Highly Efficient Synthesis of Functionalized Indolizines and Indolizinones by Copper-Catalyzed Cycloisomerizations of Propargylic Pyridines

Multicomponent Strategy to Pyrazolo[3,4-e]indolizine Derivatives under Microwave Irradiation

A simple and efficient one-pot construction of pyrazolo[3,4-e]indolizine derivatives via a diethylamine-catalyzed three-component domino reaction of arylglyoxals, cyclic 1,3-diones, and 5-aminopyrazoles under microwave irradiation is described. In this one-pot transformation, seven bonds and two new rings are efficiently formed. This synthesis was confirmed to follow the group-assisted-purification (GAP) chemistry process, which can avoid traditional recrystallization or chromatography purification methods.

Au-catalyzed ring-opening reactions of 2-(1-alkynyl-cyclopropyl)pyridines with nucleophiles

A novel method for the C-C bond cleavage of cyclopropanes was developed by gold-catalyzed cycloisomerization of 2-(1-alkynyl-cyclopropyl)pyridine with nucleophiles, which provides efficient access to structurally diverse indolizines under mild conditions. A series of N-, C- and O-based nucleophiles were involved in this reaction to afford the corresponding indolizines in modest to excellent yields.

A facile approach for the synthesis of 1,3-di- and 1,2,3-tri-substituted indolizines

A convenient synthesis of 1,3-di- and 1,2,3-tri-substituted indolizines involving the multicomponent reaction of pyridinium salt (or quinolinium salt), olefine acid, and 1,2-dichloroethane has been achieved. The process proceeds smoothly in moderate to high yields.

Cu-Catalyzed Transannulation Reaction of Pyridotriazoles with Terminal Alkynes under Aerobic Conditions: Efficient Synthesis of Indolizines

The Cu(I)-catalyzed denitrogenative transannulation reaction of pyridotriazoles with terminal alkynes en route to indolizines was developed. Compared to the previously reported Rh-catalyzed transannulation reaction, this Cu-catalyzed method features aerobic conditions and much broader scope of pyridotriazoles and alkynes.

Palladium-catalyzed direct and regioselective C–H acyloxylation of indolizines

A direct and highly regioselective C1-acyloxylation of indolizines was developed via palladium-catalyzed C–H functionalization. In this reaction, the regioselectivity was achieved in the absence of a directing group.

Palladium-catalyzed highly efficient synthesis of functionalized indolizines via cross-coupling/cycloisomerization cascade

Pd-catalyzed cross-coupling/cycloisomerization of 3-(2-pyridyl) propargyl carbonates with organoboronic acids provides a straightforward route for the synthesis of 1,3-disubstituted indolizines.

Indolizine synthesis via Cu-catalyzed cyclization of 2-(2-enynyl)pyridines with nucleophiles

Cu-catalyzed cyclization of 2-(2-enynyl)pyridines with various nucleophiles was developed, providing efficient access to indolizine derivatives.

C3 functionalization of indolizines via In(iii)-catalyzed three-component reaction

Post-functionalization at the C3 position of indolizines via In(iii)-catalyzed three-component coupling reaction with amines and aldehydes allowed rapid access to a new class of indolizines with diverse functional groups at the C3 position in good to excellent yields.

A copper-catalyzed domino reaction to construct functionalized indolizinones

An efficient one-step copper-catalyzed domino reaction to 3,8α-disubstituted indolizinone derivatives has been developed for the first time, in which cheap and readily available pyridine ketones and terminal alkynes are the starting materials.

Silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides: divergent synthesis of indolizines and pyrroles

Divergent syntheses of indolizines and 2,4-disubstituted pyrroles by the silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides are reported.

Protic-solvent-mediated cycloisomerization of quinoline and isoquinoline propargylic alcohols: syntheses of (±)-3-demethoxyerythratidinone and (±)-cocculidine

Putting the "benz" in indolizinones: A cycloisomerization approach to benz[g]indolizinones and benz[e]indolizinones provides the first general route to these unique azacycles (see example). The utility of the benzindolizinone products was demonstrated by the application of this method to the total synthesis of the Erythrina alkaloids 3-demethoxyerythratidinone and cocculidine.

