Biological and medicinal significance of benzofuran

Enantioselective Catalytic [4+1]-Cyclization of ortho-Hydroxy-para-Quinone Methides with Allenoates

The first highly asymmetric catalytic synthesis of densely functionalized dihydrobenzofurans is reported, which starts from ortho-hydroxy-containing para-quinone methides. The reaction relies on an unprecedented formal [4+1]-annulation of these quinone methides with allenoates in the presence of a commercially available chiral phosphine catalyst. The chiral dihydrobenzofurans were obtained as single diastereomers in yields up to 90 % and with enantiomeric ratios up to 95:5.

Potassium tert-Butoxide-Mediated Condensation Cascade Reaction: Transition Metal-Free Synthesis of Multisubstituted Aryl Indoles and Benzofurans

An efficient and facile method to synthesize valuable disubstituted 2-aryl indoles and benzofurans in good yields has been demonstrated, based on a tert-butoxide-mediated condensation reaction involving a vinyl sulfoxide intermediate. Products are obtained from N- or O-benzyl benzaldehydes using dimethyl sulfoxide as a carbon source. The methodology features a wide functional group tolerance and transition metal-free environment. Preliminary mechanistic studies suggest that the reaction involves a tandem aldol reaction/Michael addition/dehydrosulfenylation/isomerization sequence through an ionic protocol.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4-oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α-silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels-Alder cycloaddition through a point-chirality, axial-chirality, point-chirality transfer process.

One-Pot Allylation-Intramolecular Vinylogous Michael Addition-Isomerization Cascade of o-Hydroxycinnamates and Congeners: Synthesis of Substituted Benzofuran Derivatives

A unique intramolecular vinylogous Michael addition leading to the synthesis of heterocycles has been disclosed. Base-promoted one-pot sequential O-allylation of o-hydroxy-cinnamates or -cinnamonitrile or -chalcones with γ-bromocrotonates followed by an intramolecular conjugate addition of vinylogous Michael donors resulted in the formation of highly substituted benzofuran derivatives in good to excellent yields. The intramolecular event followed by two [1,3]-H shifts leading to aromatization appears to be the key to the success of this unprecedented transformation.

Synthesis of Benzofuran Derivatives through Cascade Radical Cyclization/Intermolecular Coupling of 2-Azaallyls

Benzofurans are among the most popular structural units in bioactive natural products, however, the synthesis of such structures by radical cyclization cascade reactions is rare. Herein, we report a mild and broadly applicable method for the construction of complex benzofurylethylamine derivatives through a unique radical cyclization cascade mechanism. Single-electron transfer (SET) from 2-azaallyl anions to 2-iodo aryl allenyl ethers initiates a radical cyclization that is followed by intermolecular radical-radical coupling. This expedient approach enables the synthesis of a range of polycyclic benzofurans that would otherwise be difficult to prepare.

A Natural Benzofuran from the Patagonic Aleurodiscus vitellinus Fungus has Potent Neuroprotective Properties on a Cellular Model of Amyloid-β Peptide Toxicity

Alzheimer's disease (AD) is characterized by amyloid plaques that form due to an increase in amyloid-β peptide (Aβ) aggregation. One strategy in the search of new treatments for AD focuses on compounds that decrease Aβ accumulation. Compounds containing a benzofuran ring have been described to play an important role in decreasing Aβ-induced toxicity; however, only synthetic benzofurans have been tested thus far. The aim of the present study was to examine the in vitro neuroprotective properties of fomannoxin (Fx), a natural benzofuran isolated from cultures of the Andean-Patagonian fungi Aleurodiscus vitellinus, and evaluate its effect on Aβ peptide. We tested the effect of Fx at a wide concentration range (10-11-10-4 M) in PC-12 cells, and found the compound did not alter cellular viability. Fx also showed a concentration-dependent effect on the Aβ-induced toxicity in PC12 cells, showing viability above 100% at 10-6 M. We then measured the effect of Fx (10-7-10-5 M) on the frequency of cytosolic Ca2+ transients in rat hippocampal neurons at both acute and chronic (24 h) times. Acute incubation with Fx increased the frequency of cytosolic Ca2+ transients to values around 200%, whereas chronic incubation with Fx increased the frequency of Ca2+ transients. Finally, the Aβ-induced decrease in intracellular Ca2+ transients was prevented when Fx (10-6 M) was co-incubated with Aβ (5×10-6 M). The results suggest a potent neuroprotective effect of this naturally occurring benzofuran against Aβ peptide toxicity that could be mediated by an interference with it binding to plasma membrane, and lead Fx as new chemical entity to develop pharmacological tools against Aβ peptide neurotoxicity.

