Regioselective synthesis of multisubstituted furans via metalloradical cyclization of alkynes with α-diazocarbonyls: construction of functionalized α-oligofurans

Transition metal catalysis in confined spaces

Transition metal catalysis plays an important role in both industry and in academia where selectivity, activity and stability are crucial parameters to control. Next to changing the structure of the ligand, introducing a confined space as a second coordination sphere around a metal catalyst has recently been shown to be a viable method to induce new selectivity and activity in transition metal catalysis. In this review we focus on supramolecular strategies to encapsulate transition metal complexes with the aim of controlling the selectivity via the second coordination sphere. As we will discuss, catalyst confinement can result in selective processes that are impossible or difficult to achieve by traditional methods. We will describe the template-ligand approach as well as the host-guest approach to arrive at such supramolecular systems and discuss how the performance of the catalyst is enhanced by confining it in a molecular container.

Iron-Catalyzed Convergent Radical Cyclization of Aldehydes with Two Alkenes to 3,4-Dihydropyrans

A novel convergent radical cyclization of an aldehyde with two alkenes has been developed. With this method, polyfunctionalized 3,4-dihydropyrans are built in an efficient and selective manner. The iron-catalyzed redox radical recombinations are proposed for the formation of a 3,4-dihydropyran skeleton.

Preparation of Furo[3,2-c]coumarins from 3-Cinnamoyl-4-hydroxy-2H-chromen-2-ones and Acyl Chlorides: A Bu3P-Mediated C-Acylation/Cyclization Sequence

A Bu3P-mediated cyclization reaction of 3-cinnamoyl-4-hydroxy-2H-chromen-2-ones though electrophilic addition of acyl chlorides towards the synthesis of highly functionalized furo[3,2-c]coumarins bearing a phosphorus ylide moiety is described. These unprecedented cyclization reaction proceeds under mild reaction conditions within short reaction times (1 min to 1 h), and can be further applied in the synthesis of alkenyl-substituted furo[3,2-c]coumarins by the treatment with carbonyl electrophiles under basic conditions.

Stereoselective Radical C-H Alkylation with Acceptor/Acceptor-Substituted Diazo Reagents via Co(II)-Based Metalloradical Catalysis

Co(II)-based metalloradical catalysis has been, for the first time, successfully applied for asymmetric intramolecular C-H alkylation of acceptor/acceptor-substituted diazo reagents. Through the design and synthesis of a new D2-symmetric chiral amidoporphyrin as the supporting ligand, the Co(II)-based metalloradical system, which operates at room temperature, is capable of 1,5-C-H alkylation of α-methoxycarbonyl-α-diazosulfones with a broad range of electronic properties, providing the 5-membered sulfolane derivatives in high yields with excellent diastereoselectivity and enantioselectivity. In addition to complete chemoselectivity toward allylic and allenic C-H bonds, the Co(II)-based metalloradical catalysis for asymmetric C-H alkylation features a remarkable degree of functional group tolerance.

Iron-catalyzed tandem cyclization of olefinic dicarbonyl compounds with benzylic Csp3–H bonds for the synthesis of dihydrofurans

A novel iron-catalyzed radical cyclization of olefinic dicarbonyl compounds with benzyl hydrocarbons and simple alkanes has been developed. This protocol provides ready access to a variety of dihydrofurans containing a quaternary carbon center in moderate to good yields.

Rh(iii)-catalyzed chelation-assisted intermolecular carbenoid functionalization of α-imino Csp3–H bonds

A Rh(iii)-catalyzed cross-coupling/cyclization cascade of α-imino Csp³–H bonds with donor/acceptor α-acyl diazocarbonyl compounds has been developed.

Synthesis of diversely substituted 2-(furan-3-yl)acetates from allenols through cascade carbonylations

Novel synthesis of diversely substituted 2-(furan-3-yl)acetatesviapalladium-catalyzed one-pot multi-component reactions of allenols, aryl iodides, alcohols, and carbon monoxide has been developed. Moreover, the 2-(furan-3-yl)acetates obtained were found to be ready intermediates for the construction of the biologically significant naphtho[1,2-b]furan-5-ol scaffold.

