Solid-phase synthesis of benzimidazoles

Activated Carbon Supported Hafnium(IV) Chloride as an Efficient, Recyclable, and Facile Removable Catalyst for Expeditious Parallel Synthesis of Benzimidazoles

A highly efficient method for parallel synthesis of a diversity of 1,2-disubstituted benzimidazoles from N-substituted phenylenediamines and aldehydes has been developed by using 10 mol% HfCl4 on activated carbon (HfCl4/C) as the catalyst. The newly reported HfCl4/C catalyst not only mediated fast and clean formation of benzimidazoles but also could be easily removed from the reaction solution and reused up to eight times. Scanning electron microscope (SEM) and thermal desorption studies showed that activated carbon could reversibly adsorb and release Hf(IV) in ethanol upon cooling and heating, thereby serving as a thermal-controlled solid support.

Benzo[d]imidazol-5-yl)-5-(substituted)-1,3,4-Oxadiazoles: Synthesis, Anticancer, Antimicrobial and In Silico Studies

Background:

Cancer is a fatal disease for mankind; continuous research is still going on for the invention of potent anticancer drugs. In this view, 1, 3, 4-Oxadiazoles are privileged molecules which attracted medicinal chemists towards their anticancer properties.

Methods:

A new series of benzo[d]imidazol-5-yl)-5-(substituted)-1,3,4-oxadiazole derivatives was synthesized in an efficient ‘one-pot’ nitro reductive cyclization using sodium dithionite as a cyclizing agent by a conventional method with good yield. All the structures of the synthesized molecules were characterized by IR, 1H NMR, HRMS and Mass spectral analysis. Anticancer activity screening against A375 melanoma cancer cell line and MDA-MB-231 breast cancer cell line along with antimicrobial activity were carried out using agar well diffusion method.

Results:

Compounds 8a and 8j of the series emerged as potent anticancer agents against A375 melanoma cancer cell line with IC50 47.06 µM and 36.76 µM, respectively. In silico studies also revealed that compounds 8a and 8j showed highest interaction with 2OH4 protein of VEGFR-2 tyrosine kinase. Substantial antibacterial and antifungal activities against the tested microorganism were observed for compounds 8j and 8g.

Conclusion:

Potent anticancer property has been observed with 1,3,4-Oxadiazole linked tetrafluro substituted benzene ring 8j indicating that future research on these type of molecules can be continued to improve the anticancer activity.

Complexes of (η5-Cp*)Ir(iii) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-substituted benzimidazole synthesis

The treatment of 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone [L1-L4] with [(η⁵-Cp*)IrCl(μ-Cl)]₂ at 25 °C followed by NH₄PF₆ results in [(η⁵-Cp*)Ir(L)Cl][PF₆] (1-4 for L = L1 to L4), authenticated with high-resolution mass spectrometry (HR-MS) and multi-nuclei nuclear magnetic resonance (NMR) imaging (¹H, ¹³C{¹H} and ⁷⁷Se{¹H}). The structures of 1-4, established with single-crystal X-ray diffraction, reveal a "piano-stool" geometry around the Ir. The Ir-thio/selenoether (Ir-S/Ir-Se) bond distances (Å) are 2.347(18)-2.355(4)/2.4663(12)-2.4663(13) and Ir-thione/selenone (Ir-S/Ir-Se) distances are 2.4146(19)-2.417(2)/2.5141(16)-2.5159(12). The reaction of 1,2-phenylenediamine with benzylic alcohols and furfuryl alcohol under mild and ambient conditions, catalyzed efficiently with complexes 1-4, generates bisimine in situ. Cyclization and rearrangement via 1,3-hydride shift triggered by its electrophilic activation with Ir(iii) species finally results in 1,2-disubstituted benzimidazole. The yield of the heterocycles in this one-pot synthesis is excellent to good. The aldehydes generated in situ by aerial oxidation of alcohols in the presence of 1-4 as catalysts are precursors to the bisimine as the protocols of this heterocycle synthesis carried out in the absence of 1,2-phenylenediamine give them in excellent-to-good yield. The oxidation of alcohols by hydrogen transfer to acetone was catalyzed efficiently with complexes 1-4 and resulted in aldehyde/ketone in excellent-to-good yield. Each catalytic process is marginally more efficient with 1 than its counterparts.

