Visible-Light-Induced Regioselective Dicarbonylation of Indolizines with Oxoaldehydes via Direct C–H Functionalization

Bichromophoric Ruthenium Complexes for Photocatalyzed Late-Stage Synthesis of Trifluoromethylated Indolizines

Indolizines are a promising class of biologically active compounds. However, photocatalytic methods for their selective derivatization are scarce in the literature. Herein, a mild, simple, and chemoselective protocol for the synthesis of 3-(trifluoromethyl)indolizine has been developed. The desired products were obtained in good to excellent yields and can be easily obtained on a gram scale. By tuning the redox properties of a Ru-based photocatalyst, it is possible to achieve competitive yields and further apply the optimized conditions to a broad variety of substrates. This method tolerates many functional groups and, therefore, can be used for late-stage functionalization. Our combined theoretical and spectroscopic findings revealed that the superior dyad-like ruthenium catalyst developed in this study has a completely different electronic nature of both key species that are crucial for efficient photoredox catalysis compared to commonly used homoleptic ruthenium complexes.

Recent Advances on the Construction of Functionalized Indolizine and Imidazo[1,2-a]pyridine Derivatives

Indolizines and imidazo[1,2-a]pyridines are commonly found in natural products, synthetic drugs, and bioactive molecules. These two types of derivatives possess good antibacterial, antiparasitic, anticancer activities, and so on. The functionalization of indolizines and imidazo[1,2-a]pyridines has always been a hot topic in organic chemistry research and has made significant progress. In recent years, our group has been dedicated to developing diverse synthetic methods for the preparation of such important compounds. 1) We have developed diverse C-H functionalization reactions for efficient modification of the parent indolizines and imidazo[1,2-a]pyridines. 2) A variety of cycloaddition reactions were established for the construction of indolizine and imidazo[1,2-a]pyridine derivatives from simple raw materials. 3) We have developed intriguing deconstruction-functionalization reactions of indolizines, enabling the reorganization of heterocyclic frameworks. This paper outlines our group's latest advancements in constructing structurally diverse indolizine and imidazo[1,2-a]pyridine derivatives. We hope that this work will offer valuable insights and inspiration for the ongoing research in the field of N-heterocyclic compounds.

Synthetic routes to access dicarbonylated aryls and heteroaryls

1,2-Dicarbonyl compounds are privileged functionalities found in natural products, pharmaceuticals, bioactive molecules, and food items, and are important precursors in catalysis, asymmetric synthesis, polymer chemistry and synthesizing functionalized heterocycles. Herein, this comprehensive review focuses on various approaches for synthesizing 1,2-dicarbonylated aryls and heteroaryls in both intermolecular and intramolecular fashion, covering the dicarbonylation of indoles, imidazoheterocycles, indolizines, aminopyrazoles, pyrroloisoquinolines, coumarins, furan, anilines, phenols, anthranils, and benzil synthesis over the last decade (since 2015). Also, the present review highlights the scope and future perspectives of the approach.

Indolizinylalanine Regioisomers: Tryptophan Isosteres with Bathochromic Fluorescence Emission

We have developed a high yielding synthesis of indolizine and directly elaborated the molecule into three optically active indolizinylalanine regioisomers. The protocols exploit metal catalyzed coupling of indolizinyl-halides with organozinc reagents derived from carbamoylated iodoalanine esters. The scalable protocols provide products in a form amenable to solid-phase peptide synthesis (SPPS). When incorporated into peptides, the indolizine heterocycle is more basic and markedly less nucleophilic than tryptophan. Its protonated vinylpyridinium form is deeply colored in solution while the neutral heterocycle is highly fluorescent. The fluorescence quantum yield of indolizine exceeds that of indole and aza-indoles in water, suggesting that indolizinylalanines could be powerful optical probes of protein structure and dynamics, functioning as true tryptophan isosteres.

