Gold-Catalyzed Cyclization of Furan-Ynes bearing a Propargyl Carbonate Group: Intramolecular Diels-Alder Reaction with In Situ Generated Allenes

Divergent Access to Polycyclic Fluorenofuran Systems via Palladium-Catalyzed Cