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

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.

Dearomative Ring-Opening of N-Fused Heteroarenes: Access to Tetrasubstituted Alkenes and Ketimines

We report a novel and efficient strategy for constructing tetrasubstituted alkenes and α-iminonitriles from 3-aminoindolizines and 3-aminoimidazo[1,2-a]pyridines. This approach involves a dearomative ring-opening of N-fused heteroaromatic amines coupled to a DDQ-mediated oxidative process under mild, metal-free conditions. The methodology demonstrates broad substrate scope, excellent functional group tolerance, and scalability, offering a versatile platform for synthesizing complex alkenes and nitrile-containing frameworks.

Synthesis of Highly Functionalized Indolizines via NIS-Promoted Spiroannulation/Ring-Opening Aromatization of Alkylidene Oxindoles with 2-(Pyridin-2-yl)acetate Derivatives

A novel NIS-promoted domino reaction of alkylidene oxindoles with 2-(pyridin-2-yl)acetate derivatives has been established, enabling the efficient and straightforward synthesis of a vast variety of highly functionalized indolizines via sequential spiroannulation and ring-opening aromatization processes. The protocol features mild reaction conditions, broad substrate scope, high efficiency, scalability, and applicability for the preparation of CF3-containing indolizines. Furthermore, the functional groups in the indolizine framework provide the feasibility for follow-up derivatization. Based on mechanistic studies, a plausible radical mechanism is proposed to elucidate the formation of indolizines.

Base-Promoted Regioselective Annulation of Pyridinium Ylides and Bromoalkynes for the Chemodivergent Synthesis of Indolizines

A base-promoted regioselective formal [3 + 2] annulation of pyridinium ylides with bromoalkynes is reported, producing a series of substituent-diverse indolizines in generally good yields. A mild K2CO3-promoted three-component formal [3 + 2] cyclization of pyridinium ylides and bromoalkynes at a 2:1 molar ratio delivered C2-acylmethylated indolizines, whereas C2-brominated indolizines were generated starting from pyridinium ylides bearing strong electron-withdrawing groups at the pyridinium unit by using 2,2,6,6-tetramethyl-1-piperidinyloxy as a dehydrogenating reagent. The current synthetic methodology offers a controllable and modular approach to access indolizines with different substitution patterns, featuring a wide substrate scope, good functional group compatibility, and complete regioselectivity without the demand of any transition-metal catalysts.

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

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.

Domino Approach to Heterocycles-Based Unsymmetrical Triarylmethanes through Heteroannulation of 2-(2-Enynyl)-pyridines with Enaminones

Herein, we report a copper-catalyzed protocol to access unsymmetrical triarylmethanes containing both indolizine and the chromone scaffolds in the same molecule via a 5-endo-dig cyclization of 2-(2-enynyl)-pyridines followed by reaction with 2-hydroxyaryl enaminones. A variety of 2-hydroxyaryl enaminones and 2-(2-enynyl)-pyridines were subjected to reaction under the optimal reaction conditions, and the respective triarylmethanes were obtained in good to excellent yields.

Pyridinium Ylide-Mediated Diastereoselective Synthesis of Spirocyclopropanyl-pyrazolones via Cascade Michael/Substitution Reaction

We have devised a highly diastereoselective formal [2 + 1] annulation reaction of arylidene/alkylidine-pyrazolones with in situ-generated supported as well as standard pyridinium ylides to construct spirocyclopropanyl-pyrazolones. The cascade approach exhibits a wide range of functional group tolerance, gram-scale capability, and substrate versatility. A diverse range of spirocyclic cyclopropanes was synthesized extensively with both mediators, and the supported pyridine was reused in subsequent cycles. Density functional theory calculations confirmed the formation of spirocyclopropane as the lower energy pathway.

One-pot C-N/C-C bond formation and oxidation of donor-acceptor cyclopropanes with tetrahydroisoquinolines: access to benzo-fused indolizines

One-pot C-N/C-C bond formation of donor-acceptor cyclopropanes (DACs) with tetrahydroisoquinolines (THIQs) has been achieved to furnish benzo-fused indolizines. These reactions involve a MgI2-catalyzed ring opening of DACs and oxidative annulation using Mn(OAc)3·2H2O. The substrate scope and functional group diversity are the important practical features.

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.

Switchable Synthesis of Spirodihydroindolizines and Indolizines from Aurones and Pyridin-2-yl Active Methylene Compounds

A novel and flexible domino reaction of aurones with pyridin-2-yl active methylene compounds promoted by I2/BF3 has been developed to afford spirodihydroindolizines and indolizines in a controllable manner. When the reaction was performed in 1,2-dichloroethane at 80 °C, a variety of spirodihydroindolizines were obtained, whereas it almost exclusively provided a series of indolizines when the reaction was performed in a mixed solvent of 1,2-dichloroethane and N,N-dimethylformamide at a relatively higher temperature of 100 °C. Being metal-free, excellent product selectivity, high atom economy, good functional group tolerance, and feasibility for large-scale synthesis are the salient features of the developed methodology.

Ag(I)-Mediated Annulation of 2-(2-Enynyl)pyridines and Propargyl Amines to Access 1-(2H-Pyrrol-3-yl)indolizines

An effective Ag(I)-mediated annulation of 2-(2-enynyl)pyridines and propargyl amines was developed, unexpectedly affording a broad range of functionalized 1-(2H-pyrrol-3-yl)indolizines in moderate to excellent yields. The developed method is characterized by operational simplicity, ready availability of starting materials, high regioselectivity, and broad substrate scope under mild reaction conditions. The Ag(I)-promoted cyclization of 2-(2-enynyl)pyridines and propargyl amines possibly results in the formation of the spiroindolizine, the ring-opening rearrangement of which may give the 1-(2H-pyrrol-3-yl)indolizine. Furthermore, a gram-scale reaction and synthetic transformations are also studied.