Unveiling Na2-Eosin Y-Catalyzed and Water-Assisted Visible-Light Activation of Oxygen Molecules for the Dicarbonylation of Pyrazole Amines

A novel approach employing visible light-mediated activation of triplet oxygen molecules has been established. The reaction occurs at room temperature between pyrazole amine and phenylglyoxal monohydrate in the presence of Na2-eosin Y. Water played the dual role of solvent and reagent/additive. Photoactivation of triplet oxygen species was used to demonstrate the initiation of the hydrogen atom transfer (HAT) process. The conversion of the reaction mixture was found to be dependent on the amount of water present. Control experiments confirmed the importance of light, the photocatalyst, oxygen, the base, and water. The process tolerated various substitutions in both pyrazole amine and phenylglyoxal derivatives, enabling the synthesis of various dicarbonylpyrazole amines 15 and pyrazolooxazine derivatives 16 in moderate to good yields. 2 equiv of phenylglyoxal 10 gave a different reaction pathway, yielding highly diastereoselective pyrazolooxazine derivatives, confirmed by X-ray diffraction analysis. Collectively, this sustainable and environmentally friendly synthetic technique offers a promising method for the efficient preparation of pyrazole-based heterocyclic compounds. The high regioselectivity observed during the formation of trans-tetrahydropyrazolo[3,4-d][1,3]oxazine has been clarified through computational methods. These investigations emphasize the underlying factors and mechanisms that encourage the formation of this specific product, providing valuable insights into the reaction's selectivity and efficiency.

Controlled and Site-Selective C-H/N-H Alkenylation, Dialkenylation, and Dehydrogenative β-Alkenylation of Various N-Heterocycles

Here, we report controlled and site-selective C-H alkenylation and dialkenylation of indolizines and pyrrolo[1,2-a]quinolines with β-alkoxyvinyl trifluoromethylketones under simple and practical conditions. Moreover, this direct C-H alkenylation strategy can also be extended to imidazo[1,2-a]pyridines. Notably, without a transition metal and external oxidant, efficient dehydrogenative β-alkenylation of tertiary amines with β-alkoxyvinyl trifluoromethylketones is presented.

Thermodynamic Controlled Regioselective C1-Functionalization of Indolizines with 3-Hydroxyisoindolinones via Brønsted Acid Catalyzed aza-Friedel-Crafts Reaction

A Brønsted acid catalyzed aza-Friedel-Crafts reaction of indolizines with 3-hydroxyisoindolinones has been established, which constructs isoindolinone derivatives bearing a tetrasubstituted stereocenter in good to high yields and enantioselectivities. Notably, this strategy provides a new access to C1-functionalization of indolizines with excellent regioselectivities. Moreover, this intriguing C1-regioselective transformation was induced under thermodynamic control.

Metal-free visible light mediated direct C-H amination of benzoxazole with secondary amines

An efficient visible light mediated, eosin Y catalyzed direct C-H oxidative amination of benzoxazoles with secondary amines has been developed, which providing a straightforward, green, and environmentally benign access to a wide variety of substituted benzoxazole-2-amines under mild reaction conditions. The biological studies such as drug-likeness and molecular docking are also carried out on the molecule.

C3 Functionalization of Indolizines via HFIP-Promoted Friedel-Crafts Reactions with (Hetero)arylglyoxals

A highly efficient Friedel-Crafts type hydroxyalkylation at the C3 position of indolizines with (hetero)arylglyoxals has been achieved by the action of hexafluoroisopropanol (HFIP) under mild reaction conditions, leading to direct access to a variety of polyfunctionalized indolizines in excellent yields. Installation of more diverse functional groups at the C3 site of indolizine scaffold was realized via further elaboration of the resulting α-hydroxyketone moiety, allowing for expansion of indolizine chemical space.

Photocatalytic C-H Disulfuration for the Preparation of Indolizine-3-disulfides

A photocatalytic C-H disulfuration of indolizines was developed, giving an approach to a wide variety of indolizine-3-disulfides with good yields. Trisulfide dioxides were explored as a high-efficient disulfuration reagent. This disulfuration reaction could be scaled up to grams. Mechanistic studies support a photoinduced pathway involving the generation of indolizine cationic radicals. A bulky alkyl substituent on terminal sulfur of trisulfide dioxide A was necessary for selective formation of disulfide over monosulfide.

