One-pot tandem approach for the synthesis of benzimidazoles and benzothiazoles from alcohols

Microwave assisted one-pot access to pyrazolo quinolinone and tetrahydroisoxazolo quinolinone derivatives via T3P®-DMSO catalysed tandem oxidative-condensation reaction

A new approach for the synthesis of two important annulated pyrazolo quinolinone and tetrahydroisoxazolo quinolinone derivatives from multicomponent reactions was achieved by using T3P®-DMSO-catalysed reactions of stable alcohols, cyclic 1,3-dicarbonyl compounds and amino derivatives of phenyl pyrazoles and isoxazole and has been reported for the first time. This reaction occurred via a tandem oxidative-condensation reaction under microwave irradiation and notable characteristics of this protocol are MCR reactions, shorter reaction time, less waste creation, ease of workup, stable precursors, broad substrate scope and functional group tolerance.

Non-metal-mediated N-oxyl radical (TEMPO)-induced acceptorless dehydrogenation of N-heterocycles via electrocatalysis

The development of protocols for direct catalytic acceptorless dehydrogenation of N-heterocycles with metal-free catalysts holds the key to difficulties in green and sustainable chemistry. Herein, an N-oxyl radical (TEMPO) acting as an oxidant in combination with electrochemistry is used as a synthesis system under neutral conditions to produce N-heterocycles such as benzimidazole and quinazolinone. The key feature of this protocol is the utilization of the TEMPO system as an inexpensive and easy to handle radical surrogate that can effectively promote the dehydrogenation reaction. Mechanistic studies also suggest that oxidative TEMPOs redox catalytic cycle participates in the dehydrogenation of 2,3-dihydro heteroarenes.

An expedient, efficient and solvent-free synthesis of T3P®-mediated amidation of benzhydrols with poorly reactive N-nucleophiles under MW irradiation

An expedient, efficient, economical, environmentally benign, and solvent free amidation protocol of benzhydrols with less reactive nitrogen nucleophiles assisted by propylphosphonic anhydride (T3P®) under microwave irradiation has been developed.

Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

N-Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine-chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal-based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non-noble metal-based heterogeneous catalysts to synthesize N-heterocycles from alcohols and N-nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid-base properties on these reactions.

Recent Developments Towards Synthesis of (Het)arylbenzimidazoles

Benzimidazole is an important heterocycle that is widely researched and utilized by the pharmaceutical industry and is one of the five most commonly used five-membered aromatic heterocyclic compounds approved by the US Food and Drug Administration. In view of their wide-ranging bioactivities, systems containing benzimidazole as one of the moieties occupy a special place among other benzimidazole derivatives. Since 2010, many improved synthetic strategies have been developed for the construction of hetaryl- and arylbenzimidazole molecular scaffolds under environmentally benign conditions. This review emphasizes the recent trends and modifications frequently used in the synthesis of derivatives of benzimidazole such as the Phillips–Ladenburg and Weidenhagen reactions, as well as entirely new methods of synthesis, involving oxidative cyclization, cross-coupling, ring distortion strategy, and rearrangements carried out under environmentally benign conditions.

1 Introduction

2 From 1,2-Diaminobenzenes with Various One-Carbon Unit Suppliers

2.1 Phillips–Ladenburg Reaction

2.1.1 With (Het)arenecarboxylic Acids

2.2.2 With (Het)arenecarboxylic Acid Derivatives

2.2 Weidenhagen Reaction

2.2.1 With (Het)arenecarbaldehydes or (Het)aryl Methyl Ketones

2.2.2 With Primary Alcohols

2.2.3 With Primary Alkylamines

2.2.4 With 2-Methylazaarenes

2.2.5 With Other One-Carbon Fragment Suppliers

3 From 2-Haloacetanilides and Amines

4 From Amidines

5 From Tetrahydroquinazolines

6 Mamedov Rearrangement

7 Conclusions and Outlook

The selective synthesis of N-arylbenzene-1,2-diamines or 1-arylbenzimidazoles by irradiating 4-methoxy-4'-substituted-azobenzenes in different solvents

