PROGRESS IN THE FISCHER INDOLE REACTION. A REVIEW

A Cascade Synthesis of Indoles

We report herein an expedient method for the regioselective synthesis of indoles from o-haloanilines and α-ketol-derived N-tosylhydrazones. This two-step, modular synthesis of N-H indoles can be carried out conveniently without purification of intermediates.

Lewis acid catalyzed [4+2] annulation of bicyclobutanes with dienol ethers for the synthesis of bicyclo[4.1.1]octanes

Bicyclic carbocycles containing a high fraction of Csp3 have become highly attractive synthetic targets because of the multiple applications they have found in medicinal chemistry. The formal cycloaddition of bicyclobutanes (BCBs) with two- or three-atom partners has recently been extensively explored for the construction of bicyclohexanes and bicycloheptanes, but applications to the synthesis of medium-sized bridged carbocycles remained more limited. We report herein the formal [4+2] cycloaddition of BCB ketones with silyl dienol ethers. The reaction occurred in the presence of 5 mol% aluminium triflate as a Lewis acid catalyst. Upon acidic hydrolysis of the enol ether intermediates, rigid bicyclo[4.1.1]octane (BCO) diketones could be accessed in up to quantitative yields. This procedure tolerated a range of both aromatic and aliphatic substituents on both the BCB substrates and the dienes. The obtained BCO products could be functionalized through reduction and cross-coupling reactions.

Consecutive four-component synthesis of trisubstituted 3-iodoindoles by an alkynylation-cyclization-iodination-alkylation sequence

A library of 19 differently substituted 3-iodoindoles is generated by a consecutive four-component reaction starting from ortho-haloanilines, terminal alkynes, N-iodosuccinimide, and alkyl halides in yields of 11-69%. Initiated by a copper-free alkynylation, followed by a base-catalyzed cyclizive indole formation, electrophilic iodination, and finally electrophilic trapping of the intermediary indole anion with alkyl halides provides a concise one-pot synthesis of 3-iodoindoles. The latter are valuable substrates for Suzuki arylations, which are exemplified with the syntheses of four derivatives, some of them are blue emitters in solution and in the solid state, in good yield.

Synthesis of Indoles from o-Haloanilines

A convenient method for the synthesis of indoles has been developed by the sequential orchestration of the cross-coupling reaction of o-haloaniline and PIFA oxidation of the resulting 2-alkenylanilines. A highlight of this two-step indole synthesis is a modular strategy which is applicable to both acyclic and cyclic starting materials. Particularly noteworthy is the regiochemistry that is complementary to the Fischer indole synthesis and related variants. Direct preparation of N-H indoles with no N-protecting group is also advantageous.

Palladium-Catalyzed Alkenyl C-H Activation/Diamination toward Tetrahydrocarbazole and Analogs Using Hydroxylamines as Single-Nitrogen Sources

A palladium-catalyzed alkenyl C-H activation/diamination reaction of cycloalkenyl bromoarenes with hydroxylamines is described. A wide range of tetrahydrocarbazoles and analogs has been prepared using fine-tuning bifunctional secondary hydroxylamines as the single-nitrogen sources. Mechanistic investigations suggest that the selective alkenyl C-H activation/diamination cascade process should build the N-heterocycles.

N1-Benzyl Tryptamine Pan-SHIP1/2 Inhibitors: Synthesis and Preliminary Biological Evaluation as Anti-Tumor Agents

Inhibition of phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase (SHIP) with small molecule inhibitors leads to apoptosis in tumor cells. Inhibitors that target both SHIP1 and SHIP2 (pan-SHIP1/2 inhibitors) may have benefits in these areas since paralog compensation is not possible when both SHIP paralogs are being inhibited. A series of tryptamine-based pan-SHIP1/2 inhibitors have been synthesized and evaluated for their ability to inhibit the SHIP paralogs. The most active compounds were also evaluated for their effects on cancer cell lines.