Iodine-Promoted Synthesis of 3-Arylindolizine-1-Carboxylates from 2-(2-Nitro-1-Arylethyl)Malonates and Pyridine

An efficient and straightforward one-pot synthetic protocol has been developed for the synthesis of 3-arylindolizine-1-carboxylates via 1,3-dipolar annulation of 2-(2-nitro-1-arylethyl)malonates with pyridine and subsequent aromatisation in the presence of molecular iodine. The structure of methyl 3-(4-methoxyphenyl)indolizine-1-carboxylate (2a) and methyl 3-iodo-2-(4-nitrophenyl)indolizine-1-carboxylate (2f) was further confirmed by X-ray single crystal analysis.

Copper-catalyzed multicomponent coupling/cycloisomerization reaction between substituted 1-formyl-9H-β-carbolines, secondary amines, and substituted alkynes for the synthesis of substituted 3-aminoindolizino[8,7-b]indoles

A copper-catalyzed efficient one step three component strategy for preparing a library of aminoindolizino[8,7-b]indoles from N-substituted 1-formyl-9H-β-carbolines, secondary amines, and substituted alkynes with high atom economy has been developed.

ORGANOCOPPER-MEDIATED TWO-COMPONENT SN2'-SUBSTITUTION CASCADE TOWARDS N-FUSED HETEROCYCLES

Organocuprates efficiently undergo reaction with heterocyclic propargyl mesylates at low temperature to produce N-fused heterocycles. The copper reagent plays a "double duty" in this cascade transformation, which proceeds through an S_N2'-substitution followed by a consequent cycloisomerization step.

Indolizinones as synthetic scaffolds: fundamental reactivity and the relay of stereochemical information

Indolizinones are under-explored N-heterocycles that react with exquisite chemo- and stereoselectivity. An exploration of the fundamental reactivity of these azabicycles demonstrates the potential to relay stereochemical information from the ring-fusion to newly formed stereocenters on the bicyclic core. The indolizinone diene undergoes selective hydrogenation and readily participates in Diels-Alder cycloadditions as well as ene reactions. The vinylogous amide embedded in the five-membered ring is resistant to reaction when the diene is in place. However, removal of the diene allows for diastereoselective hydrogenation of, and 1,4-additions to, the vinylogous amide. These fundamental reactions with indolizinones have provided a structurally diverse array of products that hold promise in the context of natural product synthesis.

Gold-catalyzed cyclizations of alkynol-based compounds: synthesis of natural products and derivatives

The last decade has witnessed dramatic growth in the number of reactions catalyzed by gold complexes because of their powerful soft Lewis acid nature. In particular, the gold-catalyzed activation of propargylic compounds has progressively emerged in recent years. Some of these gold-catalyzed reactions in alkynes have been optimized and show significant utility in organic synthesis. Thus, apart from significant methodology work, in the meantime gold-catalyzed cyclizations in alkynol derivatives have become an efficient tool in total synthesis. However, there is a lack of specific review articles covering the joined importance of both gold salts and alkynol-based compounds for the synthesis of natural products and derivatives. The aim of this Review is to survey the chemistry of alkynol derivatives under gold-catalyzed cyclization conditions and its utility in total synthesis, concentrating on the advances that have been made in the last decade, and in particular in the last quinquennium.

On the validity of Au-vinylidenes in the gold-catalyzed 1,2-migratory cycloisomerization of skipped propargylpyridines

A mechanism of the Au-catalyzed cycloisomerization of propargylpyridines has been investigated. Both DFT computational and experimental results strongly support generation of a Au-carbene via a cyclization/proton transfer sequence over the previously proposed path involving a Au-vinylidene intermediate. For the β-Si-substituted Au-carbene (G = SiR(3)), a 1,2-Si migration was shown to be kinetically favored over a 1,2-H shift. This study highlights the importance of alternative pathways that could explain reactivities commonly attributed to an alkyne-vinylidene isomerization in Au catalysis.

Palladium-catalyzed intramolecular carbopalladation/cyclization cascade: access to polycyclic N-fused heterocycles

An efficient palladium-catalyzed intramolecular carbopalladation/cyclization cascade toward tetra- and pentacyclic N-fused heterocycles has been developed. This transformation proceeds via the palladium-catalyzed coupling of aryl halides with internal propargylic esters or ethers followed by the 5-endo-dig cyclization leading to polycyclic pyrroloheterocycles in moderate to excellent yields.

Two-component approach toward a fully substituted N-fused pyrrole ring

An efficient two-component palladium-catalyzed arylation/cyclization cascade approach toward a variety of N-fused pyrroloheterocycles has been developed. This transformation proceeds via the palladium-catalyzed coupling of aryl halides with propargylic esters or ethers followed by the 5-endo-dig cyclization leading to highly functionalized pyrroloheterocycles in good to excellent yield.