Benzoic Acid-Promoted Intermolecular [3+2] Cycloaddition with Air as the Terminal Oxidant: General Approach toward Pyrrolo[2, 1-a ]isoquinolines

Photo-induced tungsten-catalyzed cascade synthesis of pyrrolo[2,1-a]isoquinoline-1,3-dicarboxylate and its reaction mechanism

A pyrrolo[2,1-a]isoquinoline core structure is prevalent in marine and other natural products. This article describes a tungsten-catalyzed [3+2] cycloaddition aromatization of dihydroisoquinoline ester and maleic anhydride or an acrylate. The photochemical reaction tolerates a range of functional groups such as ester, cyano, ketone, bromide, and alkene. It is shown that the cycloaddition-aromatization of 2-substitued acrylate catalyzed by a tungsten photocatalyst can be used to evaluate the leaving ability of the leaving group. Experiments done to determine the reaction mechanism revealed that the formation of an ion-pair intermediate generated in situ from dihydroisoquinoline ester and (Z)-4-methoxy-4-oxobut-2-enoic acid via the solvolysis of maleic anhydride with methanol is crucial for the cascade process to occur. The key cycloadduct acid intermediate derived from [3+2] cycloaddition was isolated and determined by X-ray crystallography.

Synthesis of Pyrrolo[2,1-a]isoquinolines through Cu-Catalyzed Condensation/Addition/Oxidation/Cyclization Cascade

We have developed a copper-catalyzed synthesis of pyrrolo[2,1-a]isoquinolines with terminal alkynes, aldehydes, and tetrahydroisoquinolines. A variety of pyrrolo[2,1-a]isoquinolines have been prepared in 17-69% yield via a condensation/Mannich-type addition/oxidation/cyclization cascade sequence. Modifications through simple chemical transformations provided highly functionalized molecules containing a privileged framework.

I2-DMSO-Mediated N-H/α-C(sp3)-H Difunctionalization of Tetrahydroisoquinoline: Formal [2 + 2 + 1] Annulation for the Construction of Pyrrolo[2,1-a]isoquinoline Derivatives

An I₂-DMSO-mediated cascade reaction using methyl ketones and 1,2,3,4-tetrahydroisoquinolines (THIQs) as commercially available substrates has been developed for the construction of pyrrolo[2,1-a]isoquinoline derivatives. This metal-free process involves N-H/α-C(sp³)-H difunctionalization of THIQ. Two C-C bonds and one C-N bond are formed in one pot under mild conditions. Besides, a quaternary carbon center has been constructed in this transformation efficiently.

Recent progress in the synthesis of pyrrolo[2,1-a]isoquinolines

Pyrrolo[2,1-a]isoquinolines occur frequently in a large number of bioactive natural products and pharmaceutically important molecules. The synthesis of pyrrolo[2,1-a]isoquinoline derivatives is an easy and useful way to produce artificial molecules with potential applications. A huge number of excellent methodologies for constructing pyrrolo[2,1-a]isoquinolines have been reported in the last decade. This review summarizes the recent advances in this research field covering from 2011 to 2021.

Tungsten catalysed decarboxylative [3 + 2] cycloaddition aromatization: one-pot synthesis of trifluoromethyl-pyrrolo[2,1-a]isoquinolines with visible light irradiation

Trifluoromethyl-pyrrolo[2,1-a]isoquinolines, which are difficult to prepare via traditional methods, could be readily prepared via a tungsten-catalysed decarboxylative [3 + 2] cycloaddition aromatization process.

Condensation-Based Methods for the C-H Bond Functionalization of Amines

This review aims to provide a comprehensive overview of condensation-based methods for the C-H bond functionalization of amines that feature azomethine ylides as key intermediates. These transformations are typically redox-neutral and share common attributes with classic name reactions such as the Strecker, Mannich, Friedel-Crafts, Pictet-Spengler, and Kabachnik-Fields reaction, while incorporating a redox-isomerization step. This approach provides an ideal platform to rapidly transform simple starting materials into complex amines.

Brønsted acid and Lewis acid co-promoted cascade cyclization reaction and application to the total synthesis of Erysotramidine

A strategy for the syntheses of pyrrolo [2,1-a] isoquinolines and indolizinoindolones via a Brønsted acid and Lewis acid co-promoted cascade reaction has been developed. The strategy was exemplified in the total synthesis of Erysotramidine.

CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization to access 5,6-dihydro-pyrrolo[2,1-a]isoquinolines

An efficient and enviromentally friendly CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization cascade was realized with N-substituted tetrahydroisoquinolines and electron-deficient olefins. Under the mild conditions, the reaction proceeded smoothly and displayed excellent functional group tolerance, affording 5,6-dihydro-pyrrolo[2,1-a]isoquinolines in good to high yields. This protocol exhibits a broad substrate scope to both N-alkyl tetrahydroisoquinolines and dipolarophile substrates.

Multicomponent Reaction for the Synthesis of 5,6-Dihydropyrrolo[2,1-a]isoquinolines

A multicomponent reaction between isatin, tetrahydroisoquinoline, and terminal alkyne in the presence of benzoic acid for the synthesis of N-(substituted-2-(2-phenyl-5,6-dihydropyrrolo[2,1-a]isoquinolin-3-yl)phenyl)-3,4-dihydroisoquinoline-2(1H)-carboxamides is described. This three-component reaction proceeds via sequential formation of spirooxindole, generation of isocyanate functionality via cleavage of the C2-C3 bond in the isatin subunit of spirooxindole, and addition of the second molecule of tetrahydroisoquinoline to the isocyanate group to offer title compounds. Expansion of the protocol to four-component by including an additional primary amine affords 1-substituted-3-(2-(2-phenyl-5,6-dihydropyrrolo[2,1-a]isoquinolin-3-yl)phenyl)urea in low to moderate yields. However, the reaction of intermediate spirooxindole with tetrahydroisoquinoline or any primary or secondary amine produced the title compound in excellent yields.

Acid-induced intermolecular [3+2] cycloaddition/oxidation to synthesize polysubstituted benzo[f]isoindole-4,9-diones

A novel one-pot 1,3-dipolar cycloaddition/oxidation reaction of 1,4-quinones, aromatic aldehydes, and N-substituted amino esters to construct polysubstituted benzo[ f]isoindole-4,9-diones induced by benzoic acid is reported. Based on optimized reaction conditions, various polysubstituted benzo[ f]isoindole-4,9-diones derivatives are obtained in moderate yields. This straightforward methodology employs mild reaction conditions, a green oxidant, and readily available starting materials. Notably, a wide range of substrates is tolerated.