Synthesis of Cyano-Containing Phenanthridine Derivatives via Catalyst-, Base-, and Oxidant-Free Direct Cyanoalkylarylation of Isocyanides

Synthesis of 2,4-Dicyanoalkylated Benzoxazines through the Radical-Mediated Cascade Cyclization of Isocyanides with AIBN under Metal- and Additive-Free Conditions

A general and novel method for the radical cascade cyclization of aryl isocyanides with AIBN has been described. This strategy provides straightforward access to various 2,4-dicyanoalkylated benzoxazines in moderate to good yields under metal- and additive-free conditions. The reaction can apply to a gram scale and tolerate diverse functional groups. 2,4-Dicyanoalkylated benzoxazine derivatives feature a large Stokes shift and intramolecular charge transfer properties.

A Copper-Mediated Radical α-Heteroarylation of Nitriles with Azobis(alkylcarbonitriles)

A new practical method has been developed for the α-heteroarylation of aliphatic nitriles with heteroarenes and azobis(alkylcarbonitriles) using Cu(OAc)2 as an oxidizing agent. This method allows the easy construction of nitrile-, aryl-, and dialkyl-bearing quaternary carbon centers from readily available building blocks, without requiring prefunctionalization steps. This reaction is based on adding cyanodialkyl radicals onto heteroarenes, including benzofurans, furans, pyrroles, and indoles. The resulting α-heteroaryl nitriles are useful synthetic intermediates and pharmacophores in biologically active molecules.

Metal-free 2-isocyanobiaryl-based cyclization reactions: phenanthridine framework synthesis

The development of transition metal-free 2-isocyanobiaryl-based reactions has received much attention due to the widespread presence of phenanthidine frameworks as products in pharmacological chemistry and materials science. This review article focuses on the achievements from 2013 until now in various metal-free catalyzed reactions and discusses challenging mechanisms and features of the transformations.

NHC-Catalyzed Tandem Reaction: A Strategy for the Synthesis of 2-Pyrrolidinone-Functionalized Phenanthridines

Herein, an N-heterocyclic carbene (NHC)-catalyzed tandem cyclization/addition/cyclization reaction of 2-isocyanobiaryls and α-bromo-N-cinnamylamides for the synthesis of 2-pyrrolidinone-functionalized phenanthridines is developed. This protocol features a radical cascade process, broad substrate scope, and good functional group compatibility under metal- and oxidant-free reaction conditions.

tert-Butylhydroperoxide mediated radical cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s

A novel sustainable methodology based on one-pot cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s involving a three-component radical cascade pathway has been achieved. Here, tert-butylhydroperoxide (TBHP) acts as an efficient oxidant, and it smoothly drives the reaction, producing the three-component products in very good to excellent yields. This cascade reaction eliminates the use of any base, additive, metal and hazardous cyanating agent. Additionally, this protocol exclusively delivers a stereospecific product in the case of arylalkynes. The involvement of radicals is evidenced through various radical trapping experiments.

Metal-Free Generation of γ-Cyanoalkyl Radicals by N-Heterocyclic Carbene Catalysis: Assembly of 6-Cyanoalkyl Phenanthridines

A number of γ-cyanoalkyl radicals were generated by sustainable N-heterocyclic carbene catalysis in tin-, transition-meal-, and light-free conditions, followed by insertion into biaryl isonitriles, thus leading to the rapid assembly of a variety of diversely functionalized 6-cyanoalkyl phenanthridines. A preliminary mechanism study revealed that a single-electron transfer radical process was possibly involved.

Visible Light-Promoted Radical-Mediated Ring-Opening/Cyclization of Vinyl Benzotriazoles: An Alternative Approach to Phenanthridines

A visible light-promoted radical-mediated ring-opening/cyclization of vinyl benzotriazoles has been developed. The method provides an efficient and practical approach to synthesize functionalized phenanthridines from vinyl benzotriazoles with alkyl bromides under mild conditions. Significantly, the readily available and bench-stable vinyl benzotriazoles can serve as valuable alternative radical acceptors during the synthesis of phenanthridines.

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.