Chiral-at-metal rhodium(iii) complex catalyzed enantioselective synthesis of C2-substituted benzofuran derivatives

An enantioselective C2-nucleophilic functionalization of 3-aminobenzofurans has been realized under catalysis of chiral rhodium(iii) complexes, affording a large array of C2-substituted benzofuran derivatives in high yields and enantioselectivities.

Iron-catalyzed reductive cyclization reaction of 1,6-enynes for the synthesis of 3-acylbenzofurans and thiophenes

A facile synthesis of 3-acylbenzofurans and thiophenes via iron(ii)-catalyzed reductive cyclization of 1,6-enynes has been developed.

Synthesis of hybrid polycycles containing fused hydroxy benzofuran and 1H-indazoles via a domino cyclization reaction

A trifluoroacetic acid mediated [3+2] annulation reaction between 5-hydroxy-1H-indazoles – generated in situ – and p-benzoquinones has been reported.

Synthesis and fungicidal activities of 2-{[(2-(1H-1,2,4-triazol-1-yl)-ethylidene)amino]oxy}alkanamides containing dihydrobenzofuran

In order to find new compounds with high fungicidal activity, acetamide derivatives 4a–x were rationally designed, synthesized, characterized and tested against various fungi in vivo. The bioassay results indicate that compounds 4k,m,o,r exhibit an 80% inhibition rate against Rhizoctonia solani at 500 mg/L, and compound 4j shows an 80% inhibition rate against Blumeria graminis at 500 mg/L. Therefore, compounds of 4 are promising fungicidal candidates worthy of further development.

Cyclopropylmethyl Protection of Phenols: Total Synthesis of the Resveratrol Dimers Anigopreissin A and Resveratrol-Piceatannol Hybrid

We demonstrate the versatile use of the cyclopropylmethyl group to protect phenols through the total synthesis of two benzofuran-based natural products, that is, anigopreissin A and the resveratrol-piceatannol hybrid. This protecting group is a good alternative to the conventional methyl group, owing to the feasibility of introduction, stability under a variety of conditions, and its relative ease of removal under different acidic conditions.

Benzofuran derivatives and their anti-tubercular, anti-bacterial activities

Benzofuran is a fundamental structural unit in a variety of biologically active natural products, and its derivatives display various biological properties. Some benzofuran derivatives possess unique anti-tubercular and anti-bacterial action mechanism, and exhibit excellent in vitro and in vivo activities against both drug-sensitive and drug-resistant pathogens. Moreover, several benzofuran derivatives have already used in clinics for the treatment of various diseases. Thus, benzofuran is a useful pharmacophore to develop new anti-tubercular and anti-bacterial drugs. This review covers the recent advances of benzofuran derivatives as potential anti-tubercular and anti-bacterial agents, and the structure-activity relationship is also discussed to pave the way for the further rational development of this kind of derivatives.

Benzofuran-appended 4-aminoquinazoline hybrids as epidermal growth factor receptor tyrosine kinase inhibitors: synthesis, biological evaluation and molecular docking studies

A series of 2-arylbenzo[b]furan–appended 4-aminoquinazoline hybrids were prepared and evaluated for cytotoxicity in vitro against the human lung cancer (A549), colorectal adenocarcinoma (Caco-2), hepatocellular carcinoma (C3A) and cervical cancer (HeLa) cell lines. Compounds 10d and 10j exhibited significant cytotoxicity against the C3A and Caco-2 cell lines and induced apoptosis in these cell lines. Likewise, compounds 10d and 10e exhibited significant inhibitory activity towards epidermal growth factor receptor-tyrosine kinase phosphorylation (IC₅₀ values of 29.3 nM and 31.1 nM, respectively) against Gefitinib (IC₅₀ = 33.1 nM). Molecular docking of compounds 10 into EGFR-TK active site suggests that they bind to the region of EGFR like Gefitinib does.

