Three-Component Povarov Reaction–Heteroannulation with Arynes: Synthesis of 5,6-Dihydroindolo[1,2-a]quinolines

The Povarov Reaction: A Versatile Method to Synthesize Tetrahydroquinolines, Quinolines and Julolidines

The multicomponent Povarov reaction represents a powerful approach for the construction of substances containing N-heterocyclic frameworks. By using the Povarov reaction, in addition to accessing tetrahydroquinolines, quinolines and julolidines in a single step, it is possible to form the following new bonds: two Csp 3–Csp 3 and one Csp 3–Nsp 3, two Csp 2–Csp 2 and one Csp 2–Nsp 2, and four Csp 3–Csp 3 and two Csp 3–Nsp 1, respectively. This short review discusses the main features of the Povarov reaction, including its mechanism, the reaction scope by employing different catalysts and substrates, as well as stereoselective versions.

1 Introduction

2 Mechanism of the Povarov Reaction

3 Tetrahydroquinolines

4 Quinolines

5 Julolidines

6 Concluding Remarks

Employing Arylacetylene as a Diene Precursor and Dienophile: Synthesis of Quinoline via the Povarov Reaction

A novel I₂-mediated Povarov reaction of arylacetylenes and anilines for the synthesis of 2,4-substituted quinolines has been developed, in which arylacetylene first acts as both a diene precursor and dienophile. This work further develops the Povarov reaction to expand the types of diene precursors. Preliminary mechanistic studies indicate that the I₂/DMSO system realized the oxidative carbonylation of C(sp)-H of arylacetylene and then undergoes a [4 + 2] cycloaddition reaction.

Palladium-Catalyzed Multicomponent Synthesis of Fully Substituted Alkylidene Furanones

In the presence of a catalytic amount of Pd(OAc)₂ and XantPhos, the three-component reaction of ynones, imines, and aryl iodides affords fully substituted alkylidene-furan-3(2H)-ones via a sequence of the Mannich reaction followed by chemo- and regioselective oxypalladation and reductive elimination. One carbon-oxygen and two carbon-carbon bonds are generated to afford the heterocycle in good to excellent yield.

Diastereoselective and Reversed Regioselective Annulations of N-Alkyl Anilines to Julolidines and Lilolidines

A three-component annulation reaction of N-alkyl anilines, cyclic 1,3-dicarbonyl compounds, and aryl aldehydes to julolidines and lilolidines is reported. The 6π-electrocyclization enabled the annulation to proceed with reversed regioselectivity as compared with the annulation that occurs in the Povarov reaction. Both cyclic and acyclic N-alkyl anilines participated in the reaction to provide a wide range of julolidines and lilolidines as the single regio- and diastereoisomers in good to excellent yields.

Regiodivergent Synthesis of Penta-Substituted Pyrroles through a Cascade [3 + 2] Cyclization of C-Acylimines with Activated Alkynes and Aromatic Nucleophiles

Highly robust one-pot, four-component cascade cyclization reaction of α-keto aldehydes, anilines, activated alkynes, and aromatic nucleophiles is developed to synthesize a diverse range of pharmaceutically important penta-substituted pyrroles. The reaction proceeds through the cascade cyclization of acylimines (in situ formed) with activated alkynes and aromatic nucleophiles such as indoles, pyrroles, and naphthols at room temperature under calcium(II) catalysis with high yields and broad substrate diversity.

Diiodomethane-Mediated Generation of N-Aryliminium Ions and Subsequent [4 + 2] Cycloadditions with Olefins

Herein, we report a method for in situ generation of N-aryliminium ions via reactions of N,N-dimethylanilines with diiodomethane. We used the method to prepare tetrahydroquinolines via one-pot three-component reactions between N,N-dimethylanilines, diiodomethane, and olefins. This transformation involves initial reaction of the aniline with diiodomethane to form an iodomethylammonium salt, which undergoes fragmentation accompanied by elimination of methyl iodide to give an N-aryliminium ion, which is trapped by the olefin via [4 + 2] cycloaddition to give the final product. This method for generating N-aryliminium ions requires neither a catalyst nor a strong oxidant, suggesting that it can be expected to find broad utility, especially for substrates that are sensitive to Lewis acids, transition metals, or strong oxidants.

Synthesis of 6-substituted indoloquinazolinones from arynes and 2-acyl-4-quinazolinones: a transition-metal free C–N and C–C bond formation strategy

A versatile transition-metal free synthetic strategy has been developed for the direct synthesis of 6-substituted indoloquinazolinones from 2-acyl-4-quinazolinones and aryne precursors. This cascade strategy proceeds via successive C–N and C–C bond formation in a single reaction vessel.

