A review on synthetic approaches of benzimidazoles

Mechanochemical Cascade Cyclization of Cyclopropyl Ketones with 1,2-Diamino Arenes for the Direct Synthesis of 1,2-Disubstituted Benzimidazoles†

A mechanochemical synthesis of 1,2-disubstituted benzimidazoles from donor-acceptor cyclopropyl ketones and 1,2-diaminoarenes under metal-free and solventless conditions is reported. The reaction does not require inert conditions and is promoted by a stoichiometric amount of 1,1,1,3,3,3-hexafluoroisopropanol. This cascade reaction involves ring-opening, cyclization, and retro-Mannich reaction of cyclopropyl ketones with aryl 1,2-diamines. Compared to its solution-phase counterpart, this mechanochemical approach shows fast reactivity (24 vs 1.5 h). Mechanistic investigations by electrospray ionization mass spectrometry helped us to propose the reaction mechanism.

Exploring the Synthetic and Antioxidant Potential of 1,2-Disubstituted Benzimidazoles Using [Et3NH][HSO4] Ionic Liquid Catalyst

An [Et3NH][HSO4] ionic-liquid catalyzed, intermolecular C-N bond formation for 1,2-disubstituted benzimidazole synthesis was achieved by the reaction of OPD and substituted aldehydes at ambient reaction conditions. Operational simplicity, use of easily available substrate and reagents, good yields (74-95 %) in short reaction time (4-18 min), simple work-up, and column chromatographic free synthesis are the remarkable features of this new protocol. The applicability of [Et3NH][HSO4] ionic-liquid as a green and inexpensive catalyst with good recyclability and compatibility with a broad range of functional group having heteroatom, electron-withdrawing, and electron-releasing groups manifested the sustainability, eco-friendliness, and efficiency of the present methodology. Moreover, the antioxidant studies of the synthesized compounds using DPPH and ABTS assays were appealing and several synthesized compounds showed significant antioxidant activity.

Iodine(III)-Catalyzed Oxidative Cyclization of Aryl Amines to Construct N-Alkylbenzimidazoles

An I(III)-catalyzed oxidative cyclization reaction using selectfluor as the oxidant was developed that converts ortho-substituted anilines to benzimidazoles. The mild reaction requires as little as 0.5 mol % of iodobenzene, and its scope is broad: electron-withdrawing or electron-releasing groups on the aniline portion are tolerated, and cyclic or acyclic N-alkylamines are permitted as ortho-substituents. Preliminary mechanistic investigations suggest that benzimidazole formation occurs via cationic reactive intermediates, and an intramolecular kinetic isotope effect of 1.98 ± 0.05 was measured.

The untold story of starch as a catalyst for organic reactions

Starch is one of the members of the polysaccharide family. This biopolymer has shown many potential applications in different fields such as catalytic reactions, water treatment, packaging, and food industries. In recent years, using starch as a catalyst has attracted much attention. From a catalytic point of view, starch can be used in organic chemistry reactions as a catalyst or catalyst support. Reports show that as a catalyst, simple starch can promote many heterocyclic compound reactions. On the other hand, functionalized starch is not only capable of advancing the synthesis of heterocycles but also is a good candidate catalyst for other reactions including oxidation and coupling reactions. This review tries to provide a fair survey of published organic reactions which include using starch as a catalyst or a part of the main catalyst. Therefore, the other types of starch applications are not the subject of this review.

Recent achievements in the synthesis of benzimidazole derivatives

Benzimidazoles are a class of heterocyclic compounds in which a benzene ring is fused to the 4 and 5 positions of an imidazole ring. Benzimidazole refers to the parent compound, while benzimidazoles are a class of heterocyclic compounds having similar ring structures, but different substituents. Benzimidazole derivatives possess a wide range of bioactivities including antimicrobial, anthelmintic, antiviral, anticancer, and antihypertensive activities. Many compounds possessing a benzimidazole skeleton have been employed as drugs in the market. The application of benzimidazoles in other fields has also been documented. The synthesis of benzimidazole derivatives has attracted much attention from chemists and numerous articles on the synthesis of this class of heterocyclic compound have been reported over the years. The condensation between 1,2-benzenediamine and aldehydes has received intensive interest, while many novel methods have been developed. In this article, we will give a comprehensive review of studies on the synthesis of benzimidazole, which date back to 2013. We have also tried to describe reaction mechanisms as much as we can. The work might be useful for chemists who work in the synthesis of heterocycles or drug chemistry.

N-Iodosuccinimide-Mediated Synthesis of Benzo-Fused Bisimidazoles Enabled by a One-Pot Tandem Reaction of Fluorinated Propargyl Amidines

A N-iodosuccinimide (NIS)-mediated divergent and efficient tandem reaction between fluorinated propargyl amidines and aromatic o-diamines without any metal catalyst and additive under mild reaction conditions was developed for the synthesis of benzo-fused bisimidazoles in moderate to excellent yields. Preliminary mechanistic studies suggested that this reaction proceeded by an intermediate of secondary amine derived from 5-iodomethyl imidazole, and NIS played another role of oxidation reagent to promote the formation of a benzimidazole motif.

