Highly Stereoselective Assembly of α-Carbolinone Skeletons via N-Heterocyclic Carbene-Catalyzed [4 + 2] Annulations

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.

In(OTf)3-catalyzed formal (4 + 3) cycloaddition reactions of 3-benzylideneindoline-2-thiones with 2-indolylmethanols

We report the In(OTf)3-catalyzed formal (4 + 3) cycloaddition of 3-benzylideneindoline-2-thiones with 2-indolylmethanols. This reaction not only broadens the synthetic utility of 3-benzylideneindoline-2-thiones as scarce indole-based sulfur-containing four-atom building blocks, but also provides a rapid and facile access to synthesize diindole-annulated tetrahydrothiepines.

Base-Promoted Formal (3 + 2) Cycloaddition of α-Halohydroxamates with Electron-Deficient Alkenyl-iminoindolines To Synthesize Spiro-indolinepyrrolidinones

Construction of pyrrolidinyl-spiroindoles with easily available starting materials has attracted considerable attention from the synthesis community and is in great demand. Here, we describe a base-promoted formal (3 + 2) cycloaddition of α-halohydroxamates with alkenyl-iminoindolines. The present methodology features mild reaction conditions and a broad substrate scope with up to 99% yield and excellent diastereoselectivity. The versatility of this approach is demonstrated through valuable synthetic transformations. Preliminary mechanistic studies shed light on the mechanism of this cycloaddition process.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Axially Chiral α-Carbolinones via Heterocycle Formation

An NHC-catalyzed atroposelective synthesis of axially chiral α-carbolinones from α,β-unsaturated iminoindole derivatives and α-chloroaldehydes was developed. The reaction proceeds through a cascade process including [4 + 2] annulation and then oxidative dehydrogenation with concomitant central-to-axial chirality conversion under mild conditions. The developed method opens a new avenue to efficiently access axially chiral α-carbolinones in moderate to good enantioselectivities.

TfOH-Catalyzed Cascade C-H/N-H Chemo-/Regioselective Annulation of Indole-2-carboxamides with Benzoquinones for the Construction of Anticancer Tetracyclic Indolo[2,3-c]quinolinones

An efficient TfOH-catalyzed cascade C-H/N-H annulation of indole-2-carboxamides with benzoquinones has been developed for the synthesis of tetracyclic indolo[2,3-c]quinolinones. This reaction exhibits excellent chemo-/regioselectivity, achieving functionalization of the C-3 of indole and N-H of the amide moiety to form the new C-C and C-N bonds. Various expected products were synthesized from readily available starting materials in good to high yields with a wide substrate scope and good functional group tolerance. Among all synthetic products, 3d showed the most potent cytotoxicity toward the 4T1 cancer cell line with an IC₅₀ value of 0.62 ± 0.05 μM. In vivo study demonstrated that 3d remarkably suppressed 4T1 xenograft tumor growth without body weight loss.

N-Heterocyclic Carbene (NHC)-Catalyzed Transformations Involving Azolium Enolates

The recent advances in the N-heterocyclic carbene (NHC)-organocatalyzed generation of azolium enolate intermediates and their subsequent interception with electrophiles are highlighted. The NHC-bound azolium intermediates are generated by the addition of NHCs to suitably substituted aldehydes, acid derivatives or ketenes. A broad range of coupling partners can intercept the azolium enolates to form [2+n] cycloadducts (n=2,3,4) and various α-functionalized compounds. The enantioselective synthesis of the target compounds are achieved with the use of chiral NHCs. Herein, we summarized the development that occurred in this subclass of NHC catalysis.

Sulphur ylide-mediated cyclopropanation and subsequent spirocyclopropane rearrangement reactions

The efficient construction of cyclopropyl spiroindoline skeletons and the exploration of related follow-up synthetic transformations have elicited considerable interest amongst members of the chemistry community. Here, we describe a formal (2 + 1) annulation and three-component (1 + 1 + 1) cascade cyclisation via sulphur ylide cyclopropanation under mild conditions. The spiro-cyclopropyl iminoindoline moiety can be readily transformed into another medicinally interesting pyrrolo[3,4-c]quinoline framework through a novel rearrangement process.