Application of Resin-Bound Reagents for the Synthesis of Benzimidazole Derivatives

In an efficient exploitation of solid-phase chemistry, this work describes the preparation of the new resin-bound 4-( N-arylamino)-3-aminobenzoates and their successful cyclocondensations with the sodium bisulfite adducts of different aldehydes and triethyl orthoformate or triethyl orthoacetate, followed by resin cleavage to produce the target 1,2-diaryl- and 1-aryl-2-(un)substituted benzimidazole-5-carboxylic acid derivatives, respectively. In another variation, the cyclocondensation of the resin-bound 4-( N-arylamino)-3-aminobenzoates with carbon disulfide afforded the corresponding resin-bound thioxobenzimidazoles, which upon cleavage gave the corresponding 2-thioxobenzimidazoles. S-Alkylation of the resin-bound thioxobenzimidazoles with benzyl chloride and 4-bromophenacyl bromide furnished 2-benzylsulfanyl benzimidazoles and 4-bromophenyl-2-oxoethylsulfanyl-benzimidazoles, respectively. Reacting the resin-bound 4-( N-arylamino)-3-aminobenzoates with phenyl isothiocyanate gave the open-form 4-(arylamino)-3-(3-phenylthioureido) benzoic acids. Moreover, the reaction of the resin-bound 4-( N-arylamino)-3-aminobenzoates with N,N-carbonyldiimidazole yielded the corresponding 2-bezimidazolones.

Efficient, Rapid and One Pot Synthesis of 2-substituted Benzimidazoles Using the NaOH/I2 System as an Oxidant Under Mild Conditions

A one-pot efficient synthesis of benzimidazole derivatives has been achieved from aryl aldehydes and o-phenylenediamine in the presence of the NaOH/I2 system at room temperature in acetonitrile. The simplicity of the procedure and work-up, larger scale synthesis, high yields and short reaction times are the advantages of this method.

4-Chloro-2-fluoro-5-nitrobenzoic acid as a possible building block for solid-phase synthesis of various heterocyclic scaffolds

4-Chloro-2-fluoro-5-nitrobenzoic acid is a commercially available multireactive building block that can serve as a starting material in heterocyclic oriented synthesis (HOS) leading to various condensed nitrogenous cycles. This work describes its ability for the preparation of substituted nitrogenous heterocycles having 5-7-membered cycles via polymer-supported o-phenylendiamines. Immobilization of this compound on Rink resin followed by further chlorine substitution, reduction of a nitro group and appropriate cyclization afforded benzimidazoles, benzotriazoles, quinoxalinones, benzodiazepinediones and succinimides. The method developed is suitable for the synthesis of diverse libraries including the mentioned types of heterocycles, which have significant importance in current drug discovery. In this paper, we also report limitation of these method and unsuccessful attempt to prepare an 8-membered benzodiazocine cycle.

Synthesis and biological evaluation of analogues of AKT (protein kinase B) inhibitor-IV

Inhibitors of the PI3-kinase/AKT (protein kinase B) pathway are under investigation as anticancer and antiviral agents. The benzimidazole derivative AKT inhibitor-IV (ChemBridge 5233705) affects this pathway and exhibits potent anticancer and antiviral activity. To probe its biological activity, we synthesized AKT inhibitor-IV and 21 analogues using a novel six-step route based on ZrCl(4)-catalyzed cyclization of 1,2-arylenediamines with α,β-unsaturated aldehydes. We examined effects on viability of HeLa carcinoma cells, viability of normal human cells (NHBE), replication of recombinant parainfluenza virus 5 (PIV5) in HeLa cells, and replication of the intracellular bacterium Mycobacterium fortuitum in HeLa cells. Replacement of the benzimidazole N-ethyl substitutent of AKT inhibitor-IV with N-hexyl and N-dodecyl groups enhanced antiviral activity and cytotoxicity against the cancer cell line, but these compounds showed substantially lower toxicity (from 6-fold to >20-fold) against NHBE cells and no effect on M. fortuitum, suggesting inhibition of one or more host protein(s) required for proliferation of cancer cells and PIV5. The key structural elements identified here may facilitate identification of targets of this highly biologically active scaffold.

Ethyl 4-(2-hydroxy-ethyl-amino)-3-nitro-benzoate

In the title compound, C(11)H(14)N(2)O(5), the mol-ecular structure is stabilized by an intra-molecular N-H⋯O hydrogen bond, which generates an S(6) ring motif. The nitro group is twisted slightly from the attached benzene ring, forming a dihedral angle of 5.2 (2)°. In the crystal packing, inter-molecular O-H⋯O and C-H⋯O hydrogen bonds link the mol-ecules into a three-dimensional network. The crystal studied was a non-merohedral twin, the refined ratio of the twin components being 0.264 (2):0.736 (2).

Acetic acid-promoted condensation of o-phenylenediamine with aldehydes into 2-aryl-1-(arylmethyl)-1H-benzimidazoles under microwave irradiation

An efficient and simple procedure was developed for the green synthesis of various 2-aryl-1-(arylmethyl)-1H-benzimidazoles in high yields by acetic acid-promoted condensation of o-phenylenediamine with aldehydes in air under microwave irradiation and transition metal catalystfree conditions.

