Highly Efficient Synthesis of Functionalized Indolizines and Indolizinones by Copper-Catalyzed Cycloisomerizations of Propargylic Pyridines

Divergent access to 5,6,7-perifused cycles

Nitrogen-containing heterocycles are the key components in many pharmaceuticals and functional materials. In this study, we report a transition metal-catalyzed high-order reaction sequence for synthesizing a structurally unique N-center 5,6,7-perifused cycle (NCPC). The key characteristics include the formation of a seven-membered ring by the 8π electrocyclization of various alkenes and aromatic heterocycles as π-components, in which metal carbene species are generated that further induce the cleavage of the α-C-H or -C-C bond. Specifically, the latter can react with various nucleophilic reagents containing -O, -S, -N, and -C. The stereo-controlled late-stage modification of some complicated pharmaceuticals indicates the versatility of this protocol.

Dearomative cyclization of pyridines/isoquinolines with cyclopropenones: access to indolizinones and benzo-fused indolizinones

Dearomatization reactions provide a rapid approach to construct complicated molecules that are difficult to synthesize by traditional methods from simple aromatic compounds. Herein, we report an efficient dearomative [3+2] cycloaddition reaction of 2-alkynyl pyridines with diarylcyclopropenones, leading to the synthesis of densely functionalized indolizinones in moderate to good yields under metal-free conditions. In addition, this strategy can also be employed in dearomative cyclization of isoquinolines to access a variety of benzo-fused indolizinones. Density functional theory (DFT) calculations revealed that an appropriate substituent at the 2-position of pyridine is crucial to the dearomatization process.

Copper-Catalyzed Ultrasonic-Promoted Coupling of Acetylene Analogs, Dialkyl azo dicarboxylate, and Benzazoles to Assemble Tricyclic Fused- Ring [1,2,3]triazolo[3,4-b][1,3]benzazole Analogs

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An unprecedented copper-catalyzed reaction of acetylene analogs with dialkyl azo dicarboxylate and benzazole analogs via a cross-coupling sequence was reported. A library of triazolobenzazole fused ring systems including [1,2,3] triazolo [3,4-b] [1,3] benzothiazole, [1,2,3] triazolo [3,4-b] [1,3] benzoxazole and [1,2,3] triazolo[3,4-b][1,3]benzimidazole structures were obtained in moderate to excellent yields under very mild reaction conditions. Structural confirmation of the final products became possible using different methods like spectroscopy and elemental analysis. The control experiments indicated C-H activation of acetylene by copper salts, followed by cycloaddition between a 2-(phenylethynyl)benzo[d]azol-3(2H)-yl anion and azo dicarboxylate as the key mechanistic feature. The broad substrate scope with simple and easily affordable starting materials, as well as mild reaction conditions are the noticeable attributes of this methodology, which provides facile access to the desired products.

Palladium-Catalyzed Difunctionalization of Alkenes by Relay Coupling with Propargylic Pyridines: Synthesis of Indolizine and Indolizinone-Containing Bisheterocycles

Palladium-catalyzed arylation/heteroarylation of aryl halide-tethered alkenes with propargylic pyridines has been established, which provides direct and efficient access to various oxindole, azaoxindole, dihydrobenzopyran, indole, and benzofuran-linked indolizines in good yields with a broad substrate scope and high functional group tolerance. This process enables the formation of one C-N and two C-C bonds in a single operation through an intramolecular carbopalladation and cycloisomerization sequence. Furthermore, an indolizinone-linked bisheterocyclic framework containing indole and benzofuran could be synthesized conveniently from tertiary propargylic alcohols involving methyl or phenyl migration.

Straightforward Functionalization of Sulfur-Containing Peptides via 5- and 6-endo-dig Cyclization Reactions

We present a simple and convenient method for the generation of sulfenyl electrophiles from peptides containing S–S or S–H bonds by employing N-chlorosuccinimide. The corresponding sulfenyl electrophiles are further utilized in 5- and 6-endo-dig cyclization reactions yielding indolizinium salts, indoles, benzo[b]furans, polyaromatic hydrocarbons (PAHs) and isocoumarins, as well as quinolinones bearing a glutathione moiety. PAH derivatives can be used as selective fluorescent dyes for the visualization of lipid droplets in living cells.