Transition metal-free annulative vinylene transfer via the 1,3-dipolar reaction of N-ylides: access to benzo-fused indolizines

An efficient metal-free annulative vinylene transfer protocol for the synthesis of benzo-fused indolizines via 1,3-dipolar cycloadditions of N-ylides with vinylene carbonate has been developed. Vinylene carbonate serves as an acetylene surrogate without any external oxidant involved. This transformation leads to the direct construction of versatile benzo-fused indolizine derivatives in moderate to good yields.

Acetic Acid-Catalyzed Regioselective C(sp2)-H Bond Functionalization of Indolizines: Concomitant Involvement of Synthetic and Theoretical Studies

An atom economical and environmentally benign protocol has been developed for the regioselective C(sp²)-H bond functionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing a wide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broad substrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played a crucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafast nature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated. Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzed pathway. Furthermore, regioselectivity has also been explained theoretically.

Dual Role of Oxoaldehydes: Divergent Synthesis of 3-Aryl- and 3-Aroylcoumarins

A facile and efficient synthetic approach to various valuable 3-aryl- and 3-aroylcoumarins by the direct arylation and aroylation of coumarins with glyoxals in a metal-free manner is presented. The aryl glyoxal is for the first time recognized to serve as an aryl surrogate in addition to its role as an aroyl transfer reagent via a simple switch in reaction conditions. The approach accommodates a broad substrate scope and high yields of the two types of cross-coupling reactions starting from identical starting materials.

The regioselective annulation of N-methylpyridinium ylides with alkenes enabled by palladium catalysis: access to 3-unsubstituted indolizine derivatives

The first catalytic protocol for the regioselective [3 + 2] annulation of N-methyl pyridinium ylides with alkenes to establish various valuable 3-unsubstituted indolizine derivatives is accomplished via palladium catalysis at the unactivated position.

Synthesis of α-Keto Aldehydes via Selective Cu(I)-catalyzed Oxidation of α-Hydroxy Ketones

An efficient approach to synthesize α-keto aldehydes was established through selective oxidation of α-hydroxy ketones catalyzed by Cu(I) using oxygen as oxidant. A wide array of α-keto aldehydes was prepared...

Recent advances in the synthesis of indolizine and its derivatives by radical cyclization/cross-coupling

Indolizine is a nitrogen-containing heterocycle that has a variety of potential biological activities, and some indolizine derivatives with excellent fluorescence properties can even be used as organic fluorescent molecules for biological and material applications. Thus, many approaches for their synthesis have been developed. Among them, radical-induced synthetic approaches are receiving increasing attention owing to their unique advantages, such as efficient heterocycle construction, efficient C-C or C-X bond construction, and high atom- and step-economy. This review systematically examines the current and latest synthetic strategies using radical species or radical intermediates for synthesizing indolizines and their derivatives. This review is classified into two parts based on the type of building blocks used for indolizine ring construction and the type of radical trigger for indolizine derivative construction. We anticipate that this review will provide a deep understanding of this topic, and ultimately help researchers to develop novel approaches for the synthesis of indolizine and its derivatives.

Diversity-oriented synthesis of imidazo[1,2-a][1,3,5]triazine derivatives from 2-amine-[1,3,5]triazines with ketones

An I₂-mediated annulation of 2-amino[1,3,5]triazines and ketones for the synthesis of imidazo[1,2-a][1,3,5]triazines is presented. Electron rich, or electron poor acetophenone and heterocycle ketones, as well as propiophenone, are functionalized with 2-amino-[1,3,5]triazines. Another class of imidazo[1,2-a][1,3,5]triazines tethered with an additional 1,2-dicarbonyl motif through the combination of annulation and C-H functionalization were obtained instead by changing the reaction conditions. The new methods are practically straightforward and applicable on a gram scale.