Calcium-Catalyzed Stereoselective Tandem [4+2] and [3+2] Annulation Reaction for the Synthesis of Dihydropyrrolo[1,2-a]quinolines

Calcium-catalyzed highly facile one-pot, A⁴ annulation of aldehyde, amine, alkene, and alkyne to form fused 4,5-dihydropyrrolo[1,2-a]quinolines with exclusive syn diastereoselectivity is reported. This selectivity arises from an inverse electron demand [4+2] aza-Diels-Alder cycloaddition, and the adduct further undergoes a formal [3+2] cyclization with activated alkynes. This diversity-oriented protocol is highly general and furnishes the dihydropyrrolo[1,2-a]quinoline derivatives with a broad substrate scope in good to excellent yields.

Magnesiate-Utilized/Benzyne-Mediated Approach to Indenopyridones from 2-Pyridones: An Attempt To Synthesize the Indenopyridine Core of Haouamine

An efficient, short-staged synthesis of cis-fused indeno[2,1-b]- and indeno[1,2-c]pyridin-2-ones, starting from 2-pyridones, using magnesiates of type R₃MgLi as nucleophilic and deprotonation agents, mediated by benzyne generated in situ, under optimized conditions, is described. Following the developed protocol, rare C4a-arylsubstituted indeno[2,1-b]pyridones, resembling the core of haouamine, were obtained. The protocol offering the one-pot synthesis directly from 2-pyridone is also described.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Integrating reactive distillation with continuous flow processing

A continuous flow platform has been developed that integrates reactive distillation with modern flow reactor technology.

Tandem nucleophilic addition–cycloaddition of arynes with α-iminoesters: two concurrent pathways to imidazolidines

The tandem nucleophilic addition-cycloaddition reaction of arynes with α-iminoesters gave functionalized imidazolidine derivatives with high yields, complete regioselectivities, and excellent diastereo- and enantioselectivities.

FeCl3or MeSO3H-promoted multicomponent reactions for facile synthesis of structurally diverse furan analogues

The atom-economical characteristics, mild conditions, simple operation and broad substrate scope demonstrated the synthetic value of this protocol.

Harnessing sunlight without a photosensitizer for highly efficient consecutive [3+2]/[4+2] annulation to synthesize fused benzobicyclic skeletons

A photosensitizer-free, highly efficient sunlight-promoted tandem [3+2]/[4+2] annulation of unsaturated α-bromocarbonyls with o-alkynylanilines was described, and allowed for convenient synthesis of fused benzobicyclic skeletons.

A phosphine mediated sequential annulation process of 2-tosylaminochalcones with MBH carbonates to construct functionalized aza-benzobicyclo[4.3.0] derivatives

A novel phosphine mediated sequential annulation process to construct functionalized aza-benzobicyclo[4.3.0] derivatives has been developed involving a one-pot process.

Direct Construction of 2-Aryliminochromenes from Arynes, N,S-Keteneacetals, and DMF

A concise and direct synthetic strategy for the construction of 2-aryliminochromene skeleton by cascade three-component coupling reaction of arynes, N,S-keteneacetals, and DMF in good yields has been disclosed. The process demonstrates the first example of aryne chemistry combined with N,S-keteneacetals. Using this strategy, an expeditious synthesis of biologically important arylimino-2H-chromene-3- carboxamides was achieved.

Rhodium-catalyzed pyridannulation of indoles with diazoenals: a direct approach to pyrido[1,2-a]indoles

A new rhodium-catalyzed [4 + 2]-pyridannulation of 3-substituted indoles with diazoenals resulted in the biologically important pyrido[1,2-a]indoles.

Synthesis of novel fluorescent 12a-aryl substituted indoxylisoquinolines via aryne-induced domino process

A novel effective protocol towards indolo[2,1-a]isoquinolinones via aryne-induced migration of aryl-anion in aryloxy substituted 3,4-dihydroisoquinolines is reported.

A concise synthesis of quinolinium, and biquinolinium salts and biquinolines from benzylic azides and alkenes promoted by copper(ii) species

​A copper promoted multiple aza-[4 + 2] cycloaddition reactions between benzylic azides and alkenes to form four different products including quinolinium and biquinolinium cations, biquinolines and quinolines are described.

“On-water” synthesis of novel trisubstituted 1,3-thiazolesviamicrowave-assisted catalyst-free domino reactions

A clean, efficient and catalyst-free multicomponent domino reaction of arylglyoxals, cyclic 1,3-dicarbonyls and thioamides in aqueous media under microwave conditions is reported.