Solid phase synthesis of peptides containing novel amino acids, substituted 3-benzimidazolealanines

A direct solid-phase synthesis of a series of substituted benzimidazole-containing peptides is described. The method involves on-resin formation of new amino acids containing benzimidazole derivatives in the side chain. The heterocycle conjugates were obtained by reaction between aldehydes and peptides containing beta-(3,4-diaminophenyl)alanine residue, immobilized on a polymeric solid support.

Structure-activity relationships of melanocortin agonists containing the benzimidazole scaffold

The melanocortin system has been implicated in regulating various physiological processes including pigmentation, energy homeostasis, obesity, steroidogenesis cardiovascular, and exocrine gland function. The five melanocortin receptors that belong to the super family of G protein-coupled receptors are stimulated by naturally occurring agonists. The aim of this research was focused on the design, synthesis, and pharmacological characterization of melanocortin ligands that contain the 1,2,5-trisubstituted benzimidazole scaffold. A series of benzimidazole analogues, with three points of diversity at positions 1, 2, and 5, were designed, synthesized, pharmacologically assayed at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R and resulted in ligands possessing a range of agonist activity from nm to no stimulation at up to 100 microM concentrations. This study demonstrates that the benzimidazole structure template can be appended with key melanocortin agonist amino acids for the design melanocortin receptor agonist ligands.

Novel Intramolecular Rearrangement Leading to the Synthesis of Biheterocyclic Indole−Benzoimidazole Derivatives on Solid Phase

[reaction: see text] A novel intramolecular SNAr rearrangement observed during the S-alkylation of benzoimidazole-2-thione with alpha-haloacetophenone is reported. The rearrangement led to the formation of a new benzoimidazole-based intermediate, which is further utilized for the generation of a new biheterocyclic indole-benzimidazole derivatives with a two-point diversity.

Synthesis and reactions of 7-fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid: a novel scaffold for combinatorial synthesis of coumarins

7-Fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid has been prepared as a novel scaffold for combinatorial synthesis of coumarins. The scaffold has three points of diversity. The optimal conditions for its reactions with different nucleophiles in solid phase were obtained. Sixteen coumarin derivatives with different structures were designed and synthesized in solid phase to demonstrate its application as a scaffold for combinatorial synthesis of coumarins. Thirteen of these derivatives were obtained in high yields. Many of these model compounds fluoresce. Combinatorial libraries constructed with this novel scaffold may have interesting biological or physical properties.

Non-nucleoside inhibitors of the hepatitis C virus NS5B polymerase: discovery and preliminary SAR of benzimidazole derivatives

Benzimidazole 5-carboxamide derivatives from a combinatorial screening library were discovered as specific inhibitors of the NS5B polymerase of the hepatitis C virus (HCV). Initial hit-to-lead activities taking advantage of high-throughput parallel synthetic techniques, identified a 1,2-disubstituted benzimidazole 5-carboxylic acid scaffold as the minimum core for biological activity. Potent analogues in this series inhibit the polymerase at low micromolar concentrations and provide an attractive "drug-like" lead structure for further optimization and the development of potential HCV therapeutics.

A novel and rapid encoding method based on mass spectrometry for "one-bead-one-compound" small molecule combinatorial libraries

A novel and efficient encoding method based on mass spectrometry for "one-bead-one-compound" small molecule combinatorial libraries has been developed. The topologically segregated bifunctional resin beads with orthogonal protecting groups in the outer and inner regions are first prepared according to our previously published procedure. Prior to library synthesis, the inner core of each bead is derivatized with 3-4 different coding blocks on a cleavable linker. Each functional group on the scaffold is encoded by an individual coding block containing a functional group with the same chemical reactivity. During the library synthesis, the same chemical reactions take place on the scaffold (outer layer of the bead) and coding blocks (inner core of the bead) concurrently. After screening, the coding tags in the positive beads are released, followed by molecular mass determination using matrix-assisted laser desorption ionization Fourier transform mass spectrometry. The chemical structure of library compounds can be readily identified according to the molecular masses of the coding tags. The feasibility and efficiency of this approach were demonstrated by the synthesis and screening of a model small molecule library containing 84 672 member compounds, with a model receptor, streptavidin. Streptavidin binding ligands with structural similarity (17) were identified. The decoding results were clear and unambiguous.

New efficient route for solid-phase synthesis of benzimidazole derivatives

A simple and efficient method for the solid-phase synthesis of benzimidazole libraries is described. Monoalkylation of various o-phenylenediamines on resin-bound bromoacetamide proceeded smoothly to give the monoalkyl resin-bound o-phenylenediamines in high yields. Subsequent cyclization of the diamines with various aldehydes afforded solid-supported benzimidazoles. Cleavage from the resin gave benzimidazoles in good yields. The present method enabled the introduction of the diversity on the benzene ring of imidazoles. Azabenzimidazoles, such as 4-azabenzimidazoles, 5-azabenzimidazoles, and purines, were also synthesized in good yields with high purities by the same procedure.

