Update on novel synthetic approaches towards the construction of carbazole nuclei: a review

The carbazole scaffold is a significant entity in organic compounds due to its variety of biological and synthetic applications. Traditionally, carbazole skeletons have been synthesized either via the Grabe-Ullman method, Clemo-Perkin method or Tauber method. With the passage of time, these methods have been modified and explored to accomplish the synthesis of target compounds. These methods include hydroarylations, C-H activations, annulations and cyclization reactions mediated by a variety of catalysts to construct carbazole-based compounds. This brief review article intends to provide recent updates on important methodological developments reported for the synthesis of carbazole nuclei covering 2019-2023.

Aryl glyoxal: a prime synthetic equivalent for multicomponent reactions in the designing of oxygen heterocycles

The category of bifunctional building blocks overrides many others because of their fascinating wide applicability in synthetic chemistry. Aryl glyoxal is one of the key molecules that has been extensively used in heterocyclic chemistry to afford nearly all types of five- and six-membered heterocycles, which are the structural constituents of many natural products. The multicomponent reaction is a practical strategy to utilize this wonderful moiety with different types of starting materials to obtain numerous diverse oxygen heterocycles. This review covers the advancement of aryl glyoxal as a prime synthetic equivalent in recent years for the synthesis of oxygen heterocycles.

Sulfenylation of Arenes with Ethyl Arylsulfinates in Water

A tetrabutylammonium iodide-mediated direct sulfenylation of arenes with ethyl arylsulfinates in water was developed. Various electron-rich arenes and ethyl arylsulfinates were investigated in the reaction, and a series of aryl sulfides were obtained in excellent yields. The advantages of this green protocol were simple reaction conditions (metal-free, water as the solvent, and under air), odorless and easily available sulfur reagent, broad substrate scope, and gram-scale synthesis. Moreover, the potential application of aryl sulfides was exemplified by further transformations.

I2/DMSO-Catalyzed Transformation of N-tosylhydrazones to 1,2,3-thiadiazoles

An iodine/DMSO catalyzed selective cyclization of N-tosylhydrazones with sulfur without adding external oxidant was developed for the synthesis of 4-aryl-1,2,3-thiadiazoles. In this reaction, oxidation of HI by using DMSO as dual oxidant and solvent is the key, which allowed the regeneration of I₂, ensuring thus the success of the synthesis. This protocol features by simple operation, high step-economy (one-pot fashion), broad substrate scope as well as scale-up ability.

Synthesis of mono- and bis-spirooxindole derivatives "on water" using double salt of aluminum sulfate-sulfuric acid as a reusable catalyst

The preparation of double salt of aluminum sulfate-sulfuric acid (Al₄(SO₄)₆·(H₂SO₄)·24H₂O) by the reaction of aluminum sulfate and sulfuric acid in water is described. Aluminum sulfate-sulfuric acid is characterized via some spectroscopic and microscopic techniques such as infrared spectroscopy (IR), X-ray diffraction spectroscopy (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which corroborated the structure of the double salt. This double salt is soluble in water and insoluble in organic solvents. It was employed as a new catalyst for the synthesis of spirooxindole compounds on water with good to excellent yields. The double salt could be recycled and reused without appreciable loss of activity.

Oligomeric ricinoleic acid preparation promoted by an efficient and recoverable Brønsted acidic ionic liquid