A phenotypic small-molecule screen identifies halogenated salicylanilides as inhibitors of fungal morphogenesis, biofilm formation and host cell invasion

A poorly exploited paradigm in the antimicrobial therapy field is to target virulence traits for drug development. In contrast to target-focused approaches, antivirulence phenotypic screens enable identification of bioactive molecules that induce a desirable biological readout without making a priori assumption about the cellular target. Here, we screened a chemical library of 678 small molecules against the invasive hyphal growth of the human opportunistic yeast Candida albicans. We found that a halogenated salicylanilide (N1-(3,5-dichlorophenyl)-5-chloro-2-hydroxybenzamide) and one of its analogs, Niclosamide, an FDA-approved anthelmintic in humans, exhibited both antifilamentation and antibiofilm activities against C. albicans and the multi-resistant yeast C. auris. The antivirulence activity of halogenated salicylanilides were also expanded to C. albicans resistant strains with different resistance mechanisms. We also found that Niclosamide protected the intestinal epithelial cells against invasion by C. albicans. Transcriptional profiling of C. albicans challenged with Niclosamide exhibited a signature that is characteristic of the mitochondria-to-nucleus retrograde response. Our chemogenomic analysis showed that halogenated salicylanilides compromise the potential-dependant mitochondrial protein translocon machinery. Given the fact that the safety of Niclosamide is well established in humans, this molecule could represent the first clinically approved antivirulence agent against a pathogenic fungus.

From Oxiranes to Oligomers: Architectures of U.S. FDA Approved Pharmaceuticals Containing Oxygen Heterocycles

Oxygen heterocycles are the second most common type of heterocycles that appear as structural components of U.S. Food and Drug Administration (FDA) approved pharmaceuticals. Analysis of our database of drugs approved through 2017 reveals 311 distinct pharmaceuticals containing at least one oxygen heterocycle. Most prevalent among these are pyranoses, with furanoses, macrolactones, morpholines, and dioxolanes rounding off the top five. The main body of this Perspective is organized according to ring size, commencing with three- and four-membered rings and ending with macrocycles, polymers, and unusual oxygen-containing heterocycles. For each section, all oxygen heterocycle-containing drugs are presented along with a brief discussion about structural and drug application patterns.

Sodium halides as the source of electrophilic halogens in green synthesis of 3-halo- and 3,n-dihalobenzo[b]thiophenes

A convenient methodology for the synthesis of mono- and di-halogenated benzo[b]thiophenes is described herein, which utilizes copper(II) sulfate pentahydrate and various sodium halides in the presence of substituted 2-alkynylthioanisoles. The proposed method is facile, uses ethanol as a green solvent, and results in uniquely substituted benzo[b]thiophene structures with isolated yields up to 96%. The most useful component of this methodology is the selective introduction of bromine atoms at every available position (2-7) around the benzo[b]thiophene ring, while keeping position 3 occupied by a specific halogen atom such as Cl, Br or I. Aromatic halogens are useful reactive handles; therefore, the selective introduction of halogens at specific positions would be valuable in the targeted synthesis of bioactive molecules and complex organic materials via metal-catalyzed cross coupling reactions. This work is a novel approach towards the synthesis of dihalo substituted benzo[b]thiophene core structures, which provides a superior alternative to the current methods discussed herein.