Discovery of indolizine lactones as anticancer agents and their optimization through late-stage functionalization

Indolizines fused with a seven-member lactone ring were identified as a promising scaffold in the search for new anticancer agents. Through a modular synthetic sequence, a library of cis and trans indolizines lactones had their antiproliferative activity evaluated against hormone-refractory prostate DU-145 and triple-negative breast MDA-MB-231 cancer cell lines. A methoxylated analogue was identified as an initial hit against MDA-MB-231 and late-stage functionalization of the indolizine core led to analogues within potencies up to twenty times higher than the parent precursor.

A reusable polymer anchored pyridine mediated formal [4 + 1] annulation reaction for the diastereoselective synthesis of 2,3-dihydrobenzofurans

We have developed a highly stereoselective formal [4 + 1] annulation reaction to construct trans-2,3-dihydrobenzofurans utilising in situ generated supported pyridinium ylide. This approach has excellent substrate versatility and gram-scale synthesis capability. Moreover, the polymer-anchored pyridine has been recovered and reused multiple times. The product has been transformed into valuable molecules.

Reusable Supported Pyridine-Mediated Cascade Synthesis of trans-2,3-Dihydroindoles via In Situ-Generated N-Ylide

Merrifield resin-anchored pyridines were prepared and applied as reusable mediators for trans-selective cascade synthesis of 2,3-dihydroindoles. The developed approach relied on in situ N-ylide formation followed by Michael substitution reactions. The cascade reaction was also carried out efficiently with simple pyridine. The products were further transformed into synthetically valuable compounds, and supported pyridine was reused for multiple cycles. Density functional theory calculations confirmed the trans-selectivity as the lower-energy pathway.

Synthesis of Indolizines via Tf2O-Mediated Cascade Reaction of Pyridyl-enaminones with Thiophenols/Thioalcohols

A cost-effective, highly regioselective and metal-free version for the synthesis of indolizine derivatives by means of Tf₂O-mediated cascade reaction of pyridyl-enaminones and thiophenols/thioalcohols under mild reaction conditions has been reported. Diverse electron-rich indolizine derivatives could be obtained in up to 94% yield via the selective 1,4-addition of vinyl iminium triflate tandem cyclization/aromatization, which allowed the simultaneous construction of C-N and C-S/and one example of C-Se bonds.

A recyclable stereoauxiliary aminocatalyzed strategy for one-pot synthesis of indolizine-2-carbaldehydes

Indolizine-carbaldehydes with the easily modifiable carbaldehyde group are important synthetic targets as versatile precursors for distinct indolizines. However, the efficient one-pot construction of trisubstituted indolizine-2-carbaldehydes represents a long-standing challenge. Herein, we report an unprecedented recyclable stereoauxiliary aminocatalytic approach via aminosugars derived from biomass, which enable the efficient one-pot synthesis of desired trisubstituted indolizine-2-carbaldehydes via [3+2] annulations of acyl pyridines and α,β-unsaturated aldehyde. Compared to the steric shielding effect from α-anomer, a stereoauxiliary effect favored by β-anomer of D-glucosamine is supported by control experiments. Furthermore, polymeric chitosan containing predominantly β-D-anhydroglucosamine units also shows excellent catalytic performance in aqueous solutions for the conversion of various substrates, large-scale synthesis and catalytic cycling experiments. Thus, our approach advances the existing methodologies by providing a rich library of indolizine-2-aldehydes. In addition, it delivers an efficient protocol for a set of late-stage diversification and targeted modifications of bioactive molecules or drugs, as showcased with 1,2,3-trisubstituted indolizine-2-carbaldehydes.

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.

B2pin2-Mediated Cascade Cyclization/Aromatization Reaction: Facial Access to Functionalized Indolizines

Herein, a B₂pin₂-mediated radical cascade cyclization/aromatization reaction of enaminone with pyridine is described. This strategy provides a practical way for the construction of valuable functionalized indolizines under metal-, external oxidant-, and base-free conditions, which could be compatible with various kinds of functional groups, such as halogen, π-system, heterocycle, ferrocenyl, etc. A preliminary mechanism investigation indicated that the pyridine-boryl radical formed in situ triggered the reaction to occur.

Synthesis of Ester-Substituted Indolizines from 2-Propargyloxypyridines and 1,3-Dicarbonyls

Two new complementary Au(I)-catalyzed methods for the preparation of ester-substituted indolizines from easily accessible 2-propargyloxypyridines and either acetoacetates or dimethyl malonate are reported. These reactions tolerate a wide range of functionality, allowing for diversification at three distinct positions of the product (R, R¹, R²). For electron-poor substrates, the highest yields are observed upon reaction with acetoacetates, while neutral and electron-rich substrates give higher yields upon treatment with dimethyl malonate.

Rh(ii)-catalyzed synthesis of 5H-isochromeno[3,4-b]indolizines from 4-diazoisochroman-3-imines and pyridines

A Rh(ii)-catalyzed (3 + 2) annulation of pyridines with 4-diazoisochroman-3-imines furnished 5H-isochromeno[3,4-b]indolizines with moderate to good yields and complete regioselectivity.