Tuning radical reactivity using iodine in oxidative C(sp3)–H/C(sp)–H cross-coupling: an easy way toward the synthesis of furans and indolizines

Molecular iodine was found to be an effective redox catalyst for the oxidative cross-coupling of carbonyl compounds with terminal alkynes.

A [3 + 2]-annulation approach to tetrasubstituted furans from MBH-carbonates of acetylenic aldehydes via sequential substitution/cycloisomerization

A new organo base-mediated cascade reaction has been developed for the synthesis of uniquely substituted tetrasubstituted furans, 2-furan-3-yl acrylates, starting from MBH-carbonates of acetylenic aldehydes and keto-methylenes.

Palladium-catalyzed condensation of N-aryl imines and alkynylbenziodoxolones to form multisubstituted furans

A palladium(II) catalyst promotes condensation of an N-aryl imine and an alkynylbenziodoxolone derivative to afford a multisubstituted furan, whose substituents are derived from the alkynyl moiety (2-position), the imine (3- and 4-positions), and the 2-iodobenzoate moiety (5-position), along with an N-arylformamide under mild conditions. The 2-iodophenyl group of the furan product serves as a versatile handle for further transformations. A series of isotope-labeling experiments shed light on the bond reorganization process in this unusual condensation reaction, which includes cleavage of the C-C triple bond and fragmentation of the carboxylate moiety.

DDQ-mediated oxidative coupling: an approach to 2,3-dicyanofuran (thiophene)

A facile oxidative coupling of α-carbonyl radicals to 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) for the synthesis of 2,3-dicyanofurans and thiophenes starting from readily available β-diketones, simple ketones, and β-keto thioamides in up to 95% yield in one step was developed. Mechanistic investigations revealed that a radical process could be involved in this transformation, and a water promoted C-C bond cleavage pathway is proposed for the formation of 2,3-dicyanofurans and thiophenes.

Metalloradical approach to 2H-chromenes

Cobalt(III)-carbene radicals, generated through metalloradical activation of salicyl N-tosylhydrazones by cobalt(II) complexes of porphyrins, readily undergo radical addition to terminal alkynes to produce salicyl-vinyl radical intermediates. Subsequent hydrogen atom transfer (HAT) from the hydroxy group of the salicyl moiety to the vinyl radical leads to the formation of 2H-chromenes. The Co(II)-catalyzed process can tolerate various substitution patterns and produces the corresponding 2H-chromene products in good isolated yields. EPR spectroscopy and radical-trapping experiments with TEMPO are in agreement with the proposed radical mechanism. DFT calculations reveal the formation of the salicyl-vinyl radical intermediate by a metalloradical-mediated process. Unexpectedly, subsequent HAT from the hydroxy moiety to the vinyl radical leads to formation of an o-quinone methide intermediate, which dissociates spontaneously from the cobalt center and easily undergoes an endocyclic, sigmatropic ring-closing reaction to form the final 2H-chromene product.

Zinc-catalyzed synthesis of 2-alkenylfurans via cross-coupling of enynones and diazo compounds

A ZnCl₂ catalyzed selective cross-coupling of enynones and diazo compounds, leading to a variety of 2-alkenylfurans, is reported.

Gold catalysed synthesis of 3-alkoxyfurans at room temperature

3-Alkoxyfurans can be readily prepared from aldehydes via a gold-catalysed cyclisation of propargylic alcohols bearing an acetal unit.

A convenient base-mediated synthesis of 3-aryol-4-methyl (or benzyl)-2-methylthio furans from α-oxo ketene dithioacetals and propargyl alcohols via domino coupling/annulations

A convenient base-mediated synthesis of 3-aryol-4-methyl (or benzyl)-2-methylthio furans2has been developed through the domino coupling/annulations from α-oxo ketene dithioacetals1and propargyl alcohols.