Selective Access to 3-Cyano-1H-indoles, 9H-Pyrimido[4,5-b]indoles, or 9H-Pyrido[2,3-b]indoles through Copper-Catalyzed One-Pot Multicomponent Cascade Reactions

Novel access to α-carbolines with biological applications

In recent years, the 9H-pyrido[2,3-b]indole nuclei, also named α-carboline which is found in many organic compounds such as natural products, pharmaceuticals, and materials, have intensively stimulated the research of new synthetic pathways. After a brief report published in 2015 describing novel accesses and biological applications of α-carbolines, this update reports between 2015 and 2023 on the emergence of original syntheses to this heterocyclic nucleus. Examples representing these processes are described and the biological activities of α-carbolines are mentioned when they have been prepared for therapeutic purposes.

Base-Promoted Tandem Cyclization of 2-(Indol-3-yl)naphthoquinones with Benzamidines toward Polysubstituted Pyrimido[4,5-b]indoles

A novel and unique approach for the construction of polysubstituted pyrimido[4,5-b]indoles from 2-(indol-3-yl)naphthoquinones and benzamidines is described. Our strategy, promoted by an inorganic base, involves the ring opening and recyclization of naphthoquinone and produces three nitrogen heterocyclic rings via multiple C-N bond formations in one pot under transition-metal-free conditions.

Recent progress in metal assisted multicomponent reactions in organic synthesis

To prepare complicated organic molecules, straightforward, sustainable, and clean methodologies are urgently required. Thus, researchers are attempting to develop imaginative approaches. Metal-catalyzed multicomponent reactions (MCRs) offer optimal molecular diversity, high atomic efficiency, and energy savings in a single reaction step. These versatile protocols are often used to synthesize numerous natural compounds, heterocyclic molecules, and medications. Thus far, the majority of metal-catalyzed MCRs under investigation are based on metal catalysts such as copper and palladium; however, current research is focused on developing novel, environmentally friendly catalytic systems. In this regard, this study demonstrates the effectiveness of metal catalysts in MCRs. The aim of this study is to provide an overview of metal catalysts for safe application in MCRs.

Versatile access to nitrogen-rich π-extended indolocarbazoles via a Pictet–Spengler approach

A bidirectional Pictet-Spengler Reaction allows easy access to nitrogen-rich aromatics with seven fused rings. Photophysical measurements and computational methods show significant differences to parent N-heteropolycycles with fewer nitrogen atoms.

Mn(III)-Mediated Radical Cyclization of o-Alkenyl Aromatic Isocyanides with Boronic Acids: Access to N-Unprotected 2-Aryl-3-cyanoindoles

The synthesis of N-unprotected 2-aryl-3-cyanoindoles was realized via the Mn(III)-mediated radical cascade cyclization of o-alkenyl aromatic isocyanides with boronic acids. A possible mechanism involving a sequential intermolecular radical addition, intramolecular cyclization, and cleavage of the C-C bond under mild reaction conditions is proposed. Mechanism studies show that H₂O or O₂ might provide the oxygen source for the elimination of benzaldehyde.

C-N and C-O Bond Formation in Copper-Catalyzed/Mediated sp3 C-H Activation: Mechanistic Studies from Experimental and Computational Aspects

Mechanistic studies on Cu-catalyzed/mediated sp³ C-H amidation and acetoxylation are investigated from experimental and computational aspects. The concerted metalation-deprotonation (CMD) mechanism rather than a radical-involved pathway is proved to occur in amidation and acetoxylation reactions, and this is the rare example of the CMD mechanism involved in the more challenging sp³ C-H activations. Theoretical calculations demonstrated that CMD is the rate-determining step either for methylic or benzylic positions in amidation and acetoxylation reactions, and intermolecular nucleophilic addition of acetate anions is more favorable than the ring opening of β-lactams and intramolecular acetoxylation. These mechanistic studies on the divergent and condition-dependent product formation are critical for developing Cu-promoted C-H functionalization via the CMD mechanism.

Synthesis of benzimidazoles by CuI-catalyzed three-component reaction of 2-haloaniline, ammonia and aldehyde in water

An efficient copper-catalyzed three-component reaction of 2-haloaniline, ammonia and aldehyde for the synthesis of benzimidazoles with 1,10-phenanthroline as the ligand has been developed. A variety of substituted benzimidazole derivatives can be obtained in yields up to 95%.