Benzimidazole-Based Protein Kinase Inhibitors: Current Perspectives in Targeted Cancer Therapy

Targeted therapy has emerged to be the cornerstone of advanced cancer treatment, allowing for more selectivity and avoiding the common drug toxicity and resistance. Identification of potential targets having vital role in growth and survival of cancer cells got much easier with the aid of the recent advances in high throughput screening approaches. Various protein kinases came into focus as valuable targets in cancer therapy. Meanwhile, benzimidazole-based scaffolds have gained significant attention as promising protein kinase inhibitors with high potency and varied selectivity. Great diversity of these scaffolds has inspired the medicinal chemists to inspect the effect of structural changes upon inhibitory activity on the molecular level through modeling studies. The present review gathers all the considerable attempts to develop benzimidazole-based compounds; designed as protein kinase inhibitors with anticancer activity since 2015; that target aurora kinase, CDK, CK2, EGFR, FGFR, and VEGFR-2; to allow further development and progression regarding benzimidazoles.

AlCl3-Mediated CHF2 Transfer and Cyclocondensation of Difluoromethoxy Functionalized o-Phenylenediamines to Access N-Substituted Benzimidazoles

Herein, we report for the first time a transition-metal-free frustrated Lewis pair (FLP) catalyzed CHF₂ group migration from an oxygen atom to the neighboring nitrogen atom, which led to the synthesis of N-substituted benzimidazoles at room temperature with excellent yields, broad functional group tolerance, and a short reaction time. The oxygen-attached difluoromethane acted as a C1 source in the synthesis of N-substituted benzimidazoles in the presence of AlCl₃ by cleaving one C-O bond and two C-F bonds, resulting in formation of two new C-N bonds.

A Comprehensive Account on Recent Progress in Pharmacological Activities of Benzimidazole Derivatives

Nowadays, nitrogenous heterocyclic molecules have attracted a great deal of interest among medicinal chemists. Among these potential heterocyclic drugs, benzimidazole scaffolds are considerably prevalent. Due to their isostructural pharmacophore of naturally occurring active biomolecules, benzimidazole derivatives have significant importance as chemotherapeutic agents in diverse clinical conditions. Researchers have synthesized plenty of benzimidazole derivatives in the last decades, amidst a large share of these compounds exerted excellent bioactivity against many ailments with outstanding bioavailability, safety, and stability profiles. In this comprehensive review, we have summarized the bioactivity of the benzimidazole derivatives reported in recent literature (2012-2021) with their available structure-activity relationship. Compounds bearing benzimidazole nucleus possess broad-spectrum pharmacological properties ranging from common antibacterial effects to the world's most virulent diseases. Several promising therapeutic candidates are undergoing human trials, and some of these are going to be approved for clinical use. However, notable challenges, such as drug resistance, costly and tedious synthetic methods, little structural information of receptors, lack of advanced software, and so on, are still viable to be overcome for further research.

DARK Classics in Chemical Neuroscience: Etonitazene and Related Benzimidazoles

Etonitazene and related 2-benzylbenzimidazoles are potent analgetics invented in the research laboratories of the Swiss pharmaceutical giant CIBA in the late 1950s. Though the unprecedented structure distinguishes this class of compounds from poppy-derived and other synthetic analgetics, a range of studies indicate that these drugs are selective μ opioid receptor agonists possessing morphine-like pharmacotoxicological properties in animals as well as humans. Several unscheduled members of this synthetically readily accessible class of opioids that are not controlled under the international and national drug control systems have recently emerged on the illicit drug market. Among them, isotonitazene has been implicated in at least 200 fatalities in Europe and North America. None of the 2-benzylbenzimidazole derivatives have been developed into medicines, but etonitazene and some of its derivatives have been used as receptor probes and in addiction behavior studies in animals. The unique structure has inspired research on such benzimidazoles and related benzimidazolones of which "brorphine" made its debut as one of the newest psychoactive substance to emerge on the illicit opioid drug market in mid-2019. This in-depth review provides a historical introduction, an overview on the chemistry, pharmacological profiles, adverse effects, addiction liability, regulatory status, and the impact on chemical neuroscience of the 2-benzylbenzimidazoles. Structurally related benzimidazoles with opioid and/or analgesic properties are also discussed briefly.

Preparation of 1,2-substituted benzimidazoles via a copper-catalyzed three component coupling reaction

1,2-Substituted benzimidazoles were prepared by simply stirring a mixture of copper catalysts, N-substituted o-phenylenediamines, sulfonyl azides and terminal alkynes. Particularly, the intermediate N-sulfonylketenimine occurred with two nucleophilic addition and the sulfonyl group was eliminated via cyclization. In a way, sulfonyl azides and copper catalysts activated the terminal alkynes to synthesize benzimidazoles.