Cu-Catalyzed Oxidative C(sp2)-H Cycloetherification of o-Alkenyl Arenols for the Preparation of Fused Furans

A practically efficient, copper-catalyzed approach for the synthesis of functionally embellished indeno-naphthofurans is developed from 1-(1H-inden-3-yl)naphthalen-2-ols. This intramolecular cycloetherification proceeds via C(sp2)-H oxygenation (C-H bond breaking and C-O bond forming), which enables the atom-economical synthesis of poly fused furans in high yields with large substrate diversity in the open air.

A New Method for the Synthesis of N, N-Diethyl-m-Methylbenzamide from m-Toluic Acid and Diethylamine Using 1,1'-Carbonyl-di-(1,2,4-triazole) (CDT) as Coupling Agent

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A new, simple method for the synthesis of N, N-dietyl-m-methylbenzamide (DEET) from m-toluic acid and diethylamine using 1,1'-carbonyl-di-(1,2,4-triazole) (CDT) as a coupling agent has been performed. The basic principles of activated carbonyls have been explored with the ability to prepare new amides easily. All reaction by-products are water-soluble as well as removed by filtration, the reaction could be purified easily in an aqueous solution by liquid-liquid extraction, and the product DEET was high purity. This experimental efficiency is about 94-95%, purity (HPLC): 97-98%.

Palladium-catalyzed ortho-vinylation of β-naphthols with α-trifluoromethyl allyl carbonates: one-pot access to naphtho[2,1-b]furans

Highly regio- and stereoselective palladium-catalyzed ortho-vinylation of β-naphthols (2) has been reported using easily accessible CF₃-allyl carbonates (1). The regioselective nucleophilic γ-attack of the CF₃-π-allyl-Pd-intermediate is the key to furnish (Z)-CF₃-vinylnaphthols (3) in good yields. Furthermore, we achieved a one-pot synthesis of CF₃-naphtho[2,1-b]furans (4) through an uninterrupted ortho-vinylation/oxidative radical cyclization reaction sequence.

Palladium-catalyzed dehydrogenative C–H cyclization for isoindolinone synthesis

In this paper Pd-catalyzed intramolecular dehydrogenative C(sp³)–H amidation for the synthesis of isoindolinones is described. This method features the use of a Pd/C catalyst and the addition of a stoichiometric amount of oxidant is not necessary. A mechanistic study suggested the possible formation of H₂ gas during the reaction.

Pd-catalyzed intramolecular dehydrogenative C(sp³)–H amidation for the synthesis of isoindolinones was developed. Use of Pd/C as a catalyst enables the desired cyclization to proceed smoothly without adding any stoichiometric oxidants.

Recent Achievement in the Synthesis of Benzo[b]furans

BACKGROUND

Benzo[b]furan derivatives are oxygen-containing heterocyclic compounds consisting of fused benzene and furan rings and are present in a large number of natural and non-natural compounds. This class of compounds has a wide spectrum of biological activities, such as antiarrhythmic, anticancer, inflammatory, antioxidant, antimicrobial, and antiviral. Furthermore, benzo[b]furan derivatives have also been applied in various areas, such as organic electroluminescence device materials and organic dyes, photosensitizing material, organic synthesis as building blocks or intermediates. Because of a broad range of applicability, the synthesis of benzo[b]furan derivative has drawn great attention of chemists and many studies on the synthesis of this class of compounds have been reported recently. This review will give an overview of benzo[b]furan preparation based on studies dating back to the year 2012.

OBJECTIVE

In this review, recent development in the synthesis of benzo[b]furans are discussed. There has been increasingly new methodologies for the construction of benzo[b]furans skeleton to improve efficiency or develop environmentally friendly procedures. In some studies, reaction mechanisms were also outlined.

CONCLUSION

Many methods for the synthesis of benzo[b]furans have been reported recently. Most of them involve cyclization or cycloisomerization processes. Unquestionably, more imaginative strategies for the construction of benzo[b]furan skeleton will be established in the near future. Application of known methods to natural products or drug synthesis, on industrial scale for the synthesis of economically or medicinally important benzo[ b]furans will probably be paid attention to.

Nickel catalyzed intramolecular oxidative coupling: synthesis of 3-aryl benzofurans

Recent research has been focused on the transition metal-catalyzed reactions. Herein we have developed nickel-catalyzed synthesis of 3-aryl benzofurans from ortho-alkenyl phenols via intramolecular dehydrogenative coupling. Notably, simple O₂ gas served as an oxidant, without using any sacrificial hydrogen acceptor. The strategy enabled the synthesis of 3-aryl benzofurans in good to excellent yields.

