Structure–activity relationships of arylbenzofuran H3 receptor antagonists

Palladium-catalyzed Tsuji-Trost-type reaction of benzofuran-2-ylmethyl acetates with nucleophiles

The palladium-catalyzed benzylic-like nucleophilic substitution of benzofuran-2-ylmethyl acetate with N, S, O and C soft nucleophiles has been investigated. The success of the reaction is dramatically influenced by the choice of catalytic system: with nitrogen based nucleophiles the reaction works well with Pd2(dba)3/dppf, while with sulfur, oxygen and carbo-nucleophiles [Pd(η3-C3H5)Cl]2/XPhos is more efficient. The regiochemical outcome shows that the nucleophilic substitution occurs only on the benzylic position of the η3-(benzofuryl)methyl complex. The high to excellent yields and the simplicity of the experimental procedure make this protocol a versatile synthetic tool for the preparation of 2-substituted benzo[b]furans.

QSAR Modeling of Histamine H3R Antagonists/inverse Agonists as Future Drugs for Neurodegenerative Diseases

Background

Histamine H₃ receptor (H₃R) is associated with several neuropsychological diseases, and thus it is an important target involved in several CNS disorders, such as narcolepsy, attention deficit hyperactivity disorder and schizophrenia. Since QSAR modeling is a feasible approach to explain the role of the molecular substituents in the biological activity, it can help in improving the design of better H₃R ligands for these conditions.

Methods

This article reviews papers previously published in literature to show the current status of the contribution from QSAR modeling to reach H₃R antagonists/inverse agonists.

Results

Classical and 3D-QSAR models were retrieved, showing that the steric and hydrophobic properties of the H₃R ligands are most important to reach good affinity.

Conclusion

Although QSAR methods are valuable to design better H₃R antagonists/inverse agonists, pharmacokinetics should also be considered in future models to ensure good CNS penetration.

Synthesis of 2-substituted 3-chlorobenzofurans via TMSCl-mediated nucleophilic annulation of isatin-derived propargylic alcohols

A TMSCl-mediated cascade annulation of isatin-derived propargylic alcohols for the synthesis of 2-substituted 3-chlorobenzofurans is now reported. Mechanistic investigations showed that this proceeded through a sequential Meyer-Schuster rearrangement/nucleophilic addition/intramolecular annulation. TMSCl not only acts as a promoter, but also acts as a chlorine source in this protocol.

Rapid regio- and multi-coupling reactivity of 2,3-dibromobenzofurans with atom-economic triarylbismuths under palladium catalysis

A regio- and chemoselective cross-coupling study using 2,3-dibromobenzofurans and 2,3,5-tribromobenzofuran was achieved with sub-stoichiometric loadings of triarylbismuths as atom-economic reagents under Pd-catalyzed conditions. As part of this study, various 2,3-diaryl- and 2,3,5-triarylbenzofuran products were obtained in high yields, involving one-pot operations and short reaction times.

Synthesis of multi-functionalized benzofurans through the condensation of ninhydrin and phenols using SSA as a recyclable heterogeneous acid catalyst

A simple and efficient one-pot methodology has been developed for the synthesis of biologically important multi-functionalized 3-(2[Formula: see text]-hydroxyaryl)-2-(2[Formula: see text]-carboxyphenyl)benzofurans using silica sulfuric acid (SSA) as a heterogeneous acid catalyst in DMF medium. The significant advantages of this methodology are the use of SSA as a recyclable solid acid catalyst, operational simplicity, easy availability of the starting materials, and good yield of the products with high atom-economy.

Molecular Mechanisms of Dielectrically Controlled Resolution (DCR)

It is widely believed that the chiral discrimination process is solely dependent on the stereochemistryof the relevant molecules. However, through systematic studies on several resolution systems with popularchiral selectors, we have discovered a new fact that triggers modification of this prevailing conceptof chiral resolution. The studies have demonstrated that one enantiomer of a chiral selector can recognizeboth enantiomers of a target molecule in different solvent systems with different dielectric constants.The phenomenon was termed dielectrically controlled resolution (DCR). Since DCR was observed in differentresolution systems and was not too specific to a particular system, DCR was expected to widely occurin various resolution systems. We have investigated the molecular mechanism underlying this interestingphenomenon based on X-ray analysis of the relevant diastereomeric salts. The disclosed mechanism clearlyindicates that a chiral selector can inherently recognize both enantiomers of a target moleculeand only the dielectric property of the solvent employed in the resolution process governs the selectionof the enantiomer.

A new approach to fused furan ring systems and benzofurans: Intramolecular cyclization reactions of unsaturated acyloxy sulfone derivatives

Unsaturated acyloxy sulfones 3 undergo intramolecular cyclization upon deprotonation with LHMDS in THF. Dehydration and double bond isomerization of the products upon exposure to acid, gave the fused ring furans, 4, in good yields. This strategy could be readily adapted to 12 from the cyclization reactions of acyloxy sulfones 11 prepared from phenols. Finally, this approach could be successfully modified to access dihydropyrans and benzopyrans.

