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.

Cu(II)-Catalyzed Decarboxylative (4 + 2) Annulation of Coumarin-3-Carboxylic Acids with In Situ Generated α,β-Unsaturated Carbonyl Compounds from tert-Propargylic Alcohols

Cu(II)-catalyzed decarboxylative oxidative (4 + 2) annulation of coumarin-3-carboxylic acids with tert-propargylic alcohols, via the in situ generated α,β-unsaturated carbonyl compounds by the Meyer-Schuster rearrangement, has been developed. This protocol involving indirect C-H functionalization offers access to diverse naphthochromenone architectures with good to excellent yields.

A unified approach to benzo[c]phenanthridines via the cascade dual-annulation/formylation of 2-alkynyl/alkenylbenzonitriles

A base-mediated versatile cascade dual-annulation and formylation of 2-alkenyl/alkynylbenzonitriles with 2-methylbenzonitriles has been established for the construction of four different classes of amino and amido substituted benzo[c]phenanthridines and benzo[c]phenanthrolines. The synthesized molecules could be of utmost relevance in pharmaceuticals. The transformation uses the solvent DMF as the formyl source for synthesis of the amido-substituted scaffolds. This transition-metal-free unique strategy enables the formation of multiple C-C and C-N bonds in one pot at room temperature.

Electrochemical trifluoromethylation of 2-isocyanobiaryls using CF3SO2Na: synthesis of 6-(trifluoromethyl)phenanthridines

An efficient trifluoromethylation of 2-isocyanobiaryls was developed through the constant current electrolysis, employing sodium trifluoromethanesulfinate (CF₃SO₂Na) as the trifluoromethyl source. The method enabled the syntheses of a series of 6-(trifluoromethyl)phenanthridine derivatives in moderate to high yields under metal- and oxidant-free conditions. A gram-scale synthesis highlights the synthetic versatility of the reported protocol.

Photoinduced N-Heterocyclic Nitrenium-Catalyzed Single Electron Reduction of Acyl Fluorides for Phenanthridine Synthesis

Acyl fluorides are versatile reagents in organic synthesis. However, there is no precedent to employ acyl fluorides as acyl radical precursors. We herein report an N-heterocyclic nitrenium iodide salt-catalyzed photoreduction of acyl fluorides to produce acyl radicals, which could react with 2-isocyanobiaryls to afford various carbonyl phenanthridines.

Silver-catalyzed decarboxylative cyclization for the synthesis of substituted pyrazoles from 1,2-diaza-1,3-dienes and α-keto acids

A silver-catalyzed decarboxylative cyclization process has been developed for the synthesis of substituted pyrazoles from the readily available 1,2-diaza-1,3-dienes and α-keto acids. Under the optimized conditions, a series of multisubstituted pyrazoles were well prepared in moderate to good yields. In addition, the synthetic utility of this protocol has been demonstrated by synthesizing analogs of FDA approved drugs such as anti-inflammatory drug, lonazolac and antiobesity drug, rimonabant.

Persulfate-nitrogen doped graphene mixture as an oxidant for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters and nitrite

A series of 3-nitro-4-aryl-2H-chromen-2-ones in good yields have directly been obtained from aryl alkynoate esters and nitrite by employing a mixture of K₂S₂O₈-nitrogen doped graphene as an oxidant in a watery medium at room temperature. A plausible mechanism for the reaction is also reported. It reveals that the product is formed through a cascade of nitro radical addition, spirocyclization, and ester migration. When compared to known methods for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters, this protocol is environmentally friendly, sustainable, practical and energy efficient and does not use a harmful nitro source. Furthermore, nitrogen doped graphene used in this approach can be easily recovered and reused at least four times without losing its activity.

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.