Synthesis of Diversely Functionalized 2 H-Chromenes through Pd-Catalyzed Cascade Reactions of 1,1-Dibromoolefin Derivatives with Arylboronic Acids

In this work, an unprecedented synthesis of (2 H-chromen-2-ylidene)acetates and (2 H-chromen-2-ylidene)ethanones with controlled stereoselectivity via Pd-catalyzed cascade reactions of 3-(2-(2,2-dibromovinyl)phenoxy)acrylates or 3-(2-(2,2-dibromovinyl)phenoxy)enones with aryl boronic acids has been established. This protocol combines two C-C bond forming reactions (an intermolecular Suzuki coupling followed by an intramolecular Heck coupling) in one pot under the catalysis of the same catalyst. Compared with literature methods for the preparation of 2 H-chromene derivatives, it has advantages such as easily obtainable or commercially available substrates, diverse substitution pattern of products, simple procedure, and excellent stereoselectivity. Interestingly, this cascade reaction could distinguish the subtle difference of the electron-withdrawing capacity and the size of various functional groups and thus resulted in a different chemoselectivity. In addition, the utility of the (2 H-chromen-2-ylidene)acetate thus obtained was showcased by its facile transformation into the synthetically and photophysically significant 3 H-xanthen-3-one derivative.

Copper Catalyzed Intramolecular N-Arylation of Ketene Aminals at Room Temperature: Synthesis of 2-Amino-3-cyanoindoles

Various 2-amino-3-cyanoindoles are synthesized through copper catalyzed intramolecular N-arylations of ketene aminals at room temperature for the first time with 60-99% yields within 0.1-2 h. Controlled regioselective N-arylations of unsymmetrical ketene aminals are also studied. Further, a new double heteroannulation approach is demonstrated for the synthesis of 11-aminoindolo[2,3- b]quinolines from acyclic and nonheterocyclic precursors.

Copper-Catalyzed Synthesis of Multisubstituted Indoles through Tandem Ullmann-Type C-N Formation and Cross-dehydrogenative Coupling Reactions

Multisubstituted indoles were synthesized via a one-pot tandem copper-catalyzed Ullmann-type C-N bond formation/intramolecular cross-dehydrogenative coupling process at 130 °C in DMSO. The methodology allows practical and modular assembly of indoles in good to excellent yields from readily available aryl iodides and enamines.

Practical Synthesis of Chromeno[2,3-b]indole Skeleton via an Aldehyde Group Insertion/Aromatization Strategy

The synthesis of chromeno[2,3-b]indole from simple starting materials remains a demanding process. Herein, 2-bromoindole undergoes nucleophilic attack from salicylaldehyde, followed by intramolecular insertion of an aldehyde group and aromatization to generate the desired chromeno[2,3-b]indoles. Moreover, various functional groups were tolerated and a gram-scale synthesis of the product could be achieved under the optimized condition.

Regioselective switching approach for the synthesis of α and δ carboline derivatives

A metal-free protocol for accessing both α and δ-carboline derivatives, starting from a common indolylchalcone oxime ester precursor is reported.

Substrate-driven selective mono- and bis-couplings of ortho-(OTf/I/Br) substituted gem-dibromovinylarenes

A one-pot approach with the substrate-driven selective and competitive reactivity of ortho-trifloxy, iodo or bromo functionalized gem-dibromovinylarenes was studied under Pd-catalyzed conditions.

Regio-selective synthesis of diversely substituted benzo[a]carbazoles through Rh(iii)-catalyzed annulation of 2-arylindoles with α-diazo carbonyl compounds

A novel synthetic approach toward benzo[a]carbazoles and 6-aminobenzo[a]carbazoles containing an unprotected NH unit through Rh(iii)-catalyzed cascade reactions of 2-arylindoles or 2-arylindole-3-carbonitriles with α-diazo carbonyl compounds is presented.

Copper-catalyzed one-pot reactions of acetyl chloride, o-halobenzoic acids and Wittig reagents toward 3-methyl isocoumarin synthesis

The synthesis of 3-methtyl isocoumarins is achieved via the copper-catalyzed one-pot reactions of Wittig reagents, acetyl chloride and o-iodobenzoic acids via tandem construction of CC, C–O and C–C bonds.

Cu-catalyzed sequential C–N bond formations: expeditious synthesis of tetracyclic indoloindol-3-ones

A sequence of 3 reactions in one-pot, SNAr, nitrene C–H insertion and Ullmann coupling, three C–N bond formations.