Synthesis of δ- and α-Carbolines via Nickel-Catalyzed [2 + 2 + 2] Cycloaddition of Functionalized Alkyne-Nitriles with Alkynes

Intermolecular 1,2-Difunctionalization of Nitriles via Redox Gold Catalysis: Synthesis of Benzoxazoles and Benzimidazoles

The unprecedent gold-catalyzed intermolecular 1,2-difunctionalization of nitriles with o-iodophenols or o-iodoanilines via Au(I)/Au(III) redox catalysis has been developed, providing an expedient route to the synthesis of benzoxazoles or benzimidazoles with broad substrate scope and high functional compatibility. Mechanistic investigation reveals that the Au(III)-Ar species generated via oxidative addition of o-iodophenol to MeDalphosAu+, serves as a key intermediate. Particularly, this annulation is initiated by oxidative addition, rather than the nucleophilic attack of the phenol moiety in o-iodophenol towards the nitrile. The method was also applied to the synthesis of poly aza-heterocycles via a cascade Au(I)/Au(III) and Au(I) catalysis relay.

Access to 2,3-Dihydropyridines Involving Cyano Transformation Relay through Gold Catalysis Assisted by a Lewis Acid

A gold(I)-catalyzed cyclization of enynone-nitriles with amines to 2,3-DHPs assisted by La(OTf)3 has been developed. The compatibility with abundant nucleophiles, high functional group tolerance, rapid assembly of molecular complexity, and late-stage functionalization of bioactive substances make this approach attractive for the construction of 2,3-DHPs, which have rarely been synthesized in the literature. The reaction proceeds via cascade cyclization involving gold/Lewis acid cooperative activation of the alkene and the cyano moiety.

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.

Ir/Zn-cocatalyzed chemo- and atroposelective [2+2+2] cycloaddition for construction of C─N axially chiral indoles and pyrroles

Here, an Ir/Zn-cocatalyzed atroposelective [2+2+2] cycloaddition of 1,6-diynes and ynamines was developed, forging various functionalized C─N axially chiral indoles and pyrroles in generally good to excellent yields (up to 99%), excellent chemoselectivities, and high enantioselectivities (up to 98% enantiomeric excess) with wide substrate scope. This cocatalyzed strategy not only provided an alternative promising and reliable way for asymmetric alkyne [2+2+2] cyclotrimerization in an easy handle but also settled the issues of previous [Rh(COD)2]BF4-catalyzed system on the construction of C─N axial chirality such as complex operations, limited substrate scope, and low efficiency. In addition, control experiments and theoretical calculations disclosed that Zn(OTf)2 markedly reduced the barrier of migration insertion to significantly increase reaction efficiency, which was distinctly different from previous work on the Lewis acid for improving reaction yield through accelerating oxidative addition and reductive elimination.

Nickel-Catalyzed Reductive Alkyne Hydrocyanation Enabled by Malononitrile and a Formaldehyde Additive

The development of a nickel-catalyzed reductive alkyne hydrocyanation is described using 2-methyl-2-phenylmalononitrile (MPMN), a C-bound electrophilic transnitrilation reagent. Reproducibility issues led to the detection of oxidized hemiaminal impurities within N,N-dimethylacetamide. These impurities release formaldehyde in situ, which was ultimately identified as a critical reaction additive. A range of diaryl and aryl-alkyl alkynes underwent hydrocyanation. Mechanistic experiments revealed that formaldehyde and MPMN undergo a Ni-catalyzed reductive coupling of two π-components, leading to the controlled release of glycolonitrile as the active cyanating agent.

Ring forming transformations of ynamides via cycloaddition

Ynamides are N-alkyne compounds bearing an electron withdrawing group at the nitrogen atom. They offer unique pathways for the construction of versatile building blocks owing to their exceptional balance between reactivity and stability. Recently several studies have been reported that explore and illustrate the synthetic potential of ynamides and ynamide-derived advanced intermediates in cycloadditions with different reaction partners to yield heterocyclic cycloadducts of synthetic and pharmaceutical value. Cycloaddition reactions of ynamides are the facile and preferable routes for the construction of structural motifs having striking importance in synthetic, medicinal chemistry, and advanced materials. In this systematic review, we highlighted the recently reported novel transformations and synthetic applications that involved the cycloaddition reaction of ynamides. The scope along with the limitations of the transformations are discussed in detail.

Synthesis of 5H-chromeno[3,4-c]pyridine derivatives through ruthenium-catalyzed [2 + 2 + 2] cycloaddition

Efficient access to 5H-chromeno[3,4-c]pyridines using Ru-catalyzed [2 + 2 + 2] cycloaddition of α,ω-diynes with cyanamides was developed, providing valuable tricyclic pyridine building blocks and enabling access to a biologically relevant intermediate.

