Synthesis of Pyrrolo[2,1,5-cd]indolizine Rings via Visible-Light-Induced Intermolecular [3+2] Cycloaddition of Indolizines and Alkynes

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.

Multi-Auto-Tandem Reaction to Access Site-Specific Functionalized Tricyclic Furo[3,2-c]coumarins and Naphtho[2,3-b]furan-4,9-diones

A three-component multi-auto-tandem reaction for the construction of site-specific tricyclic furo[3,2-c]coumarins via the formation of C-C, C-O, and C-S bonds in one step from 4-hydroxycoumarins/4-hydroxy-2-pyrones, ynals, and sodium sulfinates is reported. This cascade reaction efficiently produces a variety of rare C-2-functionalized furo[3,2-c]coumarins in moderate to good yields under straightforward reaction conditions. Furthermore, this protocol can be extended to a three-component coupling involving 2-hydroxy-1,4-naphthoquinone, ynals, and sodium sulfinates, yielding tricyclic naphtho[2,3-b]furan-4,9-dione derivatives. Notably, the carbonyl group and the α-position of ynals act as C-2 synthons in the specific multi-auto-tandem reaction, enabling the two aforementioned types of multicomponent transformations.

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.

Computational Insights into Palladium-Catalyzed Site-Selective Anilide and Benzamide-Type [3+2] Annulation via Double C-H Bond Activation

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

Dehydrogenation Coupling and [3 + 2] Cycloaddition of Indolizines with Allenes in the Presence of Piezoelectric Materials under Ball Milling Conditions

A wide range of indolizines with allenes proceeded smoothly under mechanochemical-induced conditions via [3 + 2] annulation process, affording various substituted pyrrolo[2,1,5-cd]indolizines with good yield. The reaction efficiency was greatly improved by using a piezoelectric material as the charge transfer catalyst. The photophysical properties of the resulting pyrrolo[2,1,5-cd]indolizine was characterized.

1-(Phenylselanyl)-2-(p-tolyl)indolizine: A selenoindolizine with potential antidepressant-like activity in mice mediated by the modulation of dopaminergic and noradrenergic systems

1-(Phenylselanyl)-2-(p-tolyl)indolizine (MeSeI) is a selenoindolizine with an antidepressant-like effect in mice by regulation of the serotonergic system. This study investigated the involvement of dopaminergic and noradrenergic systems in the antidepressant-like action of MeSeI. For this purpose, Swiss male mice were pretreated with different antagonists, after 15 min, the MeSeI was administrated by intragastric (i.g.) via; after 30 min, the mouse behavior was assessed in the forced swimming test (FST). The action of MeSeI on the activity of monoamine oxidase (MAO) was determined. The pretreatment of mice with haloperidol (0.05 mg/kg, intraperitoneally, i.p.; non-selective dopamine receptor antagonist), sulpiride (50 mg/kg, i.p.; D2 receptor antagonist), yohimbine (1 mg/kg, i.p.; α2 receptor antagonist), and propranolol (2 mg/kg, i.p.; non-selective β receptor antagonist), inhibited the anti-immobility action of MeSeI (50 mg/kg, i.g.) in the FST. This blocking effect was not observed when SCH23390 (0.01 mg/kg, i.p.; D1 receptor antagonist), and prazosin (1 mg/kg, i.p.; α1 receptor antagonist) were administered. The coadministration of subeffective doses of bupropion (3 mg/kg. i.g.; dopamine and noradrenaline reuptake inhibitor) and MeSeI (0.5 mg/kg. i.g.) reduced the immobility time in the FST. Furthermore, MeSeI inhibited MAO-A and B activities in vitro and ex vivo tests. These results suggest that MeSeI exerts its antidepressant-like effect via regulation of the D2, α2, and β1 receptors and the inhibition of MAO-A and B activities. Molecular docking investigations corroborated these results. This study provides comprehensive insights into the antidepressant-like mechanism of MeSeI in mice, suggesting its potential as a novel antidepressant candidate.