Organocatalytic Diastereodivergent Enantioselective Formal oxa-Diels-Alder Reaction of Unsaturated Ketones with Enoates Under Liquid-Assisted Grinding Conditions

Chiral heterocycles occur in many compounds of interest, but their efficient synthesis is challenging. This study concerns the enantioselective and diastereoselective synthesis of densely substituted chiral pyran derivatives. Diastereodivergence of the oxa-Diels-Alder reaction is achieved by using either a bifunctional amino-thiourea or a monofunctional quinine organocatalyst under ball-milling conditions. Liquid-assisted grinding proves a highly efficient means of affording pyrans in high yield, with high enantiomeric purities and short reaction times.

Suzuki-type cross-coupling of alkyl trifluoroborates with acid fluoride enabled by NHC/photoredox dual catalysis

The Suzuki-Miyaura cross-coupling of C(sp3)-hybridised boronic compounds still remains a challenging task, thereby hindering the broad application of alkyl boron substrates in carbon-carbon bond-forming reactions. Herein, we developed an NHC/photoredox dual catalytic cross-coupling of alkyl trifluoroborates with acid fluorides, providing an alternative solution to the classical acylative Suzuki coupling chemistry. With this protocol, various ketones could be rapidly synthesised from readily available materials under mild conditions. Preliminary mechanistic studies shed light on the unique radical reaction mechanism.

N-Heterocyclic Carbene-Promoted [4+2] Annulation of α-Chloro Hydrazones with α-Chloro Aliphatic Aldehydes to Access Enantioenriched Dihydropyridazinones

An expeditious protocol for the assembly of chiral 4,5-dihydropyridazin-3(2H)-ones from α-chloro hydrazones and α-chloro aliphatic aldehydes via N-heterocyclic carbene (NHC) catalysis is outlined. These in situ-generated 1,2-diaza-1,3-dienes undergo asymmetric [4+2] annulation with NHC-bound enolates to afford the desired products bearing a stereogenic center at the C4 position. The notable features of this approach include good to excellent enantioselectivities, high functional group tolerance, mild reaction conditions, simple operating procedures, and compatibility with gram-scale synthesis.

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones to access chromeno[2,3-b]pyridinones

N-Heterocyclic carbene-catalyzed [3 + 3] annulation of bromoenals with 2-aminochromones has been successfully developed. A structurally diverse set of chromeno[2,3-b]pyridinones was efficiently constructed in acceptable to excellent yields. The reaction features mild reaction conditions, a broad substrate scope, and easy scale-up.

Cascade C–N bond cleavage of amides/intramolecular amination reactions: an atom economical way to α-cabolin-4-ones

A novel one-pot procedure for the synthesis of 2,3-disubstituted α-carbolin-4-ones through successive C–N bond cleavage and intramolecular amination reactions involving a 1,2-acyl migration has been developed.

Design and Efficient Synthesis of RalA Inhibitors Containing the Dihydro-α-carboline Scaffold

Ras-related protein RalA is a member of the Ras small GTPases superfamily. Its activation plays an important role in regulating tumor initiation, invasion, migration, and metastasis. In this study, we designed a new type of RalA inhibitor containing a dihydro-α-carboline scaffold. The structurally new dihydro-α-carboline derivatives could be efficiently synthesized in good yields through a newly developed three-component [3+2+1] cyclization reaction. Evaluation of the biological activity showed that some of the dihydro-α-carboline derivatives can inhibit RalA/B and proliferative activities of NSCLC cell lines. The 4-(pyridin-3-yl)-dihydro-α-carboline compound (3 o) was found to be the most potent derivative, with IC₅₀ values of 0.43±0.03, 0.64±0.07, 0.93±0.10, and 1.54±0.15 μM against A549, H1299, H460, and H1975 cells, respectively. Mechanism investigation suggested that 3 o inhibits the RalA/B activation of A549, down-regulates Bcl-2, stimulates cytochrome c and PARP cleavage, and induces cell apoptosis. A molecular docking study revealed that 3 o can form stable hydrogen bonds with residues of RalA. Moreover, amide-π and alkyl-π interactions also contributed to the affinity between 3 o and RalA.