Silver-catalyzed Radical Cascade Arylthiodifluoromethylation/ Cyclization of Isonitriles for the Synthesis of 6-Phenanthridinyldifluoromethyl Aryl Thioethers

An efficient method for silver-catalyzed radical cascade arylthiodifluoromethylation/cyclization of isonitriles is disclosed. The transformation comprised addition of an arylthiodifluoromethyl radical generated in situ by the oxidative decarboxylation of arylthiodifluoroacetic salts to the isonitrile functionality to construct an ArSCF₂ -C bond, followed by intramolecular cyclization to eventually afford 6-phenanthridinyldifluoromethyl aryl thioethers. The protocol provided a variety of 6-phenanthridinyldifluoromethyl aryl thioethers in medium to excellent yields with a good functional group tolerance under mild reaction conditions.

Hypervalent Iodine(III)-Promoted Radical Oxidative C-H Annulation of Arylamines with α-Keto Acids

A novel catalyst-free radical oxidative C-H annulation reaction of arylamines with α-keto acids toward benzoxazin-2-ones synthesis under mild conditions was developed. This hypervalent iodine(III)-promoted process eliminated the use of a metal catalyst or additive with high levels of functional group tolerance. Hypervalent iodine(III) was both an oxidant and a radical initiator for this reaction. The synthetic utility of this method was confirmed by the synthesis of the natural product cephalandole A.

Photocatalytic xanthate-based radical addition/cyclization reaction sequence toward 2-biphenyl isocyanides: synthesis of 6-alkylated phenanthridines

A photocatalytic xanthate-based radical addition/cyclization reaction cascade toward 2-biphenylisocyanides is described as a practical and modular approach to 6-alkylated phenanthridines. The use of xanthates as radical precursors allowed the synthesis of diversely 6-substituted phenanthridines. Electrophilic radicals derived from nitriles, aromatic and aliphatic ketones, malonates, and amide derivatives, as well as radicals derived from phthalimidomethyl and benzylic derivatives were successfully introduced. The reaction proceeds under mild conditions without a stoichiometric amount of oxidant. Thirty novel phenanthridine scaffolds were synthesized with yields ranging from 24 to 76%.

Photochemical synthesis of 3-hydroxyphenanthro[9,10-c]furan-1(3H)-ones from α-keto acids and alkynes

A novel and efficient method for the synthesis of 3-hydroxyphenanthro[9,10-c]furan-1(3H)-ones has been achieved from α-keto acids and alkynes through photo-initiated transformation, providing a range of products in good to excellent yields.

Metal-free tandem carbene N-H insertions and C-C bond cleavages

A metal-free C-H [5 + 1] annulation reaction of 2-arylanilines with diazo compounds has been achieved, giving rise to two types of prevalent phenanthridines via highly selective C-C cleavage. Compared to the simple N-H insertion manipulation of diazo, this method elegantly accomplishes a tandem N-H insertion/SEAr/C-C cleavage/aromatization reaction, and the synthetic utility of this new transformation is exemplified by the succinct syntheses of trisphaeridine and bicolorine alkaloids.

Visible-Light-Induced Decarboxylative Cyclization/Hydrogenation Cascade Reaction to Access Phenanthridin-6-yl(aryl)methanol by an Electron Donor-Acceptor Complex

A novel and efficient visible-light-induced decarboxylative cyclization/hydrogenation cascade reaction of α-oxocarboxylic acids and 2-isocyanobiaryls has been developed. Without the need of any external photosensitizer, oxidant, and reductant, this method offers a mild and green approach for the synthesis of diverse alcohols in moderate to good yields. A mechanism indicated that an electron donor-acceptor complex-driven decarboxylation, radical addition/cyclization, and in situ photochemical reduction of ketones to alcohols could be involved in the reaction.

Expeditious synthesis of phenanthridines through a Pd/MnO2-mediated C–H arylation/oxidative annulation cascade from aldehydes, aryl iodides and amino acids

The expeditious access to phenanthridines through a Pd/MnO₂-mediated arylation/oxidative annulation cascade from aldehydes, aryl iodides and amino acids is described.

Dimerization–cyclization reactions of isocyanoaryl-tethered alkylidenecyclobutanes via a triplet biradical mediated process

A triplet biradical mediated dimerization–cyclization reaction of isocyanoaryl-tethered alkylidenecyclobutanes to construct macrocyclic skeletons including dihydroquinoline and quinoline units has been reported.