Ketones as strategic building blocks for the synthesis of natural product-inspired compounds

Natural product-inspired compound collections serve as excellent sources for the identification of new bioactive compounds to treat disease. However, such compounds must necessarily be more structurally-enriched than traditional screening compounds, therefore inventive synthetic strategies and reliable methods are needed to prepare them. Amongst the various possible starting materials that could be considered for the synthesis of natural product-inspired compounds, ketones can be especially valuable due to the vast variety of complexity-building synthetic transformations that they can take part in, their high prevalence as commercial building blocks, and relative ease of synthesis. With a view towards developing a unified synthetic strategy for the preparation of next generation bioactive compound collections, this review considers whether ketones could serve as general precursors in this regard, and summarises the opulence of synthetic transformations available for the annulation of natural product ring-systems to ketone starting materials.

Asymmetric Total Syntheses of Strychnos Alkaloids via Selective Fischer Indolization

The complex structures and important biological functions of Strychnos alkaloids have attracted a great deal of attention from synthetic chemists. Herein, we describe the concise asymmetric total syntheses of the Strychnos alkaloids, (-)-dehydrotubifoline, (-)-tubifoline, and (-)-tubifolidine, as well as the formal total synthesis of (-)-strychnine. Our strategy features the construction of the common tetracyclic pyrrolo[2,3-d]carbazole structure using regioselective Fischer indolization on unsymmetrical cyclic ketones and late-stage functionalization for divergent synthesis. We developed a stepwise Fischer indolization featuring selective formation of enol triflate to solve the challenging regioselectivity problem, leading to the common tetracyclic ring skeleton in these Strychnos alkaloids. The regioselectivity of Fischer indolization on unsymmetrical cyclic ketones was studied on the basis of different types of ring systems and supported by density functional theory calculations. Overall, our success in the construction of this tetracyclic ring secured the syntheses of Strychnos alkaloids and may provide a general method for the total syntheses of various alkaloids containing this skeleton.

Anhydrous Hydrogen Iodide-Mediated Reductive Indolization of In Situ-Generated Cyclopropyl Hydrazones

Fischer-type indolization of N-aryl-C-cyclopropyl hydrazones generated in situ followed by chemoselective reduction using tert-butyl iodide as an anhydrous HI generator was developed. This protocol provides indoles bearing carboxylic acid derivative units. A series of control experiments indicated the HI-mediated formation and reduction of spirocyclopropyl indolenines. Anhydrous HI functions as a Brønsted acid as well as a reducing agent, facilitating the successful conversion of unstable reaction intermediates and iodinated mixtures in equilibrium.

Synthesis of indole derivatives as prevalent moieties present in selected alkaloids

Indoles are a significant heterocyclic system in natural products and drugs. They are important types of molecules and natural products and play a main role in cell biology. The application of indole derivatives as biologically active compounds for the treatment of cancer cells, microbes, and different types of disorders in the human body has attracted increasing attention in recent years. Indoles, both natural and synthetic, show various biologically vital properties. Owing to the importance of this significant ring system, the investigation of novel methods of synthesis have attracted the attention of the chemical community. In this review, we aim to highlight the construction of indoles as a moiety in selected alkaloids.

Catalytic Enantioselective Desymmetrizing Fischer Indolization through Dynamic Kinetic Resolution

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl₂ (Lewis acid assisted Brønsted acid), this direct approach combines 2,2-disubstituted cyclopentane-1,3-diones with N-protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all-carbon quaternary stereocenter with good to excellent enantioselectivities.

Metal-Free [3+2] Annulation of Ynamides with Anthranils to Construct 2-Aminoindoles

A novel metal-free [3+2] annulation of ynamides with anthranils provides a facile, flexible, environmentally friendly, and atom-economical route to 2-aminoindoles. This synthetic process proceeds with efficiency, excellent regioselectivity, and wide functional group tolerance under mild conditions. Moreover, the obtained 2-aminoindole products represent a multifunctional platform for the construction of various 2-aminoindolyl frameworks.