Raw material from biomass and green preparation processes are the two key features for the development of green products. As a bio-lubricant in metalworking fluids, estolides of ricinoleic acid are considered as the promising substitute to mineral oil with a favorable viscosity and viscosity index. Thus, an efficient and sustainable synthesis protocol is urgently needed to make the product really green. In this work, an environment-friendly Brønsted acidic ionic liquid (IL) 1-butanesulfonic acid diazabicyclo[5.4.0]undec-7-ene dihydrogen phosphate ([HSO₃-BDBU]H₂PO₄) was developed as the efficient catalyst for the production of oligomeric ricinoleic acid from ricinoleic acid under solvent-free conditions. The reaction parameters containing reaction temperature, vacuum degree, amount of catalyst and reaction time were optimized and it was found that the reaction under the conditions of 190 °C and 50 kPa with 15 wt % of the [HSO₃-BDBU]H₂PO₄ related to ricinoleic acid can afford a qualified product with an acid value of 51 mg KOH/g (which corresponds to the oligomerization degree of 4) after 6 h. Furthermore, the acid value of the product can be adjusted by regulating the reaction time, implying this protocol can serve as a versatile method to prepare the products with different oligomerization degree and different applications. The other merit of this protocol is the facile product separation by stratification and decantation ascribed to the immiscibility of the product and catalyst at room temperature. It is also worth mentioning that the IL catalyst can be used at least for five cycles with high catalytic activity. As a result, the protocol based on the IL catalyst, i.e. [HSO₃-BDBU]H₂PO₄ shows great potential in industrial production of oligomeric ricinoleic acid from ricinoleic acid.

Synthesis of α-indolylacrylates as potential anticancer agents using a Brønsted acid ionic liquid catalyst and the butyl acetate solvent

In this study, new α-indolylacrylate derivatives were synthesized by the reaction of 2-substituted indoles with various pyruvates using a Brønsted acid ionic liquid catalyst in butyl acetate solvent. This is the first report on the application of pyruvate compounds for the synthesis of indolylacrylates. The acrylate derivatives could be obtained in good to excellent yields. A preliminary biological evaluation revealed their promising anticancer activity (IC₅₀ = 9.73 μM for the compound 4l) and indicated that both the indole core and the acrylate moieties are promising for the development of novel anticancer drugs. The Lipinski's rule and Veber's parameters were assessed for the newly synthesized derivatives.

4lNew α-indolylacrylate derivatives were synthesized by reaction of 2-substituted indoles with various pyruvates using a Brønsted acid ionic liquid catalyst in butyl acetate solvent. This is the first application of pyruvate compounds for the synthesis of indolylacrylates.

Pyridine-2-carboxylic acid as an effectual catalyst for rapid multi-component synthesis of pyrazolo[3,4-b]quinolinones

Green synthesis of pyrazolo[3,4-b]quinolinones was designed using bioproduct pyridine-2-carboxylic acid (P2CA) as a green and efficient catalyst. The multi-component reaction of aldehydes, 1,3-cyclodiones and 5-amino-1-phenyl-pyrazoles regioselectively produced pyrazolo[3,4-b]quinolinones in excellent yield (84–98%). Recyclization of the catalyst was also investigated. The electronic effect of the various substituents in aromatic rings indicated that the reaction proceeded through the carbocation intermediate. This newly designed protocol very quickly constructed products conventionally under milder conditions.

Green synthesis of pyrazolo[3,4-b]quinolinones was designed using bioproduct pyridine-2-carboxylic acid (P2CA) as a green and efficient catalyst.

Impact of an aryl bulky group on a one-pot reaction of aldehyde with malononitrile and N-substituted 2-cyanoacetamide

In this study, we successfully explored the effect of steric hindrance on the one-pot reaction of different aryl aldehydes with malononitrile and N-substituted 2-cyanoacetamide in the presence of piperidinium acetate as the catalyst. It involved the Knoevenagel condensation of the aldehyde and malononitrile to produce arylidene malononitrile as an intermediate, which was further treated with N-substituted 2-cyanoacetamide to give 6-amino-2-pyridone-3,5-dicarbonitrile derivatives when the less steric bulky group was involved. High steric hindrance changed the earlier reaction route and gave N-substituted 2-cyanoacrylamides via a slower route.

Impact of aryl bulky group on the one-pot reaction of aldehyde with malononitrile and N-substituted 2-cyanoacetamide is examined in the presence of piperidinium acetate as reusable catalyst.

Access to polyfunctionalized carbazoles through π-extension of 2-methyl-3-oxoacetate indoles

A structurally diverse set of polyfunctionalized carbazoles was efficiently synthesized in acceptable to excellent yields.