The solvent-controllable photoreaction of 4-methoxyazobenzenes to afford 1-aryl-1H-benzimidazoles or N-arylbenzene-1,2-diamines has been studied. The irradiation of 4-methoxyazobenzenes in DMF containing 0.5 M hydrochloric acid provided N²-aryl-4-methoxybenzene-1,2-diamines as the major product, while irradiation in acetal containing 0.16 M hydrochloric acid led to 1-aryl-6-methoxy-2-methyl-1H-benzimidazoles as the major product. A possible reaction mechanism explaining the selectivity was also discussed.

A solvent-controllable photoreaction involving 4-methoxyazobenzenes has been developed to synthesize 1-aryl-1H-benzimidazoles or N-arylbenzene-1,2-diamines in moderate to good yields.

Facile synthesis of 7-alkyl-1,2,3,4-tetrahydro-1,8-naphthyridines as arginine mimetics using a Horner-Wadsworth-Emmons-based approach

Integrin inhibitors based on the tripeptide sequence Arg-Gly-Asp (RGD) are potential therapeutics for the treatment of idiopathic pulmonary fibrosis (IPF). Herein, we describe an expeditious three-step synthetic sequence of Horner-Wadsworth-Emmons olefination, diimide reduction, and global deprotection to synthesise cores for these compounds in high yields (63-83% over 3 steps) with no need for chromatography. Key to this transformation is the phosphoramidate protecting group, which is stable to metalation steps.

Catalyst- and Additive-Free Method for the Synthesis of 2-Substituted Benzothiazoles from Aromatic Amines, Aliphatic Amines, and Elemental Sulfur

Under catalyst- and additive-free conditions, a novel, convenient, environmentally friendly method was developed for the synthesis of 2-substituted benzothiazoles via the three-component one pot reaction from aromatic amines, aliphatic amines, and elemental sulfur. The reaction achieves double C-S and one C-N bond formations via cleavage of two C-N bonds and multiple C-H bonds. Furthermore, the mechanism research shows that DMSO acts as an oxidant in the cyclization reaction.

Zirconium based porous coordination polymer (PCP) bearing organocatalytic ligand: A promising dual catalytic center for ultrasonic heterocycle synthesis

Herein, the efficient role of ultrasonic irradiation both in synthesis of Zr based porous coordination polymer (Zr-PCP) nanoparticles and boosting its catalytic activity, towards the benzimidazoles synthesis is represented. We use an amine based ligand (amino-terephthalate) for PCP and we exhibit that it can have a synergistic catalytic activity. In this work, a unique nano-engineering of cooperative and synergistic catalytic activity of zirconium, as a Lewis acid, and aminophenylene, as an organocatalyst, in the synthesis of heterocycles is presented for the synthesis of benzimidazole from cascade reaction of phenylene diamine with aldehyde at ambient temperature. Zr and amine groups of the Zr-PCP are active catalytic sites which in combination with the ultrasonic irradiation leads to a high selectivity and rapid catalytic production of benzylimidazoles. N₂ adsorption-desorption along with BJH analyses confirm the microporosity of the catalyst and recyclability shows that the catalyst is green and sustainable heterogeneous microporous catalyst.

T3P® mediated domino C(sp2)–H sulfenylation/annulation of enaminones and methylsulfinyls for the synthesis of chromone thioether derivatives

Regio-selective preparation of 3-sulfenylated chromone using T3P® has been described for the first time.

A dendritic TiO2–Co(ii) nanocomposite based on the melamine catalyzed one-pot aerobic photocatalytic synthesis of benzimidazoles

The nanocrystalline TiO₂ surface modified with cobalt melamine dendrimer amine complex under ultrasonic agitation produced a novel low band-gap photocatalyst (Co(ii)–TD@TiO₂). This catalyst system involves photooxidation of alcohols to aldehydes followed by cyclocondensation with 1,2-phenylenediamine to afford benzimidazoles in one-pot operation with good to excellent yield.

