Silver(I)-Catalyzed Domino Cyclization/Cyclopropanation/Ring-Cleavage/Nucleophilic Substitution Reaction of Enynones with Enamines: Synthesis of 4-(Furan-2-yl)-3,4-dihydro-2H-pyrrol-2-one

Regiodivergent formal [4+2] cycloaddition of nitrosoarenes with furanyl cyclopropane derivatives as 4π components

Cyclopropanes are commonly used as valuable 3-carbon building blocks. Herein, we disclose a different reactivity pattern of furanyl cyclopropanes, which serve as a 4-carbon component in Lewis acid-promoted [4+2] cycloadditions with nitrosoarenes to afford 1,2-oxazine derivatives. Importantly, the regioselectivity of the cycloaddition reaction can be controlled by the appropriate choice of the Lewis acid.

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.

Controllable Access to Diazo-functionalized 2-Methylene-2,3-dihydrofurans and Diazo-functionalized Furans from Enynones and Diazo Carbonyl Compounds

Using enynones and diazo carbonyl compounds as identical starting materials, methods for chemoselective and regioselective constructs of diazo-functionalized 2-methylene-2,3-dihydrofurans and diazo-functionalized trisubstituted furans have been established in a AgSbF6/DBU/DCE/0 °C system and a AgSbF6/DBU/Et2O·BF3/DCE/0 °C system, respectively. A Lewis acid and organic base cocontrolled reaction for the synthesis of diazo-functionalized trisubstituted furans is infrequent. For diazo-functionalized 2-methylene-2,3-dihydrofuran synthesis, the reaction possesses excellent diastereoselectivity and Z-selectivity. On the basis of Rh2(OAc)4-mediated unique decomposition of diazo-functionalized 2-methylene-2,3-dihydrofurans, an application to diastereoselective construction of a 5-methylene-4,7-dihydro-5H-furo[2,3-c]pyran frame has been achieved for the first time.

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.

Cu-Catalyzed tandem cyclization and coupling of enynones with enaminones for multisubstituted furans & furano-pyrroles

A synthetic strategy that efficiently constructs complex molecular diversity in a few steps will always be embraced by organic chemists. Here, we report a cascade reaction of enynones with enaminones via carbene insertion and aryl migration to engineer distinctive multisubstituted furans with an all-carbon quaternary center, and could extend the protocol in the same pot towards furano-pyrrole bis-heterocycles. Heterogeneity of this protocol was proved with the upshot of divergent chemical space under a relatively mild reaction environment.

Construction of the 2-Amino-1,3-selenazole Skeleton via PhICl2/KSeCN-Mediated Selenocyanation/Cyclization

The construction of 2-amino-1,3-selenazole skeleton was realized via the PhICl₂/KSeCN-enabled electrophilic selenocyanation of β-enaminones and β-enamino esters followed by intramolecular cyclization under basic conditions. Compared to the classical Hantzsch strategy that utilizes selenourea or its analogues as starting materials or crucial intermediates, this method might represent an alternative approach for the assembly of 1,3-selenazole framework through a different pathway.

Ketone Derivatives of Propargylamines as Synthetic Equivalents of Conjugated 2,4,1-Enynones in the Synthesis of Acetylenic 2-Pyrazolines and Pyrazoles

An interaction of 1,5-diaryl-3-X-pent-4-yn-1-ones (where X stands for piperidin-1-yl, morpholin-4-yl, 4-methylpiperazin-1-yl) with arylhydrazines proceeds at room temperature and results in 3-aryl-5-arylethynyl-1-phenyl-4,5-dihydro-1H-pyrazoles with up to 57-73% yields. Under similar conditions, the cyclocondensation of conjugated 2,4,1-enynones with arylhydrazine proceeds only in the presence of cyclic amines. 1,5-Diaryl-3-X-pent-4-yn-1-ones are reported as synthetic equivalents of conjugated 2,4,1-enynones in reactions with arylhydrazines. On the basis of obtained data, there are highly efficient methods developed for the synthesis of 5-arylethynyl-substituted 4,5-dihydro-1H-pyrazoles, as well as for similarly structured 1H-pyrazoles prepared by oxidation in AcOH. Presented products possess quite marked fluorescent abilities. Emission maximum wavelengths are located at 453-465 and 363-400 nm, respectively; certain compounds show extremely large Stokes shifts that may reach 91,000 cm^-1.

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.

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.

Chemo- and Enantioselective Insertion of Furyl Carbene into the N-H Bond of 2-Pyridones

Asymmetric carbene insertion reactions represent one of the most important protocols to construct carbon-heteroatom bonds. The use of donor-acceptor diazo compounds bearing an ester group is however a prerequisite for achieving high enantioselectivity. Herein, we report a chemo- and enantioselective formal N-H insertion of 2-pyridones that has been accomplished for the first time with enynones as the donor-donor carbene precursors. DFT calculations indicate an unprecedented enantioselective 1,4-proton transfer from O to C. The rhodium catalyst provides a chiral pocket in which the steric repulsion and the π-π interaction of the propeller ligand play a critical role in determining the selectivities.

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.