Metal-Free Oxidative Annulation of 2-Naphthols with Terminal Alkynes Affording 2-Arylnaphtho[2,1-b]furans

Rapid Access to a Cyclobutane-Containing Skeleton Fused with Chromane through In(III)-Catalyzed Intramolecular Annulation of 5-Hydroxy Benzochromene

Intramolecular oxidative annulation of 5-hydroxy benzochromene via In(OTf)3 and DDQ catalysis, through an ortho-quinone methide radical intermediate, afforded various cyclobutane-containing products in good to excellent yields. This oxidative annulation pathway reveals great group tolerance and broad compatibility with benzochromene. In addition, this method exhibits an excellent ability to synthesize cyclobutane-containing products in gram-scale experiments, providing a novel approach to synthesize a series of natural products, such as cannabicyclol, isoeriobrucinol A, and ranhuadujuanine A.

Serendipitous pathway to naphtho[2,1-b]furan and its thioether enabled by triflic acid and thiol-mediated reaction

We have developed a selective method for 2-phenyl naphthofuran and thio-ether derivatives using hydroxy naphthofuranone and thiols in triflic acid. Product selectivity is controlled by altering the reagent equivalents. This broad-scope, acid-mediated strategy enables diverse functionalization, overcoming regioselectivity challenges and providing a valuable tool for medicinal chemistry and organic synthesis.

BF3·OEt2-catalyzed/mediated alkyne cyclization: a comprehensive review of heterocycle synthesis with mechanistic insights

The quest for efficient and versatile methods for heterocycle synthesis continues to drive innovation in organic chemistry. In this context, the cyclization of alkynes catalyzed or mediated by boron trifluoride diethyl etherate (BF3·OEt2) has emerged as a powerful and widely applicable strategy. This review provides a comprehensive and authoritative overview of BF3·OEt2-catalyzed/mediated alkyne cyclization reactions, covering the scope, mechanisms, and applications of these processes. We discuss the synthesis of a diverse range of heterocyclic compounds, including dihydropyrans, quinolines, dehydropiperidines, oxindoles and others, and highlight the unique advantages of BF3·OEt2 as a catalyst/mediator. Recent advances, challenges, and future directions in this rapidly evolving field are also addressed. This review aims to serve as a valuable resource for synthetic chemists, inspiring further research and applications in this exciting area.

CuBr2-mediated dehydrogenative [4+2] annulation of 1-naphthyl-1,3-indandiones and alkenes

Intermolecular annulation reactions of 1-naphthyl-1,3-indandiones with alkenes proceed efficiently in the presence of a copper catalyst to generate spirocarbocycle compounds. Various spirocyclic molecules bearing an all-carbon quaternary center could be obtained by this novel method with good yields, excellent regioselectivity, and good functional group tolerance. A radical mechanism is proposed based on the HRMS analysis results of control experiments.

Copper-Catalyzed 6-endo-dig Cyclization-Coupling of 2-Bromoaryl Ketones and Terminal Alkynes toward Naphthyl Aryl Ethers in Water

The cyclization-coupling reaction of 2-bromoaryl ketones and terminal alkynes is first realized by copper catalysis, which produces polyfunctional naphthyl aryl ethers in moderate to excellent yields with broad substrate scope and good functional group tolerance. This reaction proceeds via 6-endo-dig cyclization and C(sp²)-O coupling using green H₂O as the unique solvent and 5-bromopyrimidin-2-amine as the critical additive. Mechanistically, a unique Cu(III)-acetylide probably is the key intermediate, which allows exclusive 6-endo-dig selectivity.

Cu(i) catalysis for selective condensation/bicycloaromatization of two different arylalkynes: direct and general construction of functionalized C-N axial biaryl compounds

Selective condensation/bicycloaromatization of two different arylalkynes is firstly developed under ligand-free copper(i)-catalysis, which allows the direct synthesis of C-N axial biaryl compounds in high yields with excellent selectivity and functional group tolerance. Due to the critical effects of Cu(i) catalyst and HFIP, many easily occurring undesired reactions are suppressed, and the coupled five-six aromatic rings are constructed via the selective formation of two C(sp2)-N(sp2) bonds and four C(sp2)-C(sp2) bonds. The achievement of moderate enantioselectivity verifies its potential for the simplest asymmetric synthesis of atropoisomeric biaryls. Western blotting demonstrated that the newly developed compounds are promising targets in biology and pharmaceuticals. This unique reaction can construct structurally diverse C-N axial biaryl compounds that have never been reported by other methods, and might be extended to various applications in materials, chemistry, biology, and pharmaceuticals.

