Iron-Catalyzed Oxidative C–H Functionalization of Internal Olefins for the Synthesis of Tetrasubstituted Furans

Synthesis of Trisubstituted Furans from Activated Alkenes by P(III)/P(V) Redox Cycling Catalysis

The organocatalytic formation of an underrepresented family of trisubstituted and tetrasubstituted furans from activated alkenes and acyl chlorides is reported. In a reaction sequence based on P(III)/P(V) redox cycling catalysis, the cyclic phosphine catalysts react with diacylethenes or acyl acrylates in Michael addition, followed by acylation and either an intramolecular Wittig reaction or a ring closure reaction, liberating the furans. The formed phosphine oxides are reduced in situ by phenylsilane as a terminal reductant. In the first step, 12 diacylethenes were converted to the respective trisubstituted furans. The reaction of acyl acrylates showed a surprising, catalyst-dependent alternate reaction forming tetrasubstituted furans. Two additional methods were developed, giving 14 trisubstituted furans using a phospholene catalyst and an additional 6 tetrasubstituted furans using a phosphetane catalyst. This encompassed 19 newly described compounds.

Influence of steric hindrance on the 1,4- versus 1,6-Michael addition: synthesis of furans and pentasubstituted benzenes

We described the influence of steric hindrance on the 1,4- versus 1,6-Michael addition reaction on 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles. An efficient and direct synthesis of trisubstituted furans was achieved through the reaction of 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles and acetone under mild conditions in good to moderate yield by the 1,4-Michael addition. Further exploration of the reaction with a sterically hindered aryl group containing 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles afforded biaryls by an in situ generated nucleophile through the 1,6-Michael addition. The synthetic utility of furan is further explored. These precursors are easily accessible from aryl methyl ketones. Various functional groups like alkyl, aryl, nitrile, amine, aroyl, and thiomethyl can be directly installed in the benzene and furan rings. A one-pot approach for the construction of a benzene nucleus was also developed. The structure of two compounds was confirmed by X-ray crystallography.

A silver-mediated radical process of β-keto sulfones for the synthesis of 2,3-diacyl furans

A facile and efficient method for constructing 2,3-diacyl trisubstituted furans via a silver-mediated radical process of β-keto sulfones is developed. The reaction mechanism has been carefully investigated, revealing that the transformation proceeds through a radical pathway, leading to moderate to good yields of desired products.

Trifluoromethyl-β-dicarbonyls as Versatile Synthons in Synthesis of Heterocycles

Trifluoromethyl group relishes a privileged position in the realm of medicinal chemistry because its incorporation into organic molecules often enhances the bioactivity by altering pharmacological profile of molecule. Trifluoromethyl-β-dicarbonyls have emerged as pivotal building blocks in synthetic organic chemistry due to their facile accessibility, stability and remarkable versatility. Owing to presence of nucleophilic and electrophilic sites, they offer multifunctional sites for the reaction. This review covers a meticulous exploration of their multifaceted role, encompassing an in-depth analysis of mechanism, extensive scope, limitations and wide-ranging applications in diverse organic synthesis, covering the literature from the 21st century. This comprehensive review encapsulates the applications of trifluoromethyl-β-dicarbonyls and their synthetic equivalents as precursors of complex and diverse heterocyclic scaffolds, fused heterocycles and spirocyclic compounds having medicinal and material importance. Their potent synthetic utility in cyclocondensation reactions with binucleophiles, cycloaddition reactions, C-C bond formations, asymmetric multicomponent reactions using classical/solvent-free/catalytic synthesis have been presented. Influence of unsymmetrical trifluoromethyl-β-diketones on regioselectivity of transformation is also reviewed. This review will benefit the synthetic and pharmaceutical communities to explore trifluoromethyl-β-dicarbonyls as trifluoromethyl building blocks for fabrication of heterocyclic scaffolds having implementation into drug discovery programs in the imminent future.

Rhodium(II)-Catalyzed Desaturative [3+2] Tandem Cyclization of Arylcycloalkanes with β-Dicarbonyls

Synthetically important scaffolds, fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit, play a crucial role in drug discovery. In this study, we demonstrate that rhodium(II)/N-fluorobenzenesulfonimide can catalyze the in situ generation of highly reactive alkene intermediates from commonly accessible alkanes, which undergo intermolecular [3+2] tandem cyclization with the simultaneously generated β-dicarbonyl radical to synthesize a series of fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit with a quaternary carbon center.

Tandem Cross-Coupling of Alkynyl Sulfides and Alkynyl Sulfoxides/[3,3]-Sulfonium Rearrangement to Construct Tetrasubstituted Furans

In the presence of boron trifluoride, a variety of alkynyl sulfides and alkynyl sulfoxides undergo tandem cross-coupling/[3,3]-sulfonium rearrangement followed by 5-exo-dig heterocyclization. The strategy provides concise access to novel tetrasubstituted furans in good to high yields with 100% atom-economy efficiency. Further derivatization of the resultant furans was feasible by utilizing the incorporated alkylthio groups.

Silver-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds to construct polysubstituted furans

An efficient synthesis of polysubstituted furans by Ag(i)-mediated annulation between 5-H-1,2,3-thiadiazoles and 1,3-dicarbonyl compounds is reported.

Iodine-Catalyzed Synthesis of Substituted Furans and Pyrans: Reaction Scope and Mechanistic Insights

Substituted pyrans and furans are core structures found in a wide variety of natural products and biologically active compounds. Herein, we report a practical and mild catalytic method for the synthesis of substituted pyrans and furans using molecular iodine, a simple and inexpensive catalyst. The method described is performed under solvent-free conditions at an ambient temperature and atmosphere, thus offering a facile and practical alternative to currently available reaction protocols. A combination of experimental studies and density functional theory calculations revealed interesting mechanistic insights into this seemingly simple reaction.

Unified Approach to Furan Natural Products via Phosphine-Palladium Catalysis

Polyalkyl furans are widespread in nature, often performing important biological roles. Despite a plethora of methods for the synthesis of tetrasubstituted furans, the construction of tetraalkyl furans remains non-trivial. The prevalence of alkyl groups in bioactive furan natural products, combined with the desirable bioactivities of tetraalkyl furans, calls for a general synthetic protocol for polyalkyl furans. This paper describes a Michael-Heck approach, using sequential phosphine-palladium catalysis, for the preparation of various polyalkyl furans from readily available precursors. The versatility of this method is illustrated by the total syntheses of nine distinct polyalkylated furan natural products belonging to different classes, namely the furanoterpenes rosefuran, sesquirosefuran, and mikanifuran; the marine natural products plakorsins A, B, and D and plakorsin D methyl ester; and the furan fatty acids 3D5 and hydromumiamicin.

Selectfluor-Promoted Intramolecular N-S Bond Formation of α-Carbamoyl Ketene Dithioacetals in the Presence of Water: Synthesis of Multifunctionalized Isothiazolones

A practical and efficient protocol toward fully substituted isothiazolones through Selectfluor-mediated intramolecular oxidative annulation of α-carbamoyl ketene dithioacetals has been developed in the presence of H₂O and metal-free conditions. Notably, the experimental results reveal that H₂O was crucial to the formation of new N-S bonds and the elimination of alkyl group from the sulfur atom. This protocol provides readily prepared substrates and possesses good functional group tolerance, mild reaction conditions, and operational simplicity, which provides potential access to applications in the pharmaceutical chemistry.

Transition-metal mediated carbon-sulfur bond activation and transformations: an update

Carbon-sulfur bond cross-coupling has become more and more attractive as an alternative protocol to establish carbon-carbon and carbon-heteroatom bonds. Diverse transformations through transition-metal-catalyzed C-S bond activation and cleavage have recently been developed. This review summarizes the advances in transition-metal-catalyzed cross-coupling via carbon-sulfur bond activation and cleavage since late 2012 as an update of the critical review on the same topic published in early 2013 (Chem. Soc. Rev., 2013, 42, 599-621), which is presented by the categories of organosulfur compounds, that is, thioesters, thioethers including heteroaryl, aryl, vinyl, alkyl, and alkynyl sulfides, ketene dithioacetals, sulfoxides including DMSO, sulfones, sulfonyl chlorides, sulfinates, thiocyanates, sulfonium salts, sulfonyl hydrazides, sulfonates, thiophene-based compounds, and C[double bond, length as m-dash]S functionality-bearing compounds such as thioureas, thioamides, and carbon disulfide, as well as the mechanistic insights. An overview of C-S bond cleavage reactions with stoichiometric transition-metal reagents is briefly given. Theoretical studies on the reactivity of carbon-sulfur bonds by DFT calculations are also discussed.

Synthesis of Tetrasubstituted Furans through One-Pot Formal [3 + 2] Cycloaddition Utilizing [1,2]-Phospha-Brook Rearrangement

An efficient method for the synthesis of tetrasubstituted furans was developed by utilizing the [1,2]-phospha-Brook rearrangement under Brønsted base catalysis. The two-step one-pot formal [3 + 2] cycloaddition involves the nucleophilic addition of a propargyl anion, which is catalytically generated through the [1,2]-phospha-Brook rearrangement, to an aldehyde and the subsequent intramolecular cyclization mediated by N-iodosuccinimide to provide 2,4,5-trisubstituted-3-iodofurans. The present method with readily available substrates provides new access to a wide range of well-organized tetrasubstituted furans.