Regio- and Chemoselective Copper-Catalyzed Formal [2+2+2] Cycloaddition of Primary Amines with Arylacetylenes to 2,4,5-Trisubstituted Pyridines

Disclosed herein is an efficient strategy for the synthesis of 2,4,5-trisubstituted pyridines via CuI/NBS-catalyzed formal intermolecular [2+2+2] cycloaddition of easily available primary amines and nonactivated terminal alkynes. Moreover, this given reaction features a new mode of cycloaddition with high regio- and chemoselectivity, good atom- and step-economy, broad substrate scope, and wide functional group compatibility. Further mechanism studies indicate that this transformation starts with oxidative alkynylation of the amine to form a propargylamine intermediate, followed by radical addition to the alkyne and intramolecular cycloaddition, delivering the pharmacologically interesting multisubstituted pyridines.

A review for the assembly of multi-substituted pyridines via Co-catalyzed [2+2+2] cycloaddition with nitriles

In the recent years, many methods for the facile synthesis of pyridines and their derivatives have been developed. The [2+2+2] cycloaddition reaction of alkynes and nitriles catalyzed by transition metals...

Metal-Free and Regioselective Synthesis of Functionalized α-Carbolines via [3 + 3] Annulation of Morita-Baylis-Hillman Acetates of Nitroalkenes with Iminoindolines

A facile, metal-free method for the synthesis of substituted α-carbolines from secondary Morita-Baylis-Hillman (MBH) acetates of nitroalkenes is presented. The cascade reaction of MBH acetates with tosyliminoindolines occurs regioselectively to form various α-carbolines with a wide substrate scope. The reaction involves mild conditions, and the products are formed in high yields within a short reaction time. The amenability of the reaction to scale up and synthetic applications of the products have been demonstrated.

Atroposelective Synthesis of Axially Chiral 4-Aryl α-Carbolines via N-Heterocyclic Carbene Catalysis

The first catalytic asymmetric construction of axially chiral 4-aryl α-carboline skeletons has been accomplished through an N-heterocyclic carbene (NHC)-catalyzed atroposelective formal [3 + 3] annulation of 4-nitrophenyl 3-arylpropiolates with 2-sulfonamidoindolines. The synthetic utility of the title compounds has been demonstrated by the diverse late-stage structural modifications. Density functional theory calculations were also conducted to illuminate the key factors for controlling the origin of the enantioselectivity. This strategy not only provides an efficient pathway to access axially chiral α-carboline atropisomers but also offers a novel catalytic enantioselective mode for the construction of axially chiral heterobiaryls by using NHC-bound alkynyl acylazoliums.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Nickel-Catalyzed β-Regioselective Amination/Cyclization of Ynamide-Nitriles with Amines: Synthesis of Functionalized 3-Aminoindoles and 4-Aminoisoquinolines

A highly regioselective nickel/Lewis acid catalyzed amination/cyclization of ynamide-nitriles with amines involving β-addition has been developed. The reaction offers an attractive and efficient route for the synthesis of 3-aminoindoles and 4-aminoisoquinoline derivatives. The Ts-group on the ynamide acts as a directing group to produce the alkenyl nickel species with high regioselectivity.

Pd(0)-Catalyzed Intramolecular Heck reaction of 2/3-Aryl(amino)methyl-3/2-bromoindoles: Syntheses of 3,4-Benzo[c]-β-carbolines, Benzo[4,5]isothiazolo[2,3-a]indole 5,5-Dioxides, and 1,2-Benzo[a]-γ-carbolines

One-pot synthesis of 3,4-benzo[c]-β-carbolines was achieved from 2-aryl(tosylamino)methyl-3-bromoindoles via 10 mol % Pd(OAc)₂/PPh₃-mediated intramolecular Heck coupling using K₂CO₃ as a base in DMF at 110 °C with concomitant aromatization through an elimination of tosylsulfinic acid. Under identical conditions, the isomeric 3-aryl(tosylamino)methyl-2-bromoindoles upon intramolecular Heck reaction furnished benzo[4,5]isothiazolo[2,3-a]indole 5,5-dioxides instead of the expected γ-carbolines. However, synthesis of the expected γ-carboline framework, 3-tosyl-6,9-dihydro-1,2-benzo[a]-γ-carbolines, could be achieved from 3-aryl(tosylamino)methyl-2-bromoindoles containing a mesitylene sulfonyl unit as a protecting group on the indole nitrogen.

Brønsted acid-mediated reactions of ynamides

Over the past two decades, the development and application of ynamide chemistry have received more and more attention. Ynamides have proven to be versatile reagents for organic synthesis, and have been widely applied to the rapid assembly of a diverse range of structurally complex N-containing molecules, especially the valuable N-heterocycles. In comparison with the well-established transition metal-catalyzed reactions of ynamides, metal-free ynamide transformations have relatively seldom been exploited. Recently, Brønsted acid-mediated reactions of ynamides represent significant advances in ynamide chemistry. This review summarizes the latest trends and developments of Brønsted acid-mediated reactions of ynamides, including cycloaddition, cyclization, intramolecular alkoxylation-initiated rearrangement, oxygen atom transfer reactions and hydro-heteroatom addition reactions.