Effect of Terminal Alkylation of Aryl and Heteroaryl Hydrazines in the Fischer Indole Synthesis

The effect of alkylation on the terminal position of aryl and heteroaryl hydrazines in the Fischer indole synthesis was examined. Compared to their unalkylated counterparts, reactions using alkylated hydrazines provided indole products with higher yields and faster rates. The reactions can be conducted at lower temperatures and are compatible with acid-sensitive functionality. The terminally alkylated hydrazines were readily prepared by a new two-step sequence and held as stable hydrazinium salts. The mild formation of the salts along with the favorable Fischer indole reaction conditions highlights the potential of this approach in later-stage synthetic use.

A General and Scalable Synthesis of Polysubstituted Indoles

A consecutive 2-step synthesis of N-unprotected polysubstituted indoles bearing an electron-withdrawing group at the C-3 position from readily available nitroarenes is reported. The protocol is based on the [3,3]-sigmatropic rearrangement of N-oxyenamines generated by the DABCO-catalyzed reaction of N-arylhydroxylamines and conjugated terminal alkynes, and delivers indoles endowed with a wide array of substitution patterns and topologies.

Continuous Flow Synthesis of Heterocycles: A Recent Update on the Flow Synthesis of Indoles

Indole derivatives are among the most useful and interesting heterocycles employed in drug discovery and medicinal chemistry. In addition, flow chemistry and flow technology are changing the synthetic paradigm in the field of modern synthesis. In this review, the role of flow technology in the preparation of indole derivatives is showcased. Selected examples have been described with the aim to provide readers with an overview on the tactics and technologies used for targeting indole scaffolds.

N-Trifluoromethyl Hydrazines, Indoles and Their Derivatives

Reported herein is the first efficient strategy to synthesize a broad range of unsymmetrical N-CF₃ hydrazines, which served as platform to unlock numerous currently inaccessible derivatives, such as tri- and tetra-substituted N-CF₃ hydrazines, hydrazones, sulfonyl hydrazines, and valuable N-CF₃ indoles. These compounds proved to be remarkably robust, being compatible with acids, bases, and a wide range of synthetic manipulations. The feasibility of RN(CF₃ )-NH₂ to function as a directing group in C-H functionalization is also showcased.

Metal-free and regiospecific synthesis of 3-arylindoles

A convenient, metal-free, and organic acid-base promoted synthetic method to prepare 3-arylindoles from 3-aryloxirane-2-carbonitriles and arylhydrazine hydrochlorides has been developed. In the reaction, the organic acid catalyzes a tandem nucleophilic ring-opening reaction of aryloxiranecarbonitriles and arylhydrazine hydrochlorides and Fischer indolization. The organic base triethylamine plays a crucial role in the final elimination step in the Fischer indole synthesis, affording 3-arylindoles regiospecifically. The reaction features advantages of microwave acceleration, non-metal participation, short reaction time, organic acid-base co-catalysis, and broad substrate scope.

Concerted Nucleophilic Aromatic Substitution Reactions

Recent developments in experimental and computational chemistry have identified a rapidly growing class of nucleophilic aromatic substitutions that proceed by concerted (cSN Ar) rather than classical, two-step, SN Ar mechanisms. Whereas traditional SN Ar reactions require substantial activation of the aromatic ring by electron-withdrawing substituents, such activating groups are not mandatory in the concerted pathways.

Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis

This review provides an overview of synthetic transformations that have been performed by both electro- and photoredox catalysis. Both toolboxes are evaluated and compared in their ability to enable said transformations. Analogies and distinctions are formulated to obtain a better understanding in both research areas. This knowledge can be used to conceptualize new methodological strategies for either of both approaches starting from the other. It was attempted to extract key components that can be used as guidelines to refine, complement and innovate these two disciplines of organic synthesis.