Establishment and Phytochemical Analysis of a Callus Culture from Ageratina pichinchensis (Asteraceae) and Its Anti-Inflammatory Activity

A protocol was established to produce bioactive compounds in a callus culture of Ageratina pichinchensis by using 1 mg L⁻¹ NAA with 0.1 mg L⁻¹ KIN. The phytochemical study of the EtOAc extract obtained from the callus biomass, allowed the isolation and characterization of eleven secondary metabolites, of which dihydrobenzofuran (5) and 3-epilupeol (7), showed important anti-inflammatory activity. Compound 5 inhibits in vitro the secretion of NO (IC₅₀ = 36.96 ± 1.06 μM), IL-6 (IC₅₀ = 73.71 ± 3.21 μM), and TNF-α (IC₅₀ = 73.20 ± 5.99 μM) in RAW (Murine macrophage cells) 264.7 macrophages, as well as the activation of NF-κB (40% at 150 μM) in RAW-blue macrophages, while compound 7 has been described that inhibit the in vivo TPA-induced ear edema, and the in vitro production of NO, and the PLA2 enzyme activity. In addition, quantitative GC-MS analysis showed that the anti-inflammatory metabolites 5 and 7 were not detected in the wild plant. Overall, our results indicated that A. pichinchensis can be used as an alternative biotechnological resource for obtaining anti-inflammatory compounds. This is the first report of the anti-inflammatory activity of compound 5 and its production in a callus culture of A. pichinchensis.

An elimination/Heck coupling/allylation cascade reaction: synthesis of 2,3-dihydrobenzofurans from allenoate adducts

A highly regio- and stereoselective Pd-catalyzed cascade reaction of allenoate adducts has been developed. Various allenoate adducts of phosphine-catalyzed addition of allenoates are found to be efficient substrates for the preparation of 2,3-dihydrobenzofuran. It is the first example of allenoate adducts used as ideal and practical precursors of hard-to-get functionalized 1,3-butadienes for heterocycle synthesis.

Vinylogous Elimination/Heck Coupling/Allylation Domino Reactions: Access to 2-Substituted 2,3-Dihydrobenzofurans and Indolines

A highly regio- and stereoselctive palladium-catalyzed domino reaction of functionalized aryl allyl ethers has been developed. Various aryl allyl ethers derived from the phosphine-catalyzed addition of electron-deficient allenes with phenol are found to be efficient substrates for the synthesis of 2-substituted 2,3-dihydrobenzofurans and indolines. It is the first example of aryl allyl ether used as an ideal and practical precursor of hard to get functionalized 1,3-butadiene for the heterocyclic compound synthesis.

Green synthesis and antioxidant activity of novel series of benzofurans from euparin extracted of Petasites hybridus

A novel class of benzofuran derivatives is prepared from the isocyanide-based MCR, euparin and aldehydes in the presence of ZnO-nanorods as a catalyst in excellent yields at room temperature under solvent-free conditions as a green reaction medium. Also, the antioxidant activities of some synthesised compounds such as 4a, 4b, 10a and 10b were evaluated by DPPH radical scavenging and ferric reduction activity potential (FRAP) assays. Compound 10b, was shown moderate radical scavenging activity and very good reducing activity compared to standards (BHT and TBHQ).

Synthesis of 2-arylbenzofuran-3-carbaldehydes via an organocatalytic [3+2] annulation/oxidative aromatization reaction

A novel organocatalytic [3+2] annulation/oxidative aromatization reaction of enals with 2-halophenols is developed for the synthesis of 2-arylbenzofuran-3-carbaldehydes.

Recyclable Pd/CuFe2O4 nanowires: a highly active catalyst for C–C couplings and synthesis of benzofuran derivatives

Pd/CuFe₂O₄ nanowire-catalyzed cross coupling transformations are described. Notably, these reactions showed excellent functional group tolerance. Further, the protocol is applied to a one-pot synthesis of benzofurans via a Sonogashira coupling and intramolecular etherification sequence. The catalyst was reused and found to maintain its activity and stability.

Efficient heterogeneous Pd/CuFe₂O₄ nanowires which catalyze cross coupling transformations are described. The protocol is applied to a one-pot synthesis of benzofurans via Sonogashira coupling and an intramolecular etherification sequence.

1,6-Conjugated addition-mediated [4 + 1] annulation: an approach to 2,3-dihydrobenzofurans

2,3-Dihydrobenzofurans were achieved via a 1,6-conjugated addition-mediated [4 + 1] annulation of ortho-hydroxyphenyl-substituted para-quinone methides in high yields with high diastereoselectivity.