Double C–S bond formation via C–H bond functionalization: synthesis of benzothiazoles and naphtho[2,1-d]thiazoles from N-substituted arylamines and elemental sulfur

An atom economic and environmentally friendly method for the synthesis of benzothiazoles and naphtho[2,1-d]thiazoles has been developed under metal-free conditions.

N-Iodosuccinimide involved one-pot metal-free synthesis of 2-heteroaromatic benzothiazole compounds

A metal-free and operationally simple protocol for the synthesis of structurally diverse 2-hetarylbenzothiazoles via oxidative condensation of the sp³ C–H bond with benzothiazoles has been described.

A novel approach for the synthesis of imidazo and triazolopyridines from dithioesters

Various imidazo- and triazolo-pyridines were synthesised by the intramolecular cyclization of pyridine 2-methylamine and dithioesters under mild conditions.

One-step approach for the synthesis of functionalized quinoxalines mediated by T3P®–DMSO or T3P® via a tandem oxidation–condensation or condensation reaction

An easy and efficient (T3P®)–DMSO or T3P® mediated oxidation–condensation or condensation reaction for the synthesis of quinoxalines from the different arrays of condensing partners and ortho-phenylene diamines (o-PDs) in one step has been reported.

Ligand- and catalyst-free intramolecular C-S bond formation: direct access to indalothiochromen- 4-ones

An efficient ligand- and catalyst-free intramolecular S-arylation leading to the direct synthesis of indalothiochromen-4-ones from simple dithioesters under mild conditions has been developed. This method is particularly noteworthy given its experimental simplicity, high generality, and good functional group toleration.

Recent advances in the chemistry and biology of benzothiazoles

Benzothiazole is a privileged heterocyclic scaffold having a benzene ring fused with a five-membered thiazole ring. This moiety has attracted considerable attention because of its wide range of pharmacological activities such as antitubercular, antimicrobial, antimalarial, anticonvulsant, anthelmintic, analgesic, anti-inflammatory, antidiabetic, antitumor activity, etc. In the last few years, some novel benzothiazoles have been developed with varied biological activities. To access this scaffold in high yield and to introduce diversity, a variety of new synthetic methods have been invented. In this review, we highlight the development of novel benzothiazoles for various biological activities along with the best synthetic protocols for their synthesis.

Highly diastereoselective synthesis of polycyclic amines via redox neutral C–H functionalization

Construction of polycyclic amines via C–H functionalization and 3+2 cycloaddition with high diastereoselectivities was achieved under mild conditions.

Diastereoselective synthesis of fused oxazolidines and highly substituted 1H-pyrrolo [2,1-c][1,4] oxazines via C–H functionalization

The first one pot protocol for the diastereoselective synthesis of oxazolo[2,3-c]isoquinoline was achieved by metal-free, benzoic acid catalyzed reactions under mild conditions via C–H, C–O bond functionalization.

A novel benzimidazole derivative binds to the DNA minor groove and induces apoptosis in leukemic cells

In the present study, we have synthesized various benzimidazole derivatives, evaluated their potential to act as DNA minor groove binder and tested their chemotherapeutic efficacy.

Iron(ii) bromide-catalyzed oxidative coupling of benzylamines with ortho-substituted anilines: synthesis of 1,3-benzazoles

An iron(ii) bromide-catalyzed oxidative coupling of benzylamines with 2-amino/hydroxy/mercapto-anilines has been developed, allowing the synthesis of a diversity of substituted 1,3-benzazoles in good to excellent yields.

One-pot synthesis of 2,3-substituted benzo[b]thiophenes via Cu(i) catalysed intramolecular cyclisation from dithioesters

The novel route involves CuI catalyzed C–S bond formation using dithioesters. This approach is valuable for synthesis of rolaxifene analogues.