acids was developed to construct acylated isoquinoline-dione derivatives.

Iron-mediated deuterium addition cascade cyano insertion/cyclization of N-arylacrylamides to access deuterium-labelled phenanthridines

Deuterated molecules feature important biological activities and pharmacokinetic properties for the design of pharmaceuticals.

Oxidative cascade cyclization of 2-cyano-3-arylaniline derived acrylamides with toluenes, ethers, aliphatic alcohols or simple alkanes

Oxidative radical cascade cyclization of 2-cyano-3-arylaniline derived acrylamides with a variety of hydrocarbons including toluenes, ethers, aliphatic alcohols and alkanes has been developed to construct alkyl substituted pyridophenanthridines.

Visible-light-induced radical trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides

An efficient and general visible-light-mediated trifluoromethylthiolation of N-(o-cyanobiaryl)acrylamides has been successfully accomplished using N-trifluoromethylthiosaccharin as an effective source of SCF3 radicals. The reaction was proposed to proceed via a domino radical trifluoromethylthiolation/cyano insertion/cyclization to afford the corresponding SCF3-containing ring-fused phenanthridine derivatives in moderate to good yields.

Silver-catalyzed decarboxylative radical cascade cyclization toward benzimidazo[2,1-a]isoquinolin-6(5H)-ones

A simple and efficient decarboxylative radical addition/cyclization strategy was developed, by which a wide range of benzimidazo[2,1-a]isoquinoline-6(5H)-ones were prepared in one-pot via reaction of functionalized 2-arylbenzoimidazoles and carboxylic acids in the presence of K2S2O8/AgNO3 under mild reaction conditions.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

Recent advances in iminyl radical-mediated catalytic cyclizations and ring-opening reactions

Iminyl radicals have emerged as versatile synthons for N-heterocycle constructions and ring-opening reactions.

Regioselective synthesis of polycyclic sulfones via radical-induced three-component bicyclization cascades

New radical-triggered three-component bicyclization cascades of 2-alkynylaryldiazonium tetrafluoroborates with a sulfur dioxide surrogate DABCO·(SO₂)₂ and internal alkynes have been reported for the first time, leading to 49 examples of polycyclic sulfones with moderate to good yields.

Decarboxylative cascade cyclization of α-keto acids with 2-cyano-3-arylaniline-derived acrylamides

An oxidative decarboxylative cascade cyclization of α-keto acids with 2-cyano-3-arylaniline-derived acrylamides delivering carbonyl-containing pyrido[4,3,2-gh]phenanthridines is presented.

Oxidative radical cascade cyclization involving C(sp3)–C(sp3), C(sp3)–C(sp2) and C(sp2)–N bonds formation: direct construction of cyano and methyl substituted polyheterocycles

Oxidative radical cascade cyclizations of 2-cyano-3-arylaniline derived acrylamides with acetonitrile and tert-butyl peroxybenzoate have been developed to construct cyano and methyl substituted pyrido[4,3,2-gh]phenanthridines.

Transition metal free decarboxylative fluoroalkylation of N-acrylamides with 3,3,3-trifluoro-2,2-dimethylpropanoic acid (TFDMPA)

1,1-Dimethyl-2,2,2-trifluoroethyl substituted oxindoles were prepared by a novel transition metal-free decarboxylative fluoroalkylation of activated alkenes and C–H functionalization cascade process.