Lewis acid-catalyzed domino generation/[2,3]-sigmatropic rearrangement of ammonium ylides to access chiral azabicycles

[2,3]-Sigmatropic rearrangement of ammonium ylides represents a fundamental reaction for stereoselective synthesis of nitrogenous compounds. However, its applicability is limited by the scarcity of efficient, catalytic, and mild methods for generating ammonium ylides. Here, we report silver-catalyzed domino generation/[2,3]-sigmatropic rearrangement of ammonium ylides, furnishing chiral azabicycles with bridgehead quaternary stereogenic centers in high enantiomeric purity (up to 99% ee). A combination of density functional theory calculations and experimental studies revealed that residual water in the reaction system is crucial for the mild reaction conditions by functioning as a proton shuttle to assist carbon-silver bond protonation and C2─H deprotonation to generate the ammonium ylide. This reaction has a broad application scope. Besides the diverse substituents, N-fused azabicycles of various ring sizes are also easily accessed. In addition to silver salts, this strategy has also been successfully implemented by using a stoichiometric amount of nonmetallic I2.

Base-mediated 1,3-dipolar cycloaddition of pyridinium bromides with bromoallyl sulfones: a facile access to indolizine scaffolds

An expedient and transition-metal-free synthetic strategy has been developed for the construction of substituted indolizines from a unique combination of pyridinium salts and 2-bromoallyl sulfones. This approach does not compromise with the diverse substitutions on both the pyridinium salts and 2-bromoallyl sulfones. Wide substrate scope, operational simplicity, milder reaction conditions and good to moderate yields are the merits associated with the current approach. Moreover, this method provides two products which are amenable for the generation of a library of key indolizine building blocks.

Intramolecular azavinyl carbene-triggered rearrangement of furans

An intramolecular rhodium-catalyzed reaction of 1-tosyl-1,2,3-triazoles with furans has been explored. The tosylimino functionality was found to play a significant chemical role participating in the subsequent domino-transformations of a key reaction intermediate. One of the reaction pathways leads to valuable 2-formyl- and 2-acetylpyridine building blocks, which could be obtained in one-pot starting from easily accessible (furan-2-ylmethyl)propargyl amines. An original method for the synthesis of highly functionalized indolizines from the obtained pyridines has been proposed.

Metal-Free Aminothiation of Alkynes: Three-Component Tandem Annulation toward Indolizine Thiones from 2-Alkylpyridines, Ynals, and Elemental Sulfur

A metal-free three-component annulation reaction for the synthesis of indolizine thiones via tandem C-C/C-N/C-S bond formation was developed. Various 2-alkylpyridines with aromatic ynals and elemental sulfur proceeded smoothly under catalyst-free conditions, and the desired products were obtained in moderate to excellent yields.

Catalyst-Free Annulation of 2-Pyridylacetates and Ynals with Molecular Oxygen: An Access to 3-Acylated Indolizines

A catalyst and additive-free annulation of 2-pyridylacetates and ynals under molecular oxygen was the first developed, affording 3-acylated indolizines in good to excellent yields. Molecular oxygen was used as the source of the carbonyl oxygen atom in indolizines. This approach was compatible with a wide range of functional groups, and especially it has been successfully extended to unsaturated double bonds and triple bonds, which were difficult to prepare by previous methods in a single step.

Synthesis of functionalized indolizines via gold(i)-catalyzed intramolecular hydroarylation/aromatization of pyrrole-ynes

A gold-catalyzed intramolecular hydroarylation/aromatization of pyrrole-ynes has been developed. This method provides a concise and straightforward route to functionalized indolizines through the construction of the pyridine ring of indolizines and also allows elaboration of its pyrrole moiety with or without functional groups. In addition, a wide variety of functional groups, such as aryl, alkenyl, alkynyl, pyridyl or thienyl groups, can be easily incorporated into the pyridine unit of the indolizine products under mild conditions. The utility of the indolizine products was demonstrated by their efficient transformations into various C3-functionalized indolizine derivatives.

