New achievements in the synthesis of pyrrolo[1,2-a]quinoxalines

Construction of Spirocyclic Pyrrolo[1,2-a]quinoxalines via Palladium-Catalyzed Hydrogenative Coupling of Phenols and Nitroarenes

The replacement of fossil resources with biomass resources in the construction of N-heterocycles is rapidly attracting research interest. Herein, we report palladium-catalyzed selective hydrogenative coupling of nitroarenes and phenols based on a transfer hydrogenation strategy, allowing straightforward access to spirocyclic pyrrolo- and indolo-fused quinoxalines, a class of compounds found in numerous natural alkaloids. The synthetic protocol is characterized by a broad substrate scope and the utilization of biomass-derived reactants and commercially available catalysts. In such transformations, high-pressure and explosive hydrogen are not required. This report provides a new protocol for converting biomass-derived phenols into value-added nitrogen-containing chemicals.

Synthesis, Crystal Structure and Anti-Leukemic Activity of 1,3-Dihydro-1-{1-[4-(4-phenylpyrrolo[1,2-a]quinoxalin-3-yl)benzyl]piperidin-4-yl}-2H-benzimidazol-2-one

1,3-Dihydro-1-{1-[4-(4-phenylpyrrolo[1,2-a]quinoxalin-3-yl)benzyl]piperidin-4-yl}-2H-benzimidazol-2-one has been synthesized through a multi-step pathway starting from commercially available 2-nitroaniline. A structure characterization of this new substituted pyrrolo[1,2-a]quinoxaline compound was achieved by using FT-IR, 1H-NMR, 13C-NMR, X-Ray and HRMS spectral analysis. This new pyrroloquinoxaline derivative shows an interesting cytotoxic potential against several human leukemia cell lines (HL60, K562 and U937 cells).

Pd-Catalyzed direct C-H arylation of pyrrolo[1,2-a]quinoxalines

An efficient Pd-catalyzed direct C-H arylation of pyrrolo[1,2-a]quinoxalines with aryl iodides is described, providing a selective route toward a series of 1-arylated and 1,3-diarylated pyrrolo[1,2-a]quinoxalines in good yields. This method features a broad substrate scope, good functional group tolerance and gram-scale synthesis. Furthermore, the C3-thiocyanation of the arylated product is also achieved. We believe that these novel aryl-substituted pyrrolo [1,2-a]quinoxalines will have a variety of applications in organic synthesis and medicinal chemistry.

Copper-promoted direct sulfenylation of C1–H bonds in 4-aryl pyrrolo[1,2-a]quinoxalines

Direct sulfenylation of pyrrolo[1,2-a]quinoxalines with diaryl disulfides was reported for the first time. A wide range of functionalities was compatible with reaction conditions.

Recent advances in the transition-metal-free synthesis of quinoxalines

Quinoxalines, also known as benzo[a]pyrazines, constitute an important class of nitrogen-containing heterocyclic compounds as a result of their widespread prevalence in natural products, biologically active synthetic drug candidates, and optoelectronic materials. Owing to their importance and chemists' ever-increasing imagination of new transformations of these products, tremendous efforts have been dedicated to finding more efficient approaches toward the synthesis of quinoxaline rings. The last decades have witnessed a marvellous outburst in modifying organic synthetic methods to create them sustainable for the betterment of our environment. The exploitation of transition-metal-free catalysis in organic synthesis leads to a new frontier to access biologically active heterocycles and provides an alternative method from the perspective of green and sustainable chemistry. Despite notable developments achieved in transition-metal catalyzed synthesis, the high cost involved in the preparation of the catalyst, toxicity, and difficulty in removing it from the final products constitute disadvantageous effects on the atom economy and eco-friendly nature of the transformation. In this review article, we have summarized the recent progress achieved in the synthesis of quinoxalines under transition-metal-free conditions and cover the reports from 2015 to date. This aspect is presented alongside the mechanistic rationalization and limitations of the reaction methodologies. The scopes of future developments are also highlighted.

Brønsted Acid-Catalyzed Minisci-Type Cross-Dehydrogenative Coupling of N-Heteroaromatics and Cyclic Ethers

Brønsted acid-catalyzed direct C(sp²)-H alkylation of N-heteroaromatics with cyclic ethers via a cross-dehydrogenative coupling reaction in the presence of benzoyl peroxide was developed. This methodology successfully provided an easy access to a variety of alkyl-substituted quinoxaline, benzoimidazole, pyrazine, pyrimidine, quinoline, isoquinoline, and pyridine derivatives in up to 94% yield under metal-free conditions.

Highly selective C3-H iodination of pyrrolo[1,2-a]quinoxalines

We report a C3-H direct iodination of pyrrolo[1,2-a]quinoxalines with TBAI or I2; a series of novel 3-iodopyrrolo[1,2-a]quinoxaline derivatives were obtained with excellent regioselectivity and broad substrate scope. Mechanism studies show that a catalytic amount of p-toluenesulfonic acid significantly promotes the selectivity and conversion of the reaction. Notably, the reaction can be performed on a gram scale, and the iodinated products can be further transformed into potentially biologically active pyrrolo[1,2-a]quinoxaline derivatives by palladium-catalyzed coupling reactions.

1-Phenyl-8-[[4-(pyrrolo[1,2-a]quinoxalin-4-yl)phenyl]methyl]-1,3,8-triazaspiro[4.5]decan-4-one: Synthesis, Crystal Structure and Anti-Leukemic Activity

1-Phenyl-8-[[4-(pyrrolo[1,2-a]quinoxalin-4-yl)phenyl]methyl]-1,3,8-triazaspiro[4.5]decan-4-one has been successfully synthesized via a multi-step pathway starting from 2-nitroaniline. Structure characterization of this original pyrrolo[1,2-a]quinoxaline derivative was achieved by FT-IR, 1H-NMR, 13C-NMR, X-Ray and HRMS spectral analysis. This title compound shows interesting cytotoxic potential against several human leukemia cell lines (K562, HL60, and U937 cells).

NCS-promoted thiocyanation and selenocyanation of pyrrolo[1,2-a]quinoxalines

An efficient NCS-promoted thiocyanation of pyrrolo[1,2-a]quinoxalines with NH₄SCN or KSCN was developed. Moreover, in the presence of KSeCN, the selenocyanation of pyrrolo[1,2-a]quinoxalines was also achieved.

KI/TBHP-promoted [3 + 2] cycloaddition of pyrrolo[1,2-a]quinoxalines and N-arylsulfonylhydrazones

A series of diverse fused [1,2,4]triazolo[3,4-c]quinoxalines was obtained by an efficient KI/TBHP-promoted [3 + 2] cycloaddition of pyrrolo[1,2-a]quinoxalines and N-arylsulfonylhydrazones.

α-Hydroxy acid as an aldehyde surrogate: metal-free synthesis of pyrrolo[1,2-a]quinoxalines, quinazolinones, and other N-heterocycles via decarboxylative oxidative annulation reaction

A metal-free and efficient procedure for the synthesis of pyrrolo[1,2-a]quinoxalines, quinazolinones, and indolo[1,2-a]quinoxaline has been developed. The key features of our method include the in situ generation of aldehyde from α-hydroxy acid in the presence of TBHP (tert-butyl hydrogen peroxide), and further condensation with various amines, followed by intramolecular cyclization and subsequent oxidation to afford the corresponding quinoxalines, quinazolinones derivatives in moderate to high yields.

A TBHP mediated, metal-free approach for the synthesis of quinoxalines, quinazolinones, and indolo quinoxaline was developed from alpha hydroxy acids via decarboxylation followed by condensation.