The recent impact of solid-phase synthesis on medicinally relevant benzoannelated nitrogen heterocycles

Benzoannelated heterocycles such as benzodiazepines and indoles can be prepared efficiently through cyclization on solid supports, although no single approach is currently universal for the preparation of all benzoannelated N-heterocycle chemistries. In this review, a number of synthetic strategies for the generation of benzoannelated nitrogen heterocycles using resin-bound substrates have been described. Classical heterocycle forming reactions such as the Fischer indole, the Bischler-Napieralski tetrahydroisoquinoline, the Pictet-Spengler tetrahydro-beta-carboline, the Tsuge, the Nenitzescu and the Richter cinnoline reaction are presented. In addition, the Heck, Sonogashira, Wittig, Diels-Alder, and olefin metathesis reactions have been also used. Multicomponent reactions such as the Grieco three-component assembly have been exploited for the synthesis of heterocycles. Cyclative cleavage from the solid support is particularly suitable for the synthesis of heterocycles while particular emphasis has been focused on the synthesis of libraries and the use of combinatorial chemistry techniques. In addition, the most relevant pharmacological properties of benzoannelated nitrogen heterocycles are included.

Aromatic amination/imination approach to chiral benzimidazoles

The powerful Buchwald-Hartwig amination was utilized for the construction of the benzimidazole nucleus with the substituted nitrogen atom bearing a chiral substituent. A successive amination/imination was followed by an acid-catalyzed ring closure step to give the benzimidazole ring. The products were deprotonated and acylated at the C2 position and could be alkylated on nitrogen to give chiral benzimidazolium salts.

Novel strategies for solid-phase construction of small-molecule combinatorial libraries

During the past decade we witnessed a rapid advance in the new field of chemical science, combinatorial chemistry. The pharmaceutical industries invested heavily in accelerating the development of this new technology. As a result, it has become an extremely important tool in lead identification and optimization in current pharmaceutical research. It also quickly crossed the boundaries of the original chemical discipline and demonstrated great potential in many other important areas, such as searching for novel and highly efficient catalysts and superconductive material. Researchers from both academic and industrial laboratories have directed great effort towards the development of novel strategies for combinatorial synthesis.

Traceless solid-phase synthesis of substituted benzimidazoles via a base-cleavable linker

[figure: see text] A solid-phase route to substituted benzimidazoles has been developed using a modified base-labile linker strategy to release the final products in a traceless manner. This approach permits the synthesis of diverse compounds in moderate yields and high purity.

Solid-Phase Traceless Synthesis of Selected Nitrogen-Containing Heterocyclic Compounds. The Encore Technique for Directed Sorting of Modular Solid Support

The acid lability of electron-rich N-benzylanilines has been exploited in a linker for the traceless solid-phase synthesis of benzimidazoles, 2-aminobenzimidazoles, quinoxalinones and tetrahydroquinoxalines. The target compound precursors were assembled on a solid-phase support derivatized with either a benzylamine or a benzhydrylamine linker. Exposure to an acidic reagent caused cleavage of the C(benzyl)-N(aniline) bond, releasing the product with only a hydrogen atom on the descending nitrogen. The Encore technique for directed sorting on SynPhase Lanterns has been developed and applied to combinatorial synthesis of generic drug discovery libraries.

Solid phase synthesis of purines from pyrimidines

In this paper the solid phase synthesis of various substituted purines is described starting from 4,6-dichloro-5-nitropyrimidine. The 4,6-dichloro-5-nitropyrimidine was coupled to Rink amide resin followed by displacement of the second chloride by an amino compound. Reduction of the nitro compound proved to be problematic but was achieved using lithium aluminum hydride/aluminum trichloride. The diamines (13) were then elaborated to purines by three different routes.

Traceless synthesis of benzimidazoles on solid support

Traceless solid-phase syntheses of benzimidazoles and 5-(benzimidazol-2-yl)benzimidazoles on 2-(4-formyl-3-methoxyphenoxy)ethyl polystyrene are described. No auxiliary functional groups are left in the products after ultimate cleavage and cyclization.

Synthesis of new (benzimidazolyl)piperazines with affinity for the 5-HT1A receptor via Pd(0) amination of bromobenzimidazoles

The synthesis of a new family of (benzimidazolyl)piperazines has been developed through Pd(0) mediated amination of 4- and 6-bromobenzimidazole derivatives. Preliminary studies showed that some of these compounds are potent 5-HT1A receptor ligands.