Synthesis of indolizine derivatives containing eight-membered rings via a gold-catalyzed two-fold hydroarylation of diynes

A novel strategy for a gold(i)-catalyzed synthesis of indolizine derivatives containing eight-membered rings has been developed, which may have potential usefulness as blue or green OLEDs.

Efficient synthesis and characterization of novel indolizines: exploration of in vitro COX-2 inhibitory activity and molecular modelling studies

Novel indolizine scaffolds as COX-2 inhibiting agents.

Indium(III)-catalyzed Aza-Conia-Ene Reaction for the Synthesis of Indolizines

A new indium(III)-catalyzed reaction for the synthesis of a series of indolizine scaffolds has been developed. This methodology was highly efficient, allowing a low catalyst loading of 2 mol % (down to 0.5 mol %) and rendering the products in high yields through a 5-exo-dig aza-Conia-ene reaction. Furthermore, the possibility of incorporating an electrophile into the generated pyrrolidone ring in a one-pot synergistic fashion was demonstrated. Finally, based on experimental observations, a mechanism proposal was outlined.

Metal-Free Cascade Approach toward Polysubstituted Indolizines from Chromone-Based Michael Acceptors

An efficient cascade transformation toward indolizine-based molecules has been developed. This process leads to the rapid construction of two C-N bonds and one C-C bond without the need of any metal catalysis. The approach involves easily accessible chromone-based Michael acceptors and propargylamine derivatives as starting materials. This cascade constitutes a novel and very competitive alternative to the well reported strategies using pyridine or pyrrole derivatives for accessing the indolizine ring with substituents at uncommon C-positions.

Investigation of the Pyridinium Ylide--Alkyne Cycloaddition as a Fluorogenic Coupling Reaction

The cycloaddition of pyridinium ylides with alkynes was investigated under mild conditions. A series of 13 pyridinium salts was prepared by alkylation of 4-substituted pyridines. Their reactivity with propiolic ester or amide in various reaction conditions (different temperatures, solvents, added bases) was studied, and 11 indolizines, with three points of structural variation, were, thus, isolated and characterized. The highest yields were obtained when electron-withdrawing groups were present on both the pyridinium ylide, generated in situ from the corresponding pyridinium salt, and the alkyne (X, Z = ester, amide, CN, carbonyl, etc.). Electron-withdrawing substituents, lowering the acid dissociation constant (pKa) of the pyridinium salts, allow the cycloaddition to proceed at pH 7.5 in aqueous buffers at room temperature.

Copper NPs supported on hematite as magnetically recoverable nanocatalysts for a one-pot synthesis of aminoindolizines and pyrrolo[1,2-a]quinolines

Copper mixed oxide NPs supported on a hematite surface were achieved using a facile hydrothermal method in a single step. The catalytic potential of the Cu@Fe₂O₃ NPs was explored for the synthesis of aminoindolizines and pyrrolo[1,2-a]quinolines.

Catalytic σ-activation of carbon–carbon triple bonds: reactions of propargylic alcohols and alkynes

A variety of organic transformations have been developed based on conceptually novel catalytic σ-activation of CC bonds of alkynes.

Cu ion-exchanged and Cu nanoparticles decorated mesoporous ZSM-5 catalysts for the activation and utilization of phenylacetylene in a sustainable chemical synthesis

Highly dispersed Cu nanoparticles supported on the large external surface of Meso-ZSM-5 exhibited excellent activity in the selective synthesis of indolizines, chalcones, and triazoles using phenylacetylene as one of the building blocks.

Recent advances in the synthesis of indolizines and their π-expanded analogues

Synthesis of indolizines developed during the last decade is reviewed, with special emphasis given to densely functionalized architectures, breakthrough strategies, compounds bearing electron-donating and electron-withdrawing substituents and π-expanded systems.