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.

Gold-catalyzed bicyclic annulations of 4-methoxy-1,2-dienyl-5-ynes with isoxazoles to form indolizine derivatives via an Au-π-allene intermediate

Gold-catalyzed bicyclic annulations of 4-methoxy-1,2-dienyl-5-ynes with isoxazoles afford indolizine derivatives with a structural rearrangement. The mechanism of these new annulations does not involve α-imino gold carbenes generated from gold π-alkyne intermediates. We postulate alkyne attack on gold π-allenes, yielding vinyl gold carbenes. These newly generated carbenes react with isoxazole derivatives to yield Z-3-imino-2-en-1-als, further enabling sequential cyclizations to deliver indolizine derivatives in two distinct classes.

Copper-mediated oxidative [3 + 2]-annulation of nitroalkenes and pyridinium ylides: general access to functionalized indolizines and efficient synthesis of 1-fluoroindolizines

A general method for the synthesis of substituted indolizines by copper(ii) acetate-promoted oxidative [3 + 2]-annulation of α-fluoronitroalkenes with in situ generated pyridinium ylides was developed. Application of the copper(ii) acetate-2,6-lutidine system provides efficient access to various 1-fluoroindolizines in up to 81% yield. Both electron-rich and electron-deficient nitroalkenes as well as different pyridinium and isoquinolinium salts can be involved in the reaction. Moreover, it was found that copper-mediated annulation is applicable for other α-substituted (alkyl, chloro, and ester) nitroalkenes giving rise to the corresponding indolizines. First synthesis of monofluorinated [3,2,2]cyclazines was demonstrated via oxidative annulation of 3-unsubstituted fluoroindolizines with diethyl acetylene dicarboxylate.

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.

Efficient Gold(I) Acyclic Diaminocarbenes for the Synthesis of Propargylamines and Indolizines

Mononuclear gold(I) acyclic diaminocarbenes (ADCs) were prepared by the reaction of 1,2-cyclohexanediamine with the corresponding isocyanide complexes [AuCl(CNR)] (R = Cy, t Bu). The three-component coupling of aldehydes, amines, and alkynes was investigated by using these gold(I) ADC complexes. The new gold(I) metal complexes are highly efficient catalysts for the synthesis of propargylamines and indolizines in the absence of solvent and in mild conditions. This method affords the corresponding final products with excellent yields in short reaction times. Additionally, chiral gold(I) complexes with ADCs have been prepared and tried in the enantioselective synthesis of propargylamines.

A missing member of conjugated N-heterocycles: realizing pyrido[1,2-α]azepine by reacting ruthenium alkenylcarbene complex with alkyne

Despite the excellent chemical properties of N-heterocycles, pyrido[1,2-α]azepine remains elusive due to its potential antiaromaticity and lability. Herein, we demonstrate the synthesis and characterization of the first bicyclic pyrido[1,2-α]azepine that leverages the coordination to the ruthenium center to promote the stability of N-bridged bicycle.

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 α-Formylated N-Heterocycles and Their 1,1-Diacetates from Inactivated Cyclic Amines Involving an Oxidative Ring Contraction

A novel synthesis of pyrrolidine-2-carbaldehydes or tetrahydropyridine-2-carbaldehydes from the cascade reactions of N-arylpiperidines or N-arylazepanes is presented. Mechanistically, the formation of the title compounds involves an unprecedented oxidative ring contraction of inactivated cyclic amines via Cu(OAc)₂/KI/O₂-promoted oxidative cleavage and reformation of the C-N bond. Interestingly, when PhI(OAc)₂ was used in place of KI, 1,1-diacetates of the corresponding aldehydes were directly obtained with good efficiency. To the best of our knowledge, this is the first example of regioselective C(sp³)-H bond functionalization and C(sp³)-N bond activation of saturated cyclic amines using copper salt and oxygen.

Ruthenium–indolizinone complexes as a new class of metalated heterocyclic compounds: insight into unconventional alkyne activation pathways, revelation of unexpected electronic properties and exploration of medicinal application

The first examples of metal–indolizinone complexes prepared by Ru-assisted activation of pyridine-tethered alkynes exhibit moderate cytotoxicity against several human cancer cell lines.

I2-Catalyzed intramolecular dehydrogenative aminooxygenation of alkynes to acylated imidazo[1,2-a]pyridines and indolizines

An efficient and green protocol for the construction of acylated imidazo[1,2-a]pyridines and indolizines via intramolecular dehydrogenative aminooxygenation of alkynes has been developed.

Copper-catalyzed aerobic cyclizations of tetrahydroisoquinolines with bromoketones and alkenes for the synthesis of 5,6-dihydropyrrolo[2,1-a]isoquinolines

5,6-Dihydropyrrolo[1,2-a]isoquinolines have been synthesized via copper-catalyzed three-component aerobic cyclization reaction.

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.

General C-H Arylation Strategy for the Synthesis of Tunable Visible Light-Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores

Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf₂O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

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.

Synthesis of 7-cyanoindolizine derivatives via a tandem reaction

A series of 2-substituted and 2,3-disubstituted 5-cyanoindolizine derivatives were conveniently synthesized by a one-pot tandem reaction under mild conditions in moderate yields. The reaction mechanism was proposed.

Transition-metal-free synthesis of indolizines via [3 + 2]-annulation from α-bromoenals and 2-substituted azaarenes

A transition-metal-free “one-pot” synthetic approach for the synthesis of indolizines from α-bromo-substituted enals and simple 2-substituted azaarenes has been developed.

Application of primary halogenated hydrocarbons for the synthesis of 3-aryl and 3-alkyl indolizines

A transition-metal-free synthetic route to 3-aryl and 3-alkyl indolizines from electron-deficient alkenes, pyridines and primary halogenated hydrocarbons is reported for the first time using a tandem reaction.

Au(i)/Ag(i) co-operative catalysis: interception of Ag-bound carbocations with α-gold(i) enals in the imino-alkyne cyclizations with N-allenamides

A co-operative Au/Ag catalyst system has been developed to utilize N-allenamides as nucleophilic enal equivalents for the interceptive capturing of incipient carbocations generated through π-acid-triggered imino-alkyne cyclization.

Cu(ii) complex of a new isoindole derivative: structure, catecholase like activity, antimicrobial properties and bio-molecular interactions

An isoindole based trigonal bipyramidal Cu(ii) complex shows catecholase and antimicrobial activities. It interacts with bio-relevant antipyrine derivative.