De Novo Protocol for the Construction of Benzo[a]fluorenes via Nitrile/Alkene Activation

Unprecedented chemo- and regioselective synthesis of benzo[a]fluorenes and naphthamide-substituted benzo[a]fluorenes were constructed from the reaction of (E)-2-aroyl-3-(2-(arylalkynes/alkenes)aryl)acrylonitrile scaffolds under metal-free conditions via the activation of nitriles and alkenes, respectively. A tentative reaction mechanism was proposed for this homofunctionalization of nitriles. Control experiments showed that the reaction proceeds via selective nitrile or alkene protonation, depending upon the substrates. Additionally, we demonstrated an alternative expeditious route for the synthesis of disubstituted benzo[a]fluorenes in the presence of TfOH alone.

Catalyst-Free [4+2] Cycloaddition of Ynamides with 2-Halomethyl Phenols To Construct 2-Amino-4H-Chromenes and α-Halo Enamides Simultaneously

We report the development of a facile protocol for the [4+2] cycloaddition of ynamides and 2-halomethyl phenols to afford the corresponding 2-amino-4H-chromenes and α-halo enamides under catalyst-free conditions. The reaction proceeds under mild conditions and exhibits good tolerance toward various functional groups and generates high yields. The plausible mechanism involves the formation of an active intermediate keteniminium as well as o-methylene quinone.

Metal-Catalyzed Cyclotrimerization Reactions of Cyanamides: Synthesis of 2-Aryl-α-carbolines

The synthesis of annulated 2-aryl-α-carboline heterocycles is described using transition metal catalysis. A linear strategy is described that uses Rh(I) catalysis to form the α-carboline scaffold by [2+2+2] cyclotrimerization. Alternatively, a tandem catalytic approach using a Pd(II) precatalyst afforded the same target molecules by mediating a Sonogashira reaction and a [2+2+2] cyclotrimerization in the same reaction flask. In each case, nine different 2-aryl-α-carbolines have been prepared in high to modest isolated yields.

A Mild Synthesis of 2-Substituted Benzothiazoles via Nickel-Catalyzed Intramolecular Oxidative C-H Functionalization

A highly efficient synthetic method for the preparation of 2-aminobenzothiazoles starting from arylthioureas has been reported. By using a nickel catalyst, arylthioureas undergo intramolecular oxidative C-H bond functionalization, giving the desired 2-aminobenzothiazoles in good to excellent yields. This protocol features an inexpensive catalyst, low catalyst loading, mild reaction conditions, a short reaction time, and good to excellent yields, and it can be scaled up easily to a gram scale with almost no yields decreasing.

Preparation of isoquinazolines via metal-free [4 + 2] cycloaddition of ynamides with nitriles

TfOH-mediated [4 + 2] cycloaddition of ynamides with nitriles to construct 1,2-dihydroquinazolines is realized by a direct reaction in moderate to excellent yields (up to 93%) in a stereospecific manner. A rapid and efficient strategy has been employed for the syntheses of alkyl-substituted 1,2-dihydroquinazoline derivatives, and it exhibits good functional group tolerance, has a short reaction time, shows excellent diastereoselectivity, and is a simple and high-yielding reaction.

Dehydrative Transformation of Spirooxindoles to Pyrido[2,3-b]indoles via POCl3

A two-step one-pot efficient synthesis of pyrido[2,3-b]indoles via reaction between isatin, α-amino acid, and dipolarophile has been developed. The initial 1,3-dipolar cycloaddition between the reactants that is performed in the presence of either CuI or methanol results in spirooxindoles that undergo POCl₃-mediated intramolecular dehydrative transformation to afford the title compounds.

Cobalt-catalyzed regioselective syntheses of indeno[2,1-c]pyridines from nitriles and diynes bearing propargyl fragments

A highly efficient CoI2/o-phenanthroline catalyzed cycloaddition reaction of diynes bearing TBS protected propargylic alcohol fragments with nitriles has been developed. This methodology offers regioselective access, with good functional group tolerance, to various indeno[2,1-c]pyridine derivatives in moderate to excellent yields. It was found that o-phenanthroline as a bidentate nitrogen ligand showed high efficacy in this cycloaddition reaction.

Metal-Free [2 + 2 + 2] Cycloaddition of Ynamide-Nitriles with Ynamides: A Highly Regio- and Chemoselective Synthesis of δ-Carboline Derivatives

A metal-free formal [2 + 2 + 2] cycloaddition of functionalized ynamide-nitriles with ynamides is disclosed which offers highly efficient access to polysubstituted δ-carboline derivatives under the mediation of TfOH. This strategy is highly regioselective and chemoselective and displays mild conditions, high yields, and efficiency (within 1 min) in addition to substrates scopes (56 examples).