FeCl(3)-Diorganyl dichalcogenides promoted cyclization of 2-alkynylanisoles to 3-chalcogen benzo[b]furans

A general synthesis of 3-chalcogen benzo[b]furans from the readily available 2-alkynylanisoles, via FeCl(3)/diorganyl dichalcogenides intramolecular cyclization, has been developed. Aryl and alkyl groups directly bonded to the chalcogen atom were used as cycling agents. The results revealed that the reaction significantly depends on the electronic effects of substituents in the aromatic ring bonded to the selenium atom of the diselenide species. We observed that the pathway of reaction was not sensitive to the nature of substituents in the aromatic ring of anisole since both the electron-donating and the electron-withdrawing groups delivered the products in similar yields. In addition, the obtained heterocycles were readily transformed to more complex products by using a chalcogen/lithium exchange reaction with n-BuLi followed by trapping of the lithium intermediate with aldehydes, furnishing the desired secondary alcohols in good yields.

Parallel synthesis of a multi-substituted benzo[b]furan library

The solution-phase parallel synthesis of a 121-member library of multi-substituted benzo[ b]furans is described. 2,3,5-Trisubstituted benzo[ b]furans have been prepared by the palladium-catalyzed substitution of 3-iodobenzofurans by Suzuki-Miyaura, carbonylative Suzuki, Sonogashira, Heck, and carboalkoxylation chemistry. The 3-iodobenzofurans are readily prepared in good to excellent yields by the palladium/copper-catalyzed cross-coupling of various o-iodoanisoles and terminal alkynes, followed by electrophilic cyclization with ICl.

In vitro SAR of pyrrolidine-containing histamine H3 receptor antagonists: trends across multiple chemical series

Structure-activity relationships (SAR) were analyzed within a library of diverse yet simple compounds prepared as histamine H3 antagonists. The libraries were constructed with a variety of low molecular weight pyrrolidines, selected from (R)-2-methylpyrrolidine, (S)-2-methylpyrrolidine, and pyrrolidine.

Molecular modeling of histamine H3 receptor and QSAR studies on arylbenzofuran derived H3 antagonists

Histamine H3 receptors are presynaptic autoreceptors found in both central and peripheral nervous systems of many species. The central effects of these receptors suggest a potential therapeutic role for their antagonists in treatment of several neurological disorders such as epilepsy, schizophrenia, Alzheimer's and Parkinson's diseases. The purpose of this study was to identify the structural requirements for H3 antagonistic activity via quantitative structure-activity relationship (QSAR) studies and receptor modeling/docking techniques. A combination of partial least squares (PLS) and genetic algorithm (GA) was used in the QSAR approach to select the structural descriptors relevant to the receptor binding affinity of a series of 58 H3 antagonists. The descriptors were selected out of a pool of >1000 descriptors calculated by DRAGON, Hyperchem and ACD labs suite of programs. The resulting QSAR models for rat and human H3 binding affinities were validated using different strategies. QSAR models generated in the current work suggested the role of charge transfer interactions in the ligand-receptor interaction verified using the molecular modeling of the receptor and docking two antagonists to the binding site. The 3D model of human H3 receptor was built based on bovine rhodopsin structure and evaluated by molecular dynamics (MD) simulation in a mixed water-vacuum-water environment. The results were indicative of the stability of the model relating the observed structural changes during the MD simulation to the suggested ligand-receptor interactions. The results of this investigation are expected to be useful in the process of design and development of new potent H3 receptor antagonists.

A Facile Approach to the Synthesis of Chiral 2-Substituted Benzofurans

An effective route to chiral optically active 2-substituted benzofurans directly from carboxylic acids is reported. This procedure, which allows the preparation of alpha-alkyl-2-benzofuranmethanamines from N-protected alpha-amino acids without sensible racemization phenomena, proceeds in good yields under mild conditions with the help of microwave irradiation.

Keynote review: histamine H3 receptor antagonists reach out for the clinic

Antagonists of the histamine H(1) and H(2) receptors have been successful as blockbuster drugs for treating allergic conditions and gastric ulcers, respectively. As such, histamine receptors have made a significant contribution to establishing G-Protein-coupled receptors as the favored drug targets of the industry. In this light, it can easily be understood that the discovery of a third histamine receptor subtype (H(3)R) in 1983 was greeted with considerable excitement. However, characterization of the H(3)R turned out to be far from trivial. In the past five years, molecular biology approaches have given fresh impetus to the H(3)R research field. As a result, H(3)R ligands are where they were anticipated to be 20 years ago: at the center of attention and on the verge of an anticipated breakthrough as the next generation of histaminergic blockbuster drugs. Here, we assess the status of the H(3)R medicinal chemistry programs of the various players in the field, as far as can be deduced from patent applications and scientific literature.

Synthesis of 2,3-Disubstituted Benzofurans by Platinum−Olefin-Catalyzed Carboalkoxylation of o-Alkynylphenyl Acetals

The PtCl2-catalyzed cyclization reaction of o-alkynylphenyl acetals 1 in the presence of 1,5-cyclooctadiene produces 3-(alpha-alkoxyalkyl)benzofurans 2 in good to high yields. For example, the reaction of acetaldehyde ethyl 2-(1-octynyl)phenyl acetal (1a), acetaldehyde ethyl 2-(cyclohexylethynyl)phenyl acetal (1c), and acetaldehyde ethyl 2-(phenylethynyl)phenyl acetal (1f) in the presence of 2 mol % of platinum(II) chloride and 8 mol % of 1,5-cyclooctadiene in toluene at 30 degrees C gave the corresponding 2,3-disubstituted benzofurans 2a, 2c, and 2f in 91, 94, and 88% yields, respectively.