We have developed nickel-catalyzed synthesis of 3-aryl benzofurans from ortho-alkenyl phenols via intramolecular dehydrogenative coupling. O₂ gas served as an oxidant and 3-aryl benzofurans were synthesized in good to very good yields.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Synthesis of benzofurans from the cyclodehydration of α-phenoxy ketones mediated by Eaton’s reagent

Cyclodehydration of α-phenoxy ketones promoted by Eaton’s reagent (phosphorus pentoxide–methanesulfonic acid) is used to prepare 3-substituted or 2,3-disubstituted benzofurans with moderate to excellent yields under mild conditions. The method provides a facile access to benzofurans from readily available starting materials such as phenols and α-bromo ketones. The reaction is highly efficient, which is attributed to the good reactivity and fluidity of Eaton’s reagent. The reaction can be applied to prepare naphthofurans, furanocoumarins, benzothiophenes, and benzopyrans.

Catalytic asymmetric [4+2] cycloaddition of 1-((2-aryl)vinyl)naphthalen-2-ols with in situ generated ortho-quinone methides for the synthesis of polysubstituted chromanes

An asymmetric [4+2] cycloaddition of 1-((2-aryl)vinyl)naphthalen-2-ols with in situ generated ortho-quinone methides enables the highly enantioselective synthesis of polysubstituted chromanes.

Diphenyl-Diselenide-Mediated Domino Claisen-Type Rearrangement/Cyclization of Propargylic Aryl Ethers: Synthesis of Naphthofuran-2-carboxaldehyde Derivatives

A diphenyl-diselenide-mediated Claisen-type rearrangement/cyclization of propargylic aryl ethers under metal-free conditions is developed, affording various naphthofuran-2-carboxaldehydes in moderate to excellent yield. The broad substrate scope and excellent functional group compatibility suggest that it can be a straightforward and powerful method to access naphthofuran-2-carboxaldehydes in a highly regioselective manner. Moreover, this reaction can be scaled up to the gram scale.

Naphtho[2,3-b]furan-4,9-dione synthesis via palladium-catalyzed reverse hydrogenolysis

A reverse hydrogenolysis process has been developed for two-site coupling of 2-hydroxy-1,4-naphthoquinones with olefins to produce naphtha[2,3-b]furan-4,9-diones and hydrogen (H₂).

Facile synthesis of 2-substituted benzo[b]furans and indoles by copper-catalyzed intramolecular cyclization of 2-alkynyl phenols and tosylanilines

A catalytic amount of CuCl and Cs₂CO₃ was employed to synthesize a variety of 2-substituted benzo[b]furans and indoles by an intramolecular cyclization of 2-alkynyl phenols and tosylanilines. This protocol features mild conditions, high yields and broad substrate scope, which makes it a practical method for the synthesis of 2-substituted benzo[b]furans and indoles.

A facile and inexpensive copper-catalyzed method was developed for the synthesis of 2-substituted benzo[b]furans and indoles.

Synthesis of Indoles and Benzofurans Using a Graphene Oxide-Grafted Aminobisphosphine-PdII Complex

Indoles and benzofurans have been synthesized using a graphene oxide-grafted aminobisphosphine-Pd^II complex (GO@PNP-Pd) under copper-free condition. A range of indole and benzofuran derivatives have been made by the reaction of N-protected 2-iodoanilines and 2-iodophenols with terminal acetylenes in presence of GO@PNP-Pd. The activity of the aminobisphosphine-Pd^II complex remains the same under both homogeneous and heterogeneous conditions. Finally, the recycling ability of the immobilized catalyst (GO@PNP-Pd) has been examined for five consecutive runs with appreciable conversion.