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.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Synthesis of N-, O-, and S-heterocycles from aryl/alkyl alkynyl aldehydes

In the field of heterocyclic synthesis, alkynyl aldehydes serve as privileged reagents for cyclization reactions with other organic compounds to construct a broad spectrum of N-, O-, and S-heterocycles. Due to the immense application of heterocyclic molecules in pharmaceuticals, natural products, and material chemistry, the synthesis of such scaffolds has received wide attention. The transformations occurred under metal-catalyzed, metal-free-promoted, and visible-light-mediated systems. The present review article highlights the progress made in this field over the past two decades.

Strategy Analysis of Ynones’ Radical Reactions

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This review highlights the multifaceted synthetic applications of ynones in radical reactions. Substantial progress has been made over the last decade (2010-2020) in the utilization of ynones. Herein, the chemistry of ynones is divided into three sections based on the classes of critical mechanistic insights: (1) radical addition and intramolecular cyclization; (2) radical addition and intermolecular annulation; (3) radical addition and coupling. We hope that this review will promote future research in this area.

Mechanochemically Induced Dehydrogenation Coupling and [3+2] Cycloaddition of Indolizines with Allenes Using Piezoelectric Materials

A wide range of indolizines with allenes proceeded smoothly under mechanochemically induced conditions via a [3+2] annulation process, affording various substituted pyrrolo[2,1,5-cd]indolizines in good yields. The reaction efficiency was greatly improved by using piezoelectric material as the charge transfer catalyst. The photophysical properties of the resulting pyrrolo[2,1,5-cd]indolizine were characterized.

Photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes

A facile photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes via a one-pot method has been unveiled. This photo-promoted strategy enables the formation of unsaturated NHC–boryl carboxylates under transition metal-free and radical initiator-free conditions. A wide array of pyridine-containing NHC–boryl carboxylates were directly prepared in moderate to good yields. This work contributes to a better understanding of the reactivity and photo-behavior of both indolizines and NHC–boranes.

A facile photoinduced successive oxidative ring-opening and borylation of indolizines with NHC–boranes via a one-pot method has been unveiled. This photo-promoted strategy enables the formation of unsaturated NHC–boryl carboxylates under transition metal-free conditions.

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.

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.

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.

Controllable Site-Selective Construction of 2- and 4-Substituted Pyrimido[1,2-b]indazole from 3-Aminoindazoles and Ynals

A straightforward and novel controllable site-selective construction of 2- and 4-substituted pyrimido[1,2-b]indazole from 3-aminoindazoles and ynals has been developed. The high regioselectivity of this reaction could be easily switched by converting different catalytic systems. In this way, a series of 2- and 4-substituted pyrimido[1,2-b]indazole derivatives were obtained in moderate to good yields. In addition, the photophysical properties of compound 3a prepared by the present method were discussed.

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.

Indolizines and Their Hetero/Benzo Derivatives in Reactions of [8+2] Cycloaddition

Peculiarities of [8+2] cycloaddition of acetylenes to indolizines are reviewed. Especially mentioned are indolizines with leaving groups at positions 3 and 5. Cycloaddition to aza- and benzo derivatives are reviewed, as well as 1,10-cyclizations and processes leading to cyclazines where indolizines are intermediates. Mechanistic features (adducts and cycloadducts) and theoretical aspects (one- or two-steps mechanism) are reviewed.

Palladium-Catalyzed Site-Selective [3+2] Annulation via Benzylic and meta C-H Bond Activation

The palladium-catalyzed [3+2] annulation of aromatic amides with maleimides via the activation of ortho benzylic C-H and meta C-H bonds is reported. Carboxamide and anilide type substrates that contain a 2-methylthiophenyl group both participate in this [3+2] annulation, indicating that the presence of a 2-methylthiophenyl directing group is a key for the success of the reaction. The first C-H bond activation at the benzylic C-H bond is followed by a second C-H bond activation at the meta C-H bond to give five-membered cyclic products. The cleavage of these C-H bonds is irreversible.

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.