Synthesis of trans-disubstituted-2,3-dihydrobenzofurans by a formal [4 + 1] annulation between para-quinone methides and sulfonium salts

An efficient protocol for the synthesis of trans-disubstituted-2,3-dihydrobenzofurans through [4 + 1] annulation of para-quinone methides with sulfonium salts has been developed.

Radical scavenging potency of 2-(benzofuran-2-yl)-2-oxoethyl 3-(methyl/amino)benzoate: synthesis, crystal structure and Hirshfeld surface analysis

Two novel 2-(benzofuran-2-yl)-2-oxoethyl 3-(methyl/amino)benzoates 2(a–b), were synthesized under mild conditions and characterized by spectroscopic analysis. The three-dimensional (3D) crystal structures of these compounds were further determined using single crystal X-ray diffraction technique and revealed that both compounds tend to twist away with respect to their attached carbonyl groups at the C(=O)–C–O–C(=O) connecting bridge, exhibiting a nearly perpendicular conformation with torsion angle in a range of 75–94°. In both compounds, each of the benzofuran and substituted phenyl rings are individually planar while almost perpendicular to each other. Their molecular conformation are comparable with related structures from the Cambridge Structural Database (CSD). Furthermore, the intermolecular interactions in these crystal structures were quantified and analyzed using Hirshfeld surfaces computational method. The quantitative data on the percentage contributions of overall interactions on the molecules 2a and 2b (A & B) is calculated by the 2-D fingerprint plots from the HS analysis. These structures were evaluated for their antioxidant properties by diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, ferrous ion chelating (FIC) and hydrogen peroxide radical scavenging assays (H2O2). Noteworthy, compound 2b with an amino substituent attached to the phenyl ring exhibited an IC50 value of 45.37 μg/mL in H2O2 scavenging assay, which showed better scavenging effect than the standard drug, ascorbic acid (IC50=81.02 μg/mL).

Two closely related 2-(benzo-furan-2-yl)-2-oxoethyl benzoates: structural differences and C-H⋯O hydrogen-bonded supra-molecular assemblies

The compounds 2-(1-benzo-furan-2-yl)-2-oxoethyl 2-nitro-benzoate, C17H11NO6 (I), and 2-(1-benzo-furan-2-yl)-2-oxoethyl 2-amino-benzoate, C17H13NO4 (II), were synthesized under mild conditions. Their mol-ecular structures were characterized by both spectroscopic and single-crystal X-ray diffraction analysis. The mol-ecular conformations of both title compounds are generally similar. However, different ortho-substituted moieties at the phenyl ring of the two compounds cause deviations in the torsion angles between the carbonyl group and the attached phenyl ring. In compound (I), the ortho-nitro-phenyl ring is twisted away from the adjacent carbonyl group whereas in compound (II), the ortho-amino-phenyl ring is almost co-planar with the carbonyl group. In the crystal of compound (I), two C-H⋯O hydrogen bonds link the mol-ecules into chains propagating along the c-axis direction and the chains are inter-digitated, forming sheets parallel to [20-1]. Conversely, pairs of N-H⋯O hydrogen bonds in compound (II) link inversion-related mol-ecules into dimers, which are further extended by C-H⋯O hydrogen bonds into dimer chains. These chains are inter-connected by π-π inter-actions involving the furan rings, forming sheets parallel to the ac plane.

Diversity-Oriented Synthesis of Libraries Based on Benzofuran and 2,3-Dihydrobenzofuran Scaffolds

Benzofuran and 2,3-dihydrobenzofuran scaffolds are core components in a large number of biologically active natural and synthetic compounds including approved drugs. Herein, we report efficient synthetic protocols for preparation of libraries based on 3-carboxy 2-aryl benzofuran and 3-carboxy 2-aryl trans-2,3-dihydrobenzofuran scaffolds using commercially available salicylaldehydes, aryl boronic acids or halides and primary or secondary amines. The building blocks were selected to achieve variation in physicochemical properties and statistical molecular design and subsequent synthesis resulted in 54 lead-like compounds with molecular weights of 299-421 and calculated octanol/water partition coefficients of 1.9-4.7.