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.     

A first regioselective synthesis of 3-fluoroalkylated benzofurans via palladium-catalyzed annulation of fluorine-containing internal alkynes with variously substituted 2-iodophenol

The palladium-catalyzed annulation reaction of a variety of fluorine-containing internal alkynes with 2-iodophenol derivatives was investigated. The use of P(t-Bu)₃ as a ligand on palladium was found to be crucial in this annulation reaction, resulting in the exclusive formation of 3-fluoroalkylated benzofurans in high yields. ¹⁹F NMR analysis of the reaction mixture revealed that the addition of phenol to the fluoroalkylated alkynes was followed by intramolecular Heck reaction, giving to the corresponding 3-fluoroalkylated benzofurans.     

Synthesis of benzofurans in ionic liquid by a PdCl2-catalyzed intramolecular Heck reaction

PdCl₂-catalyzed intramolecular Heck reaction was conducted in ionic liquid, 1-n-butyl-3-methylimidazolium tetraborate ([BMIm] BF₄), substituted benzofurans were obtained in modest to satisfactory yields. The ionic liquid containing Pd catalyst can be used four times with a little loss of activity.     

Palladium/2,2′-bipyridyl/Ag2CO3 catalyst for C-H bond arylation of heteroarenes with haloarenes

A Pd/bipy-based catalytic system for the C-H bond arylation of heteroarenes with haloarenes is described. The complex PdBr₂(bipy)·DMSO, whose structure was unambiguously determined by X-ray crystallography, turned out to be a general catalyst precursor for the process. The reaction is applicable to a range of electron-rich five-membered heteroarenes, such as thiophenes, thiazoles, benzofurans, and indoles.     

An efficient and general procedure for room-temperature synthesis of benzofurans under solvent-free conditions using KF/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Room temperature Rap-Stoermer condensation of α-haloacetophenone with various 2-hydroxyarylaldehydes mediated by KF/Al₂O₃ resulted in sole formation of good to excellent yields of various substituted benzofurans in the absence solvent or extra stimulant.     

A new synthesis of 2-substituted benzofurans and benzothiophens: Novel fragmentation reactions of simple alkyl groups

Flash vacuum pyrolysis of (2-methoxy- and (2-methylthiobenzoyl)alkylidenetriphenylphosphoranes results in loss of Ph₃PO and methyl radical; cyclisation of the resulting radical intermediates leads to benzofurans and benzothiophens in which the 2-substituents have undergone novel fragmentation processes.     

Gold catalysis: benzanellation versus alkylidenecyclopentenone synthesis

A series of different furan-yn-ols were prepared by a three-step sequence. Their reaction with Gagosz's catalyst Ph₃PAuNTf₂ depends strongly on the substitution pattern of the substrate and the quality of the leaving group. Benzofurans, alkylidenecyclopentenones or Meyer-Schuster type products can be obtained. Improving the leaving group quality leads to the preferred formation of benzofurans.     

Facile synthesis of 2,3-disubstituted benzofurans via DBU-promoted intramolecular annulation of ortho oxyether aroylformates

A facile synthesis of 2,3-disubstituted benzofurans from ortho oxyether aroylformates has been developed. Under the mediation of DBU, the intramolecular annulation of ortho oxyether aroylformates proceeds smoothly to provide the corresponding 2,3-disubstituted benzofurans in moderate to good yields under mild conditions. Furthermore, a one-pot two-step synthesis of 2,3-disubstituted benzofurans has also been demonstrated from readily available ortho hydroxy aroylformates.     

Highly asymmetric synthesis of (+)-corsifuran A. Elucidation of the electronic requirements in the Ruthenium–NHC catalyzed hydrogenation of benzofurans

A short and efficient synthesis of ent-corsifuran A by a highly asymmetric hydrogenation of a benzofuran precursor is reported. In addition, the electronic influence of the substituents on the asymmetric hydrogenation of benzofurans is provided. Whereas the hydrogenation of electron-deficient benzofurans was achieved under very mild conditions, the presence of electron-donating groups in the benzofuran required harsher reaction conditions for achieving full conversion to the 2,3-dihydrobenzofuran.     

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.     

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.     

Cobalt(III)‐Catalyzed Intramolecular Cross‐Dehydrogenative C−H/X−H Coupling: Efficient Synthesis of Indoles and Benzofurans

An efficient cobalt(III)‐catalyzed intramolecular cross‐dehydrogenative C?H/N?H coupling of ortho‐alkenylanilines has been developed utilizing O₂ as a terminal oxidant. The developed reaction tolerates various reactive functional groups and allows the synthesis of diverse indole derivatives in good to excellent yields. The method was successfully extended to the synthesis of benzofurans through the intramolecular cross‐dehydrogenative C?H/O?H coupling of ortho‐alkenylphenols.     

Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase

The rearrangement of the molecular ions of the isomeric 2- and 3-methyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1⁺˙?4⁺˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k_H/k_D, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.     

Silver‐Catalyzed Nitration/Annulation of α‐Alkynyl Arylols toward 3‐Nitrated Benzofurans

A silver‐catalyzed nitration/annulation of α‐alkynyl arylols is reported by using tert‐butyl nitrite (TBN) as a NO₂ radical precursor, from which a set of 3‐nitrated benzofurans were synthesized with moderate to good yields. This transformation initiated by an in situ generated NO₂ radical proceeds efficiently under mild and neutral redox conditions, which provides a new pathway toward the 3‐nitrobenzofuran framework via catalytic difunctionalization of internal alkynes.     

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.     

Acyloxyalkylidènephosphoranes—III : Etude des ω-acyloxybenzylidènetriphénylphosphoranes.nouvelle voie d'accès aux benzofurannes

The intramolecular condensation of o-acyloxybenzylidenetriphenylphosphoranes leads to acylated products in t-BuOH and to benzofurans in toluene. Mechanistic aspects are discussed. A general method is described for the synthesis of benzofurans from o-cresols, o-hydroxybenzylic alcohols, and deactivated phenols.     

An Efficient Friedel–Crafts/Oxa‐Michael/Aromatic Annulation: Rapid Access to Substituted Naphtho[2,1‐b]furan,Naphtho[1,2‐b]furan,and Benzofuran Derivatives

Substituted naphthofurans and benzofurans are easily accessible by treatment of naphthols/substituted phenols with nitroallylic acetates through a substitution–elimination process promoted by cesium carbonate. Reactions between naphthols and aromatic/heteroaromatic‐substituted nitroallylic acetates gave the desired functionalized naphthofurans in high to excellent chemical yields (14–97 %). On the other hand, treatment of phenol derivatives (i.e., 3‐dimethylamino‐, 3‐methoxy‐, and 3,5‐dimethoxyphenol) with various nitroallylic acetates afforded the corresponding benzofurans in moderate to good chemical yields (24–91 %). The reaction proceeded through an interesting Friedel–Crafts S_N2′ process followed by intramolecular oxa‐Michael cyclization and subsequent aromatization. A plot of log (k/k_H) against Hammett constants σ_p showed satisfactory linearity with a positive ρ value, indicating that the initial Friedel–Crafts‐type S_N2′ process constituted the rate‐determining step. This methodology has been applied to the synthesis of various novel C₂ and C₃ symmetric bis‐ and trisfurans by using catechol and phloroglucinol as the nucleophilic partners. The reactivity decreased when alkyl‐substituted nitroallylic acetate systems were used. This might be related to the decreased electrophilic character of these substrates.     

Furan derivatives. part 11 [1]. on substituent effects in the synthesis of 3,4,5,6-tetrahydrocyclohepta[cd]benzofurans

Tetrahydrocyclohepta[cd]benzofurans 7a-e were synthesized by the treatment of ( 5 -oxo-tetrahydro-5H-benzocyclohepten-4-yloxy)acetic acids 5a-e with sodium acetate in acetic anhydride or by heating of their esters 6a-e with sodium hydroxide or sodium hydride in dioxane. The yield of furans 7 decreased as a substituent R of acids 5 or esters 6 was changed from hydrogen to a methyl, ethyl, or isopropyl group. When R was a phenyl group furan 7e was always prepared in good yield. Sodium hydride was a useful base for synthesis of tetrahydrocyclohepta[cd]benzofurans.     

BCl3‐Induced Annulative Oxo‐ and Thioboration for the Formation of C3‐Borylated Benzofurans and Benzothiophenes

BCl₃‐induced borylative cyclization of aryl‐alkynes possessing ortho‐EMe (E=S, O) groups represents a simple, metal‐free method for the formation of C3‐borylated benzothiophenes and benzofurans. The dichloro(heteroaryl)borane primary products can be protected to form synthetically ubiquitous pinacol boronate esters or used in situ in Suzuki–Miyaura cross couplings to generate 2,3‐disubstituted heteroarenes from simple alkyne precursors in one pot. In a number of cases alkyne trans‐haloboration occurs alongside, or instead of, borylative cyclization and the factors controlling the reaction outcome are determined.     

Enantioselective reduction of benzofuryl halomethyl ketones: asymmetric synthesis of (R)-bufuralol

Enantioselective reduction of representative 2-(bromoacetyl)- and 2-(chloroacetyl)benzofurans with (−)-B-chlorodiisopinocampheylborane and by transfer hydrogenation with formic acid/triethylamine in the presence of RhCl[R,R-TsDPEN](C₅Me₅) is described. Transfer hydrogenation of the chloro ketones produced the corresponding chlorohydrins of ⩾95% ee. (R)-Bufuralol of 96% ee was prepared from the corresponding chloro ketone by transfer hydrogenation–substitution.