Visible-Light-Promoted Oxidative Annulation of Naphthols and Alkynes: Synthesis of Functionalized Naphthofurans

A visible-light-mediated site-selective oxidative annulation of naphthols with alkynes for the synthesis of functionalized naphthofurans has been developed. The reaction relies on the in situ formation of an electron donor acceptor pair between phenylacetylene and thiophenol as the light-absorbing system to obviate the requirement of an added photocatalyst. The protocol facilitates the transformation of 1-naphthol and 2-naphthol as well as 1,4-naphthoquinone into a wide variety of highly functionalized naphthofurans.

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.

Domino Reaction of 2,4-Diyn-1-ols with 1,3-Dicarbonyl Compounds: Direct Access to Aryl/Heteroaryl-Fused Benzofurans and Indoles

A domino propargylation/furanylation (intramolecular exo-dig-cyclization)/benzannulation reaction of 2,4-diyn-1-ols with 1,3-dicarbonyl compounds has been developed for the first time. This provides a novel and effective method for the preparation of aryl/heteroaryl-fused benzofurans from easily accessible starting materials in a single step. The methodology was extended to pyrrolyl-benzannulation to obtain aryl/heteroaryl-fused indoles. Further, application of this approach in the synthesis of eustifoline D and dictyodendrin structural frameworks has been demonstrated.

Access to Corrole-Appended Persubstituted Benzofurans by a Multicomponent Reaction: The Dual Role of p-Chloranil

A multicomponent reaction among dipyrranes, aryl-propargyl aldehydes, and p-chloranil leading to 10-(benzofuran-2-yl)corroles is described. p-Chloranil was identified as a crucial reagent playing a twofold role: an oxidant taking part in the formation of the corrole macrocycle and a component undergoing heteroannulation to the incipient 10-arylethynylcorrole. A series of corroles bearing persubstituted benzofuran-2-yl moieties have been synthesized, and their fundamental electronic properties have been studied via UV-vis absorption and fluorescence spectroscopies.

Phosphorous Acid-Catalyzed Alkylation of Phenols with Alkenes

A H₃PO₃-catalyzed alkylation of phenols with alkenes is achieved in a facile, efficient, and selective manner. The reaction shows a unique selectivity, i.e., excellent regioselectivity, thorough suppression of overalkylation, without alkylation of a simple phenyl ring, and can selectively provide ortho-, meta-, or para-alkylated phenol derivatives in good to excellent yields. This feature along with mild reaction conditions, sensitive functional group tolerance, and scale-up synthesis and late modification of phenolic bioactive compounds make it an ideal and practical alternative for the modification of phenols.

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.

Palladium-catalyzed benzofuran and indole synthesis by multiple C-H functionalizations

Heterocyclic compounds are commonly found in the core structures of several pharmaceuticals, natural products, and agrochemicals, thus spurring intensive research for conducting their synthesis in a mild and simpler way. Over the years, a host of different strategies has been introduced in an effort to synthesize these heterocyles. In this context, significant attention has been gained by methodologies that ensure both step as well as atom efficiency. Synthesis of heterocyclic moieties via multiple C-H activations was found to fulfill these expectations besides guaranteeing the use of starting materials that are easily procurable. This review is focused on the current development in the field of benzofuran and indole synthesis using multiple C-H functionalization strategies.

Transition-metal-free base catalyzed intramolecular cyclization of 2-ynylphenols for efficient and facile synthesis of 2-substituted benzo[b]furans

An efficient and practical transition-metal-free synthesis of 2-substituted benzo[b]furans catalyzed by readily accessible base was developed with broad substrates scope.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Manganese(iii) acetate catalyzed oxidative amination of benzylic C(sp3)-H bonds with nitriles

Mn-Catalyzed oxidative amination of benzylic C(sp³)-H bonds with nitriles is disclosed, which enables the synthesis of a broad range of secondary amides in moderate to excellent yields under mild conditions. The interaction between Mn(iii) and DDQ facilitates the oxidation and makes it highly efficient and selective.

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.

FeCl3/ZnI2-Catalyzed regioselective synthesis of angularly fused furans

The FeCl₃/ZnI₂-catalyzed synthesis of angularly fused furans by intermolecular coupling between enols and alkynes has been developed in ambient air.