Annulation Cascades of N-Allyl- N-((2-bromoaryl)ethynyl)amides Involving C-H Functionalization

An annulation cascades of N-allyl- N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C-H functionalization for producing 2,3-functionalized indoles has been first developed by means of Cu catalysis. The method is enabled by the formation of the ketenimine intermediates to deliver 2,3-disubstituted indoles through a sequence of aza-Claisen rearrangement, C-H functionalization, Ullmann C-N coupling, and cyclization.

Synthesis of Annulated Indolines by Reductive Fischer Indolization

Cyclic β-oxoesters and arylhydrazine derivatives were converted at ambient temperature under modified Fischer indolization conditions to furnish annulated indolines with quaternary bridgehead carbon center. Reaction conditions were Brønsted acidic (with CF₃ CO₂ H), but also reductive (with Et₃ SiH). The latter reduced intermediate iminium ions under formation of the 2,3-dihydroindole product constitutions. Racemic products (13 examples) were obtained as single diastereoisomers with relative cis-configuration, which was proved in two cases by single-crystal X-ray structure analysis. Conversion of 4-bromo-1-indanone-2-carboxylate and 4-bromophenylhydrazine gave indeno[1,2-b]indolines with either 1-bromo, 8-bromo, or 1,8-dibromo substitution, which were further diversified by Suzuki coupling reactions with several arylboronic acids (nine examples).

Effects of Substituents on Metastable-State Photoacids: Design, Synthesis, and Evaluation of their Photochemical Properties

Recently, metastable-state photoacids have been widely used to control proton transfer in numerous chemical and biological processes as well as applications with visible light. Generally, substituents have a great influence on the photochemical properties of molecules, which will further affect their applications. Yet, the effects of substituents on metastable-state photoacids have not been studied systematically. In this work, 16 metastable-state photoacid derivatives were designed and synthesized on the basis of substituents having a large range of σ-π electron-donor-acceptor capabilities. The effects of substituents on the color display [or maximum absorption band(s)], solubility, pK_a values, dark/photoacidity, photosensitivity, and relaxation kinetic(s) were investigated in detail. This study will be helpful for the targeted design and synthesis of promising photoacids and the application of their photocontrolled proton-release processes in functional materials/devices.

A new convenient synthetic route towards 2-(hetero)aryl-substituted thieno[3,2-b]indoles using Fischer indolization

A number of 2-(hetero)aryl-substituted thieno[3,2-b]indoles have been successfully prepared using an efficient transition-metal-free strategy, involving the Fiesselmann synthesis of methyl 5-(hetero)aryl-3-hydroxythiophene-2-carboxylates from 2-bromo-3-(hetero)arylacrylates and methyl thioglycolate, and the transformation of the synthesized 3-hydroxyesters into the corresponding thiophen-3(2H)-ones, followed by their treatment with arylhydrazines to directly form the targeted structures via Fischer indolization. At the same time, structural variety of the obtained thieno[3,2-b]indoles has been achieved due to a wide range of available starting materials, including both 2-bromo-3-(hetero)arylacrylates and arylhydrazines. In addition, two π-extended molecules, namely 1,4-bis(4H-thieno[3,2-b]indol-2-yl)benzene and 2,5-bis(4H-thieno[3,2-b]indol-2-yl)thiophene, have been synthesized in line with the current approach towards 2-(hetero)arylated thieno[3,2-b]indoles.

Synthesis of Hydrazines via Radical Generation and Addition of Azocarboxylic tert-Butyl Esters

A new chemistry of azo compounds that is a radical generation and addition in situ of azocarboxylic tert-butyl esters to synthesize hydrazines has been described. The protocol provides a novel strategy for the synthesis of various hydrazines. The advantages of the transformation include broad substrate scope, benign conditions, and convenient operation.

Understanding and Interrupting the Fischer Azaindolization Reaction

Experimental and computational studies pertaining to the Fischer azaindolization reaction are reported. These studies explain why pyridylhydrazines are poorly reactive in Fischer indolization reactions, in addition to the origin of hydrazine substituent effects. Additionally, an interrupted variant of Fischer azaindolization methodology is disclosed, which provides a synthetic entryway into fused azaindoline scaffolds.