One-Pot Synthesis of 2-(Het)aryl/alkyl-3-cyanobenzofurans from 2-(2-Bromoaryl)-3-(het)aryl-3-(methylthio)acrylonitriles and Benzyl Carbamate

A one-pot synthesis of 2-(het)aryl/alkyl-3-cyanobenzofurans has been reported. The overall sequence involves base-induced reaction of 2-(2-bromoaryl)-3-(het)aryl-3-(methylthio)acrylonitriles with benzyl carbamate, generating α-(het)aroyl-α-(2-bromoaryl)acetonitriles, which are in situ subjected to copper-catalyzed intramolecular O-arylation, furnishing the 2-(het)aryl/alkyl-3-cyanobenzofurans in high yields. A probable mechanism for the base-induced cleavage of 2-(2-bromoaryl)-3-(het)aryl-3-(methylthio)acrylonitriles to α-(het)aroyl-α-(2-bromoaryl)acetonitriles in the presence of benzyl carbamate has been proposed.

Metal- and Reagent-Free Anodic C-C Cross-Coupling of Phenols with Benzofurans leading to a Furan Metathesis

Heterobiaryls consisting of a phenol and a benzofuran motif are of significant importance for pharmaceutical applications. An attractive sustainable, metal- and reagent-free, electrosynthetic, and highly efficient method, that allows access to (2-hydroxyphenyl)benzofurans is presented. Upon the electrochemical dehydrogenative C-C cross-coupling reaction, a metathesis of the benzo moiety at the benzofuran occurs. This gives rise to a substitution pattern at the hydroxyphenyl moiety which would not be compatible by a direct coupling process. The single-step protocol is easy to conduct in an undivided electrolysis cell, therefore scalable, and inherently safe.

Hydrazone–Pd-catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates

The hydrazone–palladium catalyzed direct intermolecular reaction of o-alkynylphenols with allylic acetates gave the corresponding 2-substituted-3-allylbenzofuran derivatives at room temperature.

Oxidant- and hydrogen acceptor-free palladium catalyzed dehydrogenative cyclization of acylhydrazones to substituted oxadiazoles

An efficient method for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles has been developed through palladium(0) catalyzed dehydrogenative cyclization ofN-arylidenearoylhydrazides without oxidants and hydrogen acceptors.

Pd/C-catalyzed dehydrogenation of 2-cinnamoylbenzoic acids to 3-benzylidene-3H-isochroman-1,4-diones

Pd/C, a widely accepted hydrogenation catalyst, is found to catalytically dehydrogenate 2-cinnamoylbenzoic acids to 3-benzylidene-3-H-isochroman-1,4-diones with H₂ as the only byproduct.

An Approach to the Synthesis of 1-Propenylnaphthols and 3-Arylnaphtho[2,1-b]furans

A simple and efficient strategy for the synthesis of 1-propenylnaphthols from readily accessible 3-arylallylnaphthyl ethers has been developed. By using K₂CO₃ as base and 2-methoxyethanol as solvent, direct access to a wide range of 1-propenylnaphthols can be achieved in generally good yield (up to 99%) with high stereoselectivity toward the Z isomer. The control experiments indicate that the reaction proceeds through a sequential Claisen rearrangement/isomerization process. Furthermore, starting from the same material, the highly valuable 3-arylnaphtho[2,1-b]furans can be obtained in N,N-dimethylformamide and in the presence of Ag₂O as the oxidant via a one-pot sequential Claisen rearrangement/isomerization/cyclization reaction. Mechanistic studies confirm that 1-propenylnaphthols are the key intermediates to form the 3-arylnaphtho[2,1-b]furans. In addition, these two operationally simple and practical protocols could be scaled up to a gram level.

C-H Functionalization via Remote Hydride Elimination: Palladium Catalyzed Dehydrogenation of ortho-Acyl Phenols to Flavonoids

Although deprotonation of electron-poor C-H bonds to carbon anions with bases has long been known and widely used in organic synthesis, the hydride elimination from electron-rich C-H bonds to carbon cations or partial carbocations for the introduction of nucleophiles is a comparatively less explored area. Here we report that the carbonyl β-C(sp³)-H bond hydrogens of ortho-acyl phenols could be substituted by intramolecular phenolic hydroxyls to form O-heterocycles, followed by dehydrogenation of the O-heterocycle into flavonoids. The cascade reaction is catalyzed by Pd/C without added oxidants and sacrificing hydrogen acceptors.

Oxidative annulations triggered by a simple Lewis acid: facile synthesis of benzofurans

The first example of Lewis acid promoted efficient one-pot synthesis of benzofurans is presented.

Formal arylation of NH3 to produce diphenylamines over supported Pd catalysts

In the presence of Pd/Al₂O₃, various diphenylamines could be synthesized through acceptorless formal arylation using urea as a nitrogen source and cyclohexanones as arylation sources.