Bifunctional Brønsted Base Catalyzed [3 + 3] Annulations of Indolin-2-imines and α,β-Unsaturated Imides: An Enantioselective Approach to α-Carbolinones

Asymmetric construction of α-carbolinones with easily available starting materials has recently attracted considerable attention from the synthesis community, and the development of effective catalysis for this target is in great demand. Here, a bifunctional Brønsted base catalyzed asymmetric [3 + 3] cyclization of indolin-2-imines and α,β-unsaturated N-acylated succinimides was developed by using the strategy of noncovalent bonding catalysis. With this organocatalytic protocol, a variety of tetrahydro-α-carbolinones bearing different substituents were synthesized with up to 99% yield and up to 96:4 er.

Regiodivergent construction of medium-sized heterocycles from vinylethylene carbonates and allylidenemalononitriles

Medium-sized heterocycles exist in a broad spectrum of biologically active natural products and medicinally important synthetic compounds. The construction of medium-sized rings remains challenging, particularly the assembly of different ring sizes from the same type of substrate. Here we report palladium-catalyzed, regiodivergent [5 + 4] and [5 + 2] annulations of vinylethylene carbonates and allylidenemalononitriles. We describe the production of over 50 examples of nine- and seven-membered heterocycles in high isolated yields and excellent regioselectivities. We demonstrate the synthetic utility of this approach by converting a nine-membered ring product to an interesting polycyclic caged molecule via a [2 + 2] transannulation. Mechanistic studies suggest that the [5 + 2] annulation proceeds through palladium-catalyzed ring-opening/re-cyclization from the [5 + 4] adducts.

Here we report palladium-catalyzed, regiodivergent [5 + 4] and [5 + 2] annulations of vinylethylene carbonates and allylidenemalononitriles affording over 50 medium-sized heterocycles in high isolated yields with excellent regioselectivities.

Dearomative [4 + 2] annulations between 3-nitroindoles and enals through oxidative N-heterocyclic carbene catalysis

A novel intermolecular dearomative [4 + 2] annulation of 3-nitroindoles and enals under oxidative N-heterocyclic carbene catalysis has been developed. This protocol was also suitable for the oxidative cyclisation of 2-nitrobenzothiophenes with enals.

Controllable synthesis of pyrido[2,3-b]indol-4-ones or indolo[3,2-b]quinolines via formal intramolecular C(sp2)-H functionalization

A novel Fe-catalyzed protocol for the controllable synthesis of pyrido[2,3-b]indol-4-ones or indolo[3,2-b]quinolines has been developed by using indole-2-carboxylic derivatives as starting materials. Indole-2-carboxenamines were transformed into pyrido[2,3-b]indol-4-ones through intramolecular N-H/C-H coupling, in which a carbonyl 1,2-migration was involved. Whereas, when indole-2-carboxarylamines were employed, indolo[3,2-b]quinolones were produced through direct N-H/C-H coupling. The desired products were obtained under mild reaction conditions in moderate to good yields with wide substrate scope. The natural product quindolinone was conveniently prepared by this reaction.

Synthetic approach to skeletally diverse nitrogen heterocycles from dicyano-2-methylenebut-3-enoates

Divergent synthesis of three types of azaheterocycles, including vinylaziridines, pyrroline and 2-aminopyridines, has been achieved through [2 + 1], formal-[4 + 1] and [5 + 1] annulations.