Ag(I)-Catalyzed Tandem Reaction of Conjugated Ene-yne-ketones in the Presence of PhI(OAc)2 and Triethylamine: Synthesis of 2-Alkenylfurans

Synthesis of Highly Substituted Furans via Intermolecular Enynone-Aldehyde Cross-Coupling/Cyclization Catalyzed by N-Heterocyclic Carbenes

A new strategy for facile access to multifunctionalized furans via N-heterocyclic carbene-catalyzed cross-coupling/cyclization of ynenones with aldehydes has been explored. This protocol features readily obtainable starting materials, mild and metal-free conditions, broad substrate scope, good functional group tolerance, excellent yields, and easy scale-up. Synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and postmodifications of the product.

Access to 2-Alkenyl-furans via a Cascade of Pd-Catalyzed Cyclization/Coupling Followed by Oxidative Aromatization with DDQ

An unprecedented DDQ-mediated oxidative aromatization of 2-bezylidene-dihydrofurans yielding 2-alkenyl-furans is disclosed. Integration of this transformation with a prior Pd-catalyzed reaction of α-propargylic-β-ketoesters and (hetero)aryl halides into a one-pot cascade process opens a direct modular route to highly substituted 2-vinyl-furans. Experimental and computational studies reveal that the crucial step of the oxidative-aromatization involves facile hydride transfer from the dihydrofuran ring to the O-center of DDQ.

Highly Diastereoselective Synthesis and Application of Functionalized 2,3-Dihydrofuran Derivatives from Enynones and Bis(diazo) Compounds

A highly efficient Ag(I)-catalyzed cascade Michael addition/cyclization of enynones with 1,3-(bis)diazo compounds has been established, providing functionalized 2,3-dihydrofuran derivatives containing a diazo group and an acetylenic bond with excellent diastereoselectivity. Transformation of the diazo group and hydration of the carbon-carbon triple bond have been performed successfully in different reaction systems.

Silver(I)-Catalyzed Tandem Reaction of Enynones and 4-Alkenyl Isoxazoles: Synthesis of 2-(Furan-2-yl)-1,2-dihydropyridines

Silver(I)-catalyzed tandem reaction of enynones with 4-alkenyl isoxazoles provides access to 2-(furan-2-yl)-1,2-dihydropyridines. No competitive cyclopropanation of alkenes and O-H insertion via (2-furyl)carbene complexes were observed. The cascade reaction proceeds via the formation of (2-furyl)metal carbene intermediate, the N-O bond cleavage of 4-alkenyl isoxazoles/rearrangement, subsequent 6π electrocyclic reaction, and [1,5] H-shift. The successive construction of both 1,2-dihydropyridine skeleton and furan frame has been achieved in the one-pot reaction. A broad range of readily available enynones and 4-alkenyl isoxazoles are suitable to this protocol; however, when R³ is the alkyl group such as n-Bu and Me, a complicated mixture was generated without the desired products. In addition, in the case of R⁴ = bulky group such as R₃^'SiOCH₂, the reaction gave an in situ oxo-product of (2-furyl)silver carbene. An atom-economic strategy for the synthesis of 2-(furan-2-yl)-1,2-dihydropyridines has been established.

Palladium-catalyzed acetalization/cyclization of enynones with alcohols: rapid access to functionalized dihaloalkenyl dihydrofurans

A novel Pd-catalyzed acetalization/cyclization of enynones and alcohols for the construction of dihaloalkenyl dihydrofuran derivatives is described.

Preparation of 4-(Nitromethyl)furan Derivatives and Their Application in the Syntheses of Bis(furan-2-yl)oximes

An efficient method for the preparation of tetrasubstituted furans, which contains a nitromethyl group at the 4-position, has been developed. The applications of 4-(nitromethyl)furans on the synthesis of highly functionalized bis(furyl)oxime were explored for the first time.

AgOTf/I2-Mediated Cyclization/Cross-Coupling/Isomerization of Enynones with Phosphorus Ylides: An Expedient Route to Stereoselective Synthesis of (E)-2-Alkenylfurans

Ag(I)-catalyzed cascade reactions involving enynone cyclization and cross-coupling with phosphorus ylides have been achieved for the first time. Subsequent treatment of the reaction mixture with I₂ afforded the corresponding (E)-α-alkenylfurans in 73-95% yields with excellent stereoselectively. A reasonable mechanism has been proposed.

Oxymetalation or oxidative cyclization? mechanism of Pd-catalyzed annulation of enynones

Enynones are powerful synthons for constructing furan derivatives in the presence of transition metal catalysts. Unlike the conventional intramolecular nucleophilic attack with the activation of coinage metals, we propose that enynones undergo an oxidative cyclization process with a Pd(0) species. The full catalytic cycle involves oxidative cyclization, isocyanide insertion, and reductive elimination, which was supported by DFT calculations. Geometric and electronic analyses confirmed the oxidative cyclization process, which proceeds via a Pd(ii) intermediate.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Synthesis of 2-alkenylfurans via a Ag(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides

A silver(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides to construct carbon-carbon double bonds has been developed. The strategy provides a novel method for the synthesis of 2-alkenylfurans. This is the first cross-coupling reaction between metal-carbene complexes and iodonium ylides.

Syntheses of Z-Iodovinylfurans and 2-Acyl Furans via Controllable Cyclization of Ynenones

A divergent synthesis of Z-iodovinylfurans and 2-acyl furans promoted by NIS via controllable cyclization of ynenones is reported. The reaction proceeded by sequential 5-exo-dig electrophilic cyclization to intermediate 2-(iodomethylene)-2H-furanium cation D, providing a range of synthetically valuable and useful trisubstituted furan derivatives 2 and 3 in moderate to excellent yields. This approach is metal-free, mild, and atom-economic, with good selectivity and high stereoselectivity.