Mechanochemical Synthesis of 1,2-Diketoindolizine Derivatives from Indolizines and Epoxides Using Piezoelectric Materials

A simple and efficient mechanochemical-induced approach for the synthesis of 1,2-diketoindolizine derivatives has been developed. BaTiO₃ was used as the piezoelectric material in this transformation. This method features no usage of solvent, simple experimental operation, scalable potential, and high conversion efficiency, which make it attractive and practical.

Direct 3-Acylation of Indolizines by Carboxylic Acids for the Practical Synthesis of Red Light-Releasable Caged Carboxylic Acids

To enhance the practicality of photouncaging system using 3-acyl-2-methoxyindolizines, direct acylation of indolizines with carboxylic acids was developed using condensation reagents, generally used for peptide coupling. This method allowed for caging a broad range of carboxylic acids with indolizines. The method enabled a facile synthesis of water-soluble caged bioactive carboxylic acids having an intramolecular photosensitizer. The efficient release of carboxylic acids from the synthesized caged compounds upon red light irradiation was confirmed in neutral buffered solutions.

Oxidation Potential-Guided Electrochemical Radical-Radical Cross-Coupling Approaches to 3-Sulfonylated Imidazopyridines and Indolizines

Oxidation potential-guided electrochemical radical-radical cross-coupling reactions between N-heteroarenes and sodium sulfinates have been established. Thus, simple cyclic voltammetry measurement of substrates predicts the likelihood of successful radical-radical coupling reactions, allowing the simple and direct synthetic access to 3-sulfonylated imidazopyridines and indolizines. The developed electrochemical radical-radical cross-coupling reactions to sulfonylated N-heteroarenes boast the green synthetic nature of the reactions that are oxidant- and metal-free.

Regioselective C-H dithiocarbamation of indolizines with tetraalkylthiuram disulfide under metal-free conditions

An efficient and straightforward metal-free regioselective C-H dithiocarbamation of indolizines with tetraalkylthiuram disulfide has been described. A series of indolizine-dithiocarbamate derivatives were easily accessed in moderate to good yields with a broad scope. In addition, imidazo[1,2-a]pyridines were also well tolerated to afford diverse imidazoheterocycle-dithiocarbamate products, which are expected to be utilized for drug discovery. Of note, the reaction could be readily scaled up, and shows its practical value in organic synthesis.

Indolizine synthesis via radical cyclization and demethylation of sulfoxonium ylides and 2-(pyridin-2-yl)acetate derivatives

A novel radical cross-coupling/cyclization of 2-(pyridin-2-yl)acetate derivatives and sulfoxonium ylides is developed, which provides a straightforward access to structurally diverse methylthio-substituted indolizine.

Lewis acid-catalyzed regioselective C–H carboxamidation of indolizines with dioxazolones via an acyl nitrene type rearrangement

An efficient, direct, and novel Lewis acid-catalyzed regioselective C–H carboxamidation of indolizines with dioxazolones via an acyl nitrene type rearrangement under metal-free conditions has been documented.

Indolizine synthesis via copper-catalyzed cyclization of gem-difluoroalkenes and 2-(pyridin-2-yl)acetate derivatives

A novel and versatile approach to construct substituted indolizines through copper-catalyzed coupling cyclization of 2-(pyridin-2-yl)acetate with gem-difluoroalkenes has been developed.

Recent applications of Rose Bengal catalysis in N-heterocycles: a short review

The visible light harnessing ability of Rose Bengal, an organic dye, has been extensively employed in organic chemistry over the last few years. In visible light mediated reactions, this photoredox catalyst operates through multiple pathways and has the ability to provide distinctly different and valuable results. The most significant of these results are bond creation, bond functionalization, particularly for C–H and C–heteroatom bonds, and cross couplings. It is crucial to study these cases whenever these bond formations and couplings lead to the formation of heterocyclic compounds or their functionalization. The diverse biological activity and medicinal applications of heterocyclic compounds is an extensively explored area. This review primarily attempts to demonstrate the synthetic potential of Rose Bengal for synthesis and site selective functionalization of nitrogen containing heterocycles.

The recent applications of Rose Bengal as a photocatalyst for the synthesis and functionalization of N-heterocycles have been discussed.