DABCO-Promoted Efficient and Convenient Synthesis of Benzofurans

An efficient and convenient synthesis of benzofurans has been described from phenacyl halides and o-hydroxy benzaldehyde in the presence of DABCO. The procedure is applicable for a variety of phenacyl halides and provide a variety of benzofurans. DABCO act as a base and as well as nucleophile.     

BCl3-promoted synthesis of benzofurans

Lewis acidic nature of boron trichloride (BCl₃) to coordinate to the carbonyl functionality was exploited for the synthesis of benzofurans via dehydrative cyclization. This mild and efficient procedure allowed for facile access to a number of highly substituted benzofurans in a regioselective manner. The structural requirement for the successful cyclodehydration was examined in the cases, where competitive demethylation could occur.     

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.     

Preparation of functional benzofurans, benzothiophenes, and indoles using ester, thioester, and amide via intramolecular Wittig reactions

Preparation of new types of highly functional benzofurans, benzothiophenes, and indoles is realized via intramolecular Wittig reactions with the corresponding ester, thioester, and amide functionalities. The key intermediates, phosphorus ylides, presumably result from the addition of Bu(3)P toward aldehydes followed by acylation and deprotonation. Synthesis of functional benzofurans directly starting from salicylic aldehyde derivatives with acid chlorides in a one-step procedure is also developed.     

Direct Assembly of 3,4‐Difunctionalized Benzofurans and Polycyclic Benzofurans by Phenol Dearomatization and Palladium‐Catalyzed Domino Reaction

A method to directly convert 2‐alkynylphenols to 3,4‐difunctionalized benzofurans and polycyclic benzofurans was developed. This protocol involves a hypervalent‐iodine‐mediated oxidative dearomatization to break the aromaticity of 2‐alkynylphenols, and a palladium‐catalyzed domino reaction to install two functional groups at the C3 and the C4 positions and restore the aromaticity of benzofurans.     

The first total synthesis of Cicerfuran utilizing a one-pot synthesis of hydroxylated benzofurans

A simple one-pot procedure was elaborated for the preparation of hydroxylated benzofurans from halogenated phenols and was successfully applied to the first total synthesis of Cicerfuran, a natural defence agent of wild chickpea.     

Benzofuran trimers for organic electroluminescence

Four linear benzofuran trimers have been prepared by a two-stage synthetic procedure. They were tested as materials for organic electroluminescence (OEL). Precursor phenylene ethynylene oligomers were formed in the first stage, then after removal of the phenolic hydroxyl protecting groups, a base was used to promote the cyclization of ortho-hydroxy phenylene ethynylenes to benzofurans. Both acetate esters and tert-butyl carbonates were employed as protecting groups. tert-Butyl and n-hexyl substituents on the benzofurans were used to modulate solubility, aggregation, and film-forming properties; two tert-butyl groups prevented aggregation in the solid state, thus maintaining emission in the blue region of the visible spectrum. The OEL characteristics of the tert-butyl-substituted benzofuran trimer were explored, and blue emission was observed. The two-stage synthetic procedure employed for the preparation of these benzofuran trimers may be applied to a wide variety of benzofuran oligomer and polymer targets.     

Palladium-catalyzed direct arylation of polysubstituted benzofurans

An efficient access to 2-substituted 3-arylbenzofurans through a palladium-catalyzed C3 direct arylation of 2-substituted benzofurans with aryl bromides is described. The scope and limitation of this reaction was studied. The method tolerates a variety of functional groups on the aryl halide and has been successfully extended to polysubstituted benzofurans to obtain the corresponding 3-arylbenzofurans with good to excellent yields.     

Palladium-catalyzed oxidative cyclization of 3-phenoxyacrylates: an approach to construct substituted benzofurans from phenols

In this paper, a novel and applicable synthesis of benzofurans from commercially available phenols and propiolate through the direct oxidative cyclization has been developed. In the presence of Pd(OAc)(2)/PPh(3) and CF(3)CO(2)Ag, (E)-type 3-phenoxyacrylates underwent reaction smoothly to generate the corresponding benzofurans in good yields in benzene at 110 °C under the air pressure. In addition, this transformation of phenols into benzofurans can also be carried out in one pot. The process was simple and efficient. A tentative mechanism of palladium-catalyzed oxidative cyclization of 3-phenoxyacrylates was proposed.     

Novel Route to 2-Trifluoromethylated Benzofurans

A novel route for the synthesis of a variety of 2-trifluoromethylbenzofurans is reported. By selection of solvents, the key intermediates, 2-chloro-3,3,3-trifluoropropenyl phenyl acetates, were cyclized either to give 2-trifluoromethyl-substituted benzofurans or to yield trifluoromethyl modified o-alkynylphenols. The latter intermediates could also be cyclized to give 3-iodo-2-trifluoromethyl-substituted benzofurans.     

Synthesis of Conjugated Systems Based on 3-(5-Methylfur-2-yl)-2-(3-oxobutyl)benzofurans

Methods were developed for the synthesis of 3-butyl-2-(3-oxobutenyl)benzofurans and 3-(3-furylbenzofuran-2-yl)acrylic acids on the basis of 3-furyl-2-(3-oxobutyl)benzofurans.     

