Gold-catalyzed 1,2-migration of silicon, tin, and germanium en route to C-2 substituted fused pyrrole-containing heterocycles

Trialkylborane-Mediated Multicomponent Reaction for the Diastereoselective Synthesis of Anti-δ,δ-Disubstituted Homoallylic Alcohols

The trialkylborane/O₂-mediated reaction of propargyl acetates having a tributylstannyl group at an alkyne terminus with aldehydes in a THF-H₂O solvent system gave anti-δ,δ-disubstituted homoallylic alcohols with good to high diastereoselectivity. Intriguingly, two alkyl groups derived from trialkylborane were embedded into the reaction product. The trialkylborane plays a key role not only as a radical initiator but also as a source of alkyl radicals.

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.

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.

One-Pot Synthesis of Indolizines via Sequential Rhodium-Catalyzed [2 + 1]-Cyclopropanation, Palladium-Catalyzed Ring Expansion, and Oxidation Reactions from Pyridotriazoles and 1,3-Dienes

An efficient, one-pot synthetic method for producing functionalized indolizine derivatives was developed via a Rh-catalyzed [2 + 1]-cyclopropanation, Pd-catalyzed ring expansion, and subsequent oxidation using manganese dioxide from pyridotriazoles and 1,3-dienes.

Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylanilides: Synthesis of 3-Substituted Indoles by 1,2-Carbon Migration

We developed ruthenium-catalyzed cycloisomerization of alkynylanilides that gave 3-substituted indoles in high yields. The reaction proceeded via the disubstituted vinylidene ruthenium complex that was formed by the 1,2-carbon migration.

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.

Chloroacetate Promotes and Participates in the Oxidative Annulation of Pyridines/Isoquinoline by Using Oxygen as the Oxidant

The aerobic oxidative annulation of chalcones, pyridines/isoquinoline and ethyl chloroacetate to indolizines was achieved by cascade reaction. Various functional groups on chalcones were tolerated. And different pyridines derivatives could also be suitable substrates. Ethyl chloroacetate is an essential component in participating of the oxidative annulation process. Overall, this protocol is very practical and efficient by using molecular oxygen as oxidant with high selectivity for the annulation product.

Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition

Cationic gold complexes in which gold is bound to a formally divalent carbon atom, typically formulated as gold carbenes or α-metallocarbenium ions, have been widely invoked in a range of gold-catalyzed transformations, most notably in the gold-catalyzed cycloisomerization of 1,n-enynes. Although the existence of gold carbene complexes as intermediates in gold-catalyzed transformations is supported by a wealth of indirect experimental data and by computation, until recently no examples of cationic gold carbenes/α-metallocarbenium ions had been synthesized nor had any cationic intermediates generated via gold-catalyzed enyne cycloaddition been directly observed. Largely for this reason, there has been considerable debate regarding the electronic structure of these cationic complexes, in particular the relative contributions of the carbene (LAu(+)[double bond, length as m-dash]CR2) and α-metallocarbenium (LAu-CR2(+)) forms, which is intimately related to the extent of d → p backbonding from gold to the C1 carbon atom. However, over the past ∼ seven years, a number of cationic gold carbene complexes have been synthesized in solution and generated in the gas phase and cationic intermediates have been directly observed in the gold-catalyzed cycloaddition of enynes. Together, these advances provide insight into the nature and electronic structure of gold carbene/α-metallocarbenium complexes and the cationic intermediates generated via gold-catalyzed enyne cycloaddition. Herein we review recent advances in this area.

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.

The fluxional amine gold(III) complex as an excellent catalyst and precursor of biologically active acyclic carbenes

A new amine gold(III) complex [Au(C6F5)2(DPA)]ClO4 with the di-(2-picolyl)amine (DPA) ligand has been synthesised. In the solid state the complex has a chiral amine nitrogen because the ligand coordinates to the gold centre through one nitrogen atom from a pyridine and through the NH moiety, whereas in solution it shows a fluxional behaviour with a rapid exchange between the pyridine sites. This complex can be used as an excellent synton to prepare new gold(III) carbene complexes by the reaction with isocyanide CNR. The resulting gold(III) derivatives have unprecedented bidentate C^N acyclic carbene ligands. All the complexes have been spectroscopically and structurally characterized. Taking advantage of the fluxional behaviour of the amine complex, its catalytic properties have been tested in several reactions with the formation of C-C and C-N bonds. The complex showed excellent activity with total conversion, without the presence of a co-catalyst, and with a catalyst loading as low as 0.1%. These complexes also present biological properties, and cytotoxicity studies have been performed in vitro against three tumour human cell lines, Jurkat (T-cell leukaemia), MiaPaca2 (pancreatic carcinoma) and A549 (lung carcinoma). Some of them showed excellent cytotoxic activity compared with the reference cisplatin.

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.

Synthesis of substituted indolizines via radical oxidative decarboxylative annulation of 2-(pyridin-2-yl)acetate derivatives with α,β-unsaturated carboxylic acids

A copper mediated radical oxidative annulation of 2-(pyridin-2-yl)acetate derivatives with α,β-unsaturated carboxylic acids is developed.

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.

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.