Double Gold-Catalysed Annulation of Indoles by Enynones

Cu(OTf)2/HFIP catalyzed regioselective cycloisomerization of indole-C3-functionalized alkynols to carbazoles

We report here a simple and atom economic cycloisomerization reaction of indole-tethered alkynols for constructing diverse carbazoles using Cu(OTf)2/HFIP as the excellent promoter system. The reaction proceeds through a one-pot, domino process of spiro cyclization and 1,2-migration followed by aromatization to deliver carbazoles.

Catalyst- and Substrate-Controlled Regiodivergent Synthesis of Carbazoles through Gold-Catalyzed Cyclizations of Indole-Functionalized Alkynols

A wide variety of regioselectively substituted carbazole derivatives can be synthesized by the gold-catalyzed cyclization of alkynols bearing an indol-3-yl and an additional group at the homopropargylic positions. The regioselectivity of the process can be controlled by both the oxidation state of the gold catalyst and the electronic nature of the substituents of the alkynol moiety. The 1,2-alkyl migration in the spiroindoleninium intermediate, generated after indole attack to the activated alkyne, is favored with gold(I) complexes and for electron-rich aromatic substituents at the homopropargylic position, whereas the 1,2-alkenyl shift is preferred when using gold(III) salts and for alkyl or non-electron-rich aromatic groups.

A General Synthesis of Cross-Conjugated Enynones through Pd Catalyzed Sonogashira Coupling with Triazine Esters

The palladium-catalyzed Sonogashira coupling of α, β-unsaturated acid derivatives offers a diversity-oriented synthetic strategy for cross-conjugated enynones. However, the susceptibility of the unsaturated C-C bonds adjacent to the carbonyl group toward Pd catalysts makes the direct conversion of α, β-unsaturated derivatives as acyl electrophiles to cross-conjugated ketones rare. This work presents a highly selective C-O activation approach to prepare cross-conjugated enynones using α, β-unsaturated triazine esters as acyl electrophiles. Under base and phosphine ligand-free conditions, NHC-Pd(II)-Allyl precatalyst alone catalyzed the cross-coupling of α, β-unsaturated triazine esters with terminal alkynes efficiently, yielding 31 cross-conjugated enynones with diverse functional groups. This method demonstrates the potential of triazine-mediated C-O activation for preparing highly functionalized ketones.

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.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Reactivity of Cross-Conjugated Enynones in Cyclocondensations with Hydrazines: Synthesis of Pyrazoles and Pyrazolines

The cyclocondensation of cross-conjugated enynones, dienynones, and trienynones (easily available due to low-cost starting compounds) with arylhydrazines leads to the regioselective synthesis of pyrazole derivatives (dihetaryl-substituted ethens, buta-1,3-diens, and hexa-1,3,5-triens) or results in 4,5-dihydro-1H-pyrazoles in good yield. The reaction path is controlled by the character of the substituent in enynone: the pyrazoles are obtained from the reaction of substrates that contain five-membered heteroaromatic substituents with arylhydrazines, and the 4,5-dihydro-1H-pyrazoles are obtained from the reaction of 1,5-diphenylpent-1-en-4-yn-3-one with arylhydrazines consistently. Despite the presence of a substituent, cyclocondensation of 2-hydrazinylpyridine with all of examined cross-conjugated enynones leads to the formation of pyrazoles. The reaction does not require special conditions (temperature, catalyst, inert atmosphere). The cyclocondensation pathways are determined by the electronic effect of an electron-rich five-membered heteroaromatic ring in the substrate. The synthesis allows use of various substituents and functional groups in enynone and hydrazine. The present method features high yields and simplicity of the product purification. The obtained pyrazoles possess fluorescent properties with a quantum yield up to 31%.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Modular Synthesis of Polycyclic Alkaloid Scaffolds via an Enantioselective Dearomative Cascade

The polycyclic core of the akuammiline alkaloids can be synthesized from simple tryptamine and tryptophol derivatives via a Ag(I)-catalyzed enantioselective dearomative cyclization cascade sequence. The complex tetracyclic scaffolds are prepared via a rapid, versatile, three-step modular synthesis from simple commercially available indole derivatives in high yields and enantiomeric excess (up to 99% yield and >99% ee).