Direct synthesis of highly functionalized furans from donor-acceptor cyclopropanes via DBU-mediated ring expansion reactions

A DBU-mediated, unprecedented formal ring expansion reaction of 2-acyl-3-arylcyclopropane-1,1-dicarbonitriles for the synthesis of multisubstituted furan derivatives is reported. This transformation represents the regioselective ring-opening reaction of cyclopropane-1,1-dicarbonitriles and annulation using an intramolecular addition cascade reaction protocol for the synthesis of fully substituted furans includes use of readily available starting materials, mild reaction conditions, and it is transition-metal catalyst free, has good functional tolerance, and broad substrate scope.

Mn-mediated sequential three-component domino Knoevenagel/cyclization/Michael addition/oxidative cyclization reaction towards annulated imidazo[1,2-a]pyridines

The sequential three-component reaction between o-hydroxybenzaldehydes, N-(cyanomethyl)pyridinium salts and a nucleophile towards substituted chromenoimidazopyridines under oxidative conditions has been developed. The employment of Mn(OAc)₃·2H₂O or KMnO₄ as stoichiometric oxidants allowed the use of a wide range of nucleophiles, such as nitromethane, (aza)indoles, pyrroles, phenols, pyrazole, indazole and diethyl malonate. The formation of the target compounds presumably proceeds through a domino Knoevenagel/cyclization/Michael addition/oxidative cyclization reaction sequence.

Transition-metal-free C–C σ-bond activation of α-aryl ketones and subsequent Zn-catalyzed intramolecular cyclization: synthesis of tetrasubstituted furans

Atom-economical synthesis of tetrasubstituted furans from α-aryl ketones and alkynones through base-promoted cleavage of unstrained C–C single bonds of ketones and subsequent Zn-catalyzed cyclization.

Oxidant-controlled divergent transformations of 3-aminoindazoles for the synthesis of pyrimido[1,2-b]-indazoles and aromatic nitrile-derived dithioacetals

An oxidant-controlled divergent reactivity of 3-aminoindazoles is presented, which enables the [3 + 3] annulation and C–H arylation of ketene dithioacetals in disparate pathways to assemble pyrimido[1,2-b]-indazoles and aromatic nitrile-derived dithioacetals.

Synthesis of 2-acetyl trisubstituted furans via copper-mediated deacylation cleavage of unstrained C(sp3)–C(sp2) bonds

A copper-mediated tandem addition/cyclization/carbon–carbon cleavage reaction for the convenient synthesis of 2-acyl trisubstituted furans has been developed.

Iron-Catalyzed Oxyalkylation of Terminal Alkynes with Alkyl Iodides

A general oxyalkylation of terminal alkynes enabled by iron catalysis has been developed. Primary and secondary alkyl iodides acted as the alkylating reagents and afforded a range of α-alkylated ketones under mild reaction conditions. Acetyl tert-butyl peroxide (TBPA) was used as the radical relay precursor, providing the initiated methyl radical to start the radical relay process. Preliminary mechanistic studies were conducted, and late-stage functionalizations of natural product derivatives were performed.

Palladium-Catalyzed C-S Bond Cleavage with Allenoates: Synthesis of Tetrasubstituted 2-Alkenylfuran Derivatives

Palladium-catalyzed C-S cleavage of tetrasubstituted internal alkene α-oxo ketene dithioacetals was realized with allenoates as the coupling partners, efficiently affording tetrasubstituted 2-alkenylfuran derivatives with excellent regioselectivity under mild conditions. Allenoates acted as C1 synthons in the desulfurative [4 + 1] annulation.

Metal-Free Vinyl C-H Sulfenylation/Alkyl Thiolation of Ketene Dithioacetals for the Synthesis of Polythiolated Alkenes

The sulfenylation of the vinyl C-H bond in ketene dithioacetals leading to the synthesis of polythiolated alkenes is achieved via the promotion of molecular iodine. In addition, alkyl thiols also exhibit tolerance to the C-H bond elaboration reaction for the synthesis of corresponding alkylthiolated ketene dithioacetals.

Cu(OTf)2-Catalyzed Pummerer Coupling of β-Ketosulfoxides

The copper(II) trifluoromethanesulfonate-catalyzed Pummerer reaction of β-ketosulfoxides with 1,3-dicarbonyl compounds or π-nucleophiles such as phenols, arenes, and tetraallylsilane is reported. The mild conditions provide an efficient entry to a novel class of polysubstituted 3-alkylthiofuran and polysubstituted 3-thiobenzofuran heterocycles from readily available materials.