Photocatalyzed alkylative cyclization of 2-isocyanobiphenyls with unactivated alkyl iodides

We herein report the first photocatalyzed radical cascade cyclization of 2-isocyanobiaryls with unactivated alkyl iodides. This simple protocol operates under mild reaction conditions and affords 6-alkyl phenanthridines in good yields. To elucidate the reaction mechanism, Stern-Volmer quenching studies were carried out and these studies revealed that the photocatalyst is not directly involved in a single electron transfer process with the alkyl iodide.

Metal-free three-component cyanoalkylation of quinoxalin-2(1H)-ones with vinylarenes and azobis(alkylcarbonitrile)s

A K2S2O8-mediated C3 cyanoalkylation of quinoxalin-2(1H)-ones via a three-component radical cascade reaction with vinylarenes and azobis(alkylcarbonitrile)s has been achieved.

Mn-mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides with carbazate: access to quinazoline-2-carboxylates

Mn-TBHP mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides using methyl carbazate has been described.

Visible-light-induced selectivity controllable synthesis of diamine or imidazoline derivatives by multicomponent decarboxylative radical coupling reactions

A visible-light-induced and photoredox-catalyzed three-component selectivity controllable synthesis of vicinal diamines and imidazoles from readily available starting materials under mild reaction conditions has been realized.

Iodine-promoted radical alkyl sulfuration of imidazopyridines with dialkyl azo compounds and elemental sulfur

Dialkyl azo compounds were found to be effective alkyl radical sources for direct alkyl sulfuration with imidazopyridines using elemental sulfur under metal-free conditions. Iodine, an inexpensive and mild reagent, could promote alkyl sulfuration. A variety of quaternary cyanoalkyl radicals were successfully coupled with elemental sulfur. A subsequent C-H sulfuration of imidazopyridines afforded a diverse array of imidazopyridine derivatives bearing cyanoalkylthio groups. The cyano group could be modified and further underwent condensation with 2-aminothiazole to afford an interesting heterocyclic amide. Control experiments showed that iodine could greatly suppress the self-coupling of cyanoalkyl radicals, thus making the sulfuration proceed smoothly.

A Synthetic Route to Sodium α-Aminoalkanesulfinates and Their Application in the Generation of α-Aminoalkyl Radicals for Radical Addition Reactions

The synthesis of sodium α-aminoalkanesulfinates and their synthetic utility as α-aminoalkyl radical precursors are reported. A variety of α-aminoalkanesulfinates were readily obtained from the reaction between the anions of N-Boc-protected alkylamines and 1,4-diazabicyclo[2.2.2]octanebis(sulfur dioxide). Treatment of sodium α-aminoalkanesulfinates with (diacetoxyiodo)benzene easily generated the corresponding α-aminoalkyl radicals under mild conditions, which were then applied in radical 1,2-addition to imines, radical 1,4-addition to electron-deficient olefins, and radical addition/cyclization to 2-isocyanobiphenyls.

Boryl Radicals-Triggered Selective C-H Functionalization for the Synthesis of Diverse Phenanthridine Derivatives

A boryl radical-triggered C-H functionalization of aliphatic ethers/amines or DMF with isocyanides is developed to deliver diverse phenanthridine derivatives in good to excellent yields. The substrate scope is broad, and a wide range of functional groups are tolerated under the standard conditions. The rapid removal of HBPin species by 4-cyanopyridine 1-oxide provides the driving force for this reaction. This new method should make boryl radicals widely applicable in organic synthesis.

Cobalt-catalyzed radical cyclization of isocyanides forming phenanthridine derivatives

A neutral cobalt(ii) compound catalyzes the radical cyclization of 2-isocyano-biphenyls with various alkyl halides, providing a series of phenanthridine-fused polycycles.

Use of unprotected amino acids in metal-free tandem radical cyclization reactions: divergent synthesis of 6-alkyl/acyl phenanthridines

We demonstrated a metal-free protocol for the construction of C–C bonds utilizing unprotected amino acids as radical sources.

Visible-light promoted γ-cyanoalkyl radical generation: three-component cyanopropylation/etherification of unactivated alkenes

A photoredox approach to generate distal cyano-substituted alkyl radicals through C–C bond cleavage of cyclobutanone oximes was developed.