Gold catalysed transformation of 2-arylindoles to terphenyl amines via 3-dienyl indoles and Brønsted acid promoted formation of 2-carboxyindoles to 3-indenylindoles via 3-allenylindoles

Gold-catalysed intramolecular reaction of unprotected/protected 3-dienyl indoles leads to the formation of functionalised terphenylamines with high efficiency; this reaction can also be accomplished in one pot by starting with 2-arylindole and propargylic alcohols. This reaction occurs via 3-dienyl indoles and proceeds likely by gold-catalysed cascade cyclisation, aromatisation and fragmentation of the indole heterocyclic moiety. We have also found that PTSA can work well giving comparable yields, but at least one mole equivalent of PTSA is required. On the other hand, 3-indenylindoles have been synthesised from indole-2-carboxylates (2-carboxyindoles) and propargylic alcohols through Brønsted acid mediation. Many of these indenylindoles exhibit isomerism (probably diastereomers) in the solution state (1H NMR). Two examples of indolo-indenyl acetates have been prepared by reacting the above terphenylanilines with PhI(OAc)2. Key products are characterised by X-ray crystallography.

Catalytic dehydrogenative dual functionalization of ethers: dealkylation–oxidation–bromination accompanied by C–O bond cleavage via aerobic oxidation of bromide

Catalytic dehydrogenative dual functionalization (DDF) of ethers via oxidation, dealkylation, and α-bromination by the aerobic oxidation of bromide was developed to obtain the corresponding α-bromo ketones in high yields.

Synthesis of 1,4-enyne-3-ones via palladium-catalyzed sequential decarboxylation and carbonylation of allyl alkynoates

A novel and efficient palladium-catalyzed sequential decarboxylation and carbonylation of allyl alkynoates for constructing functionalized 1,4-enyne-3-ones has been demonstrated.

Modelling flavoenzymatic charge transfer events: development of catalytic indole deuteration strategies

The formation and chemistry of flavin-indole charge transfer (CT) complexes has been studied using a model cationic flavin. The ability to form a CT complex is sensitive to indole structure as gauged by spectroscopic, kinetics and crystallographic studies. Single crystals of sufficient quality of a flavin-indole CT complex, suitable for X-ray diffraction, have been grown, allowing solid-state structural analysis. When CT complex formation is conducted in d4-methanol, an efficient and synthetically useful C-3 indole deuteration is observed.

Catalyst-Driven Scaffold Diversity: Selective Synthesis of Spirocycles, Carbazoles and Quinolines from Indolyl Ynones

Medicinally relevant spirocyclic indolenines, carbazoles and quinolines can each be directly synthesised selectively from common indolyl ynone starting materials by catalyst variation. The high yielding, divergent reactions all proceed by an initial dearomatising spirocyclisation reaction to generate an intermediate vinyl-metal species, which then rearranges selectively by careful choice of catalyst and reaction conditions.

From Heteroaromatic Acids and Imines to Azaspirocycles: Stereoselective Synthesis and 3D Shape Analysis

Heteroaromatic carboxylic acids have been directly coupled with imines using propylphosphonic anhydride (T3P) and NEt(iPr)2 to form azaspirocycles via intermediate N-acyliminium ions. Spirocyclic indolenines (3H-indoles), azaindolenines, 2H-pyrroles and 3H-pyrroles were all accessed using this metal-free approach. The reactions typically proceed with high diastereoselectivity and 3D shape analysis confirms that the products formed occupy areas of chemical space that are under-represented in existing drugs and high throughput screening libraries.

Exploring the gold mine: [Au]-catalysed transformations of enynals, enynones and enynols

This review highlights reactions of enynals, enynones and enynols using gold catalysis which often lead to cycloaddition or cyclisation products.

Gold(i)-catalysed cascade reactions in the synthesis of 2,3-fused indole derivatives

Unactivated alkenes and 1,3-unsubstituted indoles in gold(i)-catalysed hydroaminative/arylative cascade cyclizations.