Transition-metal-free synthesis of CMe2CF3-containing chroman-4-ones via decarboxylative trifluoroalkylation

(NH₄)₂S₂O₈-mediated decarboxylative trifluoroalkylation of alkenes with 3,3,3-trifluoro-2,2-dimethylpropanoic acid under metal-free conditions has been described.

Recent Organic Transformations with Silver Carbonate as a Key External Base and Oxidant

Silver carbonate (Ag2CO3), a common transition metal-based inorganic carbonate, is widely utilized in palladium-catalyzed C–H activations as an oxidant in the redox cycle. Silver carbonate can also act as an external base in the reaction medium, especially in organic solvents with acidic protons. Its superior alkynophilicity and basicity make silver carbonate an ideal catalyst for organic reactions with alkynes, carboxylic acids, and related compounds. This review describes recent reports of silver carbonate-catalyzed and silver carbonate-mediated organic transformations, including cyclizations, cross-couplings, and decarboxylations.

Thio radical-induced denitrogenative annulation of 1-azido-2-isocyanoarenes to construct 2-thiolated benzimidazoles

A method for the synthesis of 2-thiolated benzimidazoles is described starting from thiols and 1-azido-2-isocyanoarenes. The isocyano group works as an acceptor of various thio radicals, followed by denitrogenative annulation of the resulting imidoyl radical intermediates to the azido group, with nitrogen loss as the only process involving high bond-forming efficiency. The one-pot method for the synthesis of these products with high functional group tolerance in the benzimidazole-based ring is not available in previous literature.

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.

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.

Convenient synthesis of 6-alkyl phenanthridines and 1-alkyl isoquinolines via silver-catalyzed oxidative radical decarboxylation

A convenient and efficient protocol for the synthesis of 6-alkyl phenanthridines and 1-alkyl isoquinolines has been developed. The reaction relies on the coupling of 2-isocyanobiphenyls and vinyl isonitriles with alkyl radicals formed by the silver-catalyzed decarboxylation of stoichiometric aliphatic carboxylic acids, and affords diverse phenanthridine and isoquinoline derivatives under mild reaction conditions. The experiment of β-scission of cyclobutylcarbinyl radicals is used to shed light on the reaction mechanism.

Metal-free oxidative isocyanides insertion with aromatic aldehydes to aroylated N-heterocycles

A metal-free, mild and efficient protocol for the construction of aroylated N-heterocycles via radical cascade aroylation of phenyl or vinyl isocyanides with aromatic aldehydes has been developed. Both phenanthridine and isoquinoline derivatives are delivered quickly in moderate to good yields with high regioselectivity.

Copper-Catalyzed Decarboxylative Disulfonylation of Alkynyl Carboxylic Acids with Sulfinic Acids

A copper-catalyzed decarboxylative disulfonylation of alkynyl carboxylic acids with sulfinic acids in aqueous solution has been developed. The reaction provides a straightforward and practical access to (E)-1,2-disulfonylethenes, which are important building blocks in synthetic organic chemistry, and exhibits a good functional group tolerance and excellent stereoselectivity. A possible mechanism for the transformation is proposed.

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.

Synthetic access to isoxazoline-functionalized isoquinolines via microwave-assisted iminoxyl radical-participated cascade cyclization of vinyl isocyanides

The synthesis of isoxazoline-functionalized isoquinolines via radical cascade cyclization of vinyl isocyanides with β,γ-unsaturated ketoximes has been described.

Construction of a 4H-pyrido[4,3,2-gh]phenanthridin-5(6H)-one skeleton via a catalyst-free radical cascade addition/cyclization using azo compounds as radical sources

The new radical addition/cyano insertion/homolytic aromatic substitution cascade reaction initiated by the thermal homolysis of azo compounds under catalyst-free conditions produced polycyclic phenanthridine derivatives.

Direct synthesis of 3-acylbenzothiophenes via the radical cyclization of 2-alkynylthioanisoles with α-oxocarboxylic acids

A radical cascade cyclization strategy has been developed to provide a direct route to access 3-acylbenzothiophenes from simple chemical feedstocks.