An Easy Access to Benzofurans via DBU Induced Condensation Reaction of Active 2‐Hydroxy Acetophenones with Phenacyl Chlorides: A Novel Class of Antioxidant Agents

A convenient and efficient one‐pot synthesis of benzofurans 3a , 3b , 3c , 3d , 3e , 3f , 3g , 3h , 3i , 3j , 3k , 3l , 3m , 3n , 3o , 3p , 3q , 3r , 3s , 3t has been described from 2‐hydroxy acetophenones and phenacyl chlorides in the presence of DBU. The procedure was applicable for a variety of phenacyl chlorides and provides a variety of benzofurans with higher yields. DBU acts as a base and as well as nucleophiles. All the derivatives were subjected to in vitro antioxidant screenings against representative 2,2′‐diphenyl‐1‐picryl‐hydrazyl and 2,2′‐azino‐bis(3‐ethylbenzthiazoline‐6‐sulfonic acid) radicals and results worth for further investigations.     

Metal-free I2O5-mediated oxidative synthesis of sulfonylated benzofurans through cyclization reaction of 1,6-enynes and arylsulfonylhydrazides

A simple I₂O₅-mediated method for the construction of sulfonylated benzofurans has been developed under metal-free conditions. The present reaction can be efficiently achieved through the oxidative cyclization of 1,6-enynes and arylsulfonylhydrazides, which provides an attractive approach to a series of sulfonylated benzofurans in moderate to good yields.     

Preparation of functional benzofurans and indoles via chemoselective intramolecular Wittig reactions

A general preparation of new types of benzofurans, benzothiophenes and indoles is realized via chemoselective intramolecular Wittig reactions with the corresponding ester, thioester and amide functionalities using in situ formed phosphorus ylides as key intermediates. The reaction conditions are very mild, and numerous Michael acceptors and commercially available acid chlorides can be applied very efficiently in a one-step procedure.     

Benzofuran derivatives. Part 4 , synthesis of benzofurans and 2,3,4,5-tetrahydro-1-benzoxepin-3,5-diones

By treatment of ethyl 4- or 5-substituted 2-acetylphenoxyacetates 1 with potassium hydroxide in dry dioxane, benzofurans 2–7 and 2,3,4,5-tetrahydro-1-benzoxepin-3,5-diones 8 were obtained. The relative yields of benzofurans 2–7 and 2,3,4,5-tetrahydro-1-benzoxepin-3,5-diones 8 varied with the types of 4- or 5-substituents. The electron-donating 4-methoxyl group favored the formation of benzoxepins. On the other hand, electron-withdrawing substituents such as the 4-nitro group favored the formation of benzofurans. When esters 1 were treated with sodium amide, 2,3-dihydrobenzofurans 2 were obtained exclusively regardless of 4- or 5-substituents.     

Catalytic Asymmetric Dearomatization by Visible‐Light‐Activated [2+2] Photocycloaddition

A novel method for the catalytic asymmetric dearomatization by visible‐light‐activated [2+2] photocycloaddition with benzofurans and one example of a benzothiophene is reported, thereby providing chiral tricyclic structures with up to four stereocenters including quaternary stereocenters. The benzofurans and the benzothiophene are functionalized at the 2‐position with a chelating N‐acylpyrazole moiety which permits the coordination of a visible‐light‐activatable chiral‐at‐rhodium Lewis acid catalyst. Computational molecular modeling revealed the origin of the unusual regioselectivity and identified the heteroatom in the heterocycle to be key for the regiocontrol.     

Selenium heterocycles XXVI. Synthesis of theino[3,4-b]benzofuran and selenolo[3,4-b] benzofuran. Two novel heterocycles

Starting from the readily available aryl 3-methyl-2-benzo[b] furyl ketones a series of 3-sub-stituted thieno[3,4-b] benzofurans and 3-substituted selenolo[3,4-b] benzofurans were prepared in high yield. The parent compound, thieno[3,4-b] benzofuran was prepared through the reaction of thioacetamide with 2-chloromethyl-3-formylbenzo[b] furan in moderate yield.     

Practical,Modular, and General Synthesis of Benzofurans through Extended Pummerer Annulation/Cross‐Coupling Strategy

Operationally simple, efficient, and widely applicable Pummerer annulations of simple phenols with ketene dithioacetal monoxides, with the aid of trifluoroacetic anhydride, have been shown to provide a variety of benzofurans having a methylthio group at the 2‐position. Subsequent and newly developed nickel‐catalyzed arylation at the methylthio group culminates in diversity‐oriented synthesis of multisubstituted benzofurans. Our extended Pummerer annulation/cross‐coupling sequence is powerful enough to synthesize biologically active natural products as well as highly fluorescent benzofuran derivatives.     

One-pot synthesis of benzofurans via palladium-catalyzed enolate arylation with o-bromophenols

A one-pot synthesis of benzofurans which utilizes a palladium-catalyzed enolate arylation is described. The process demonstrates broad substrate scope and provides differentially substituted benzofurans in moderate to excellent yields. The utility of the method is further demonstrated by the synthesis of the natural product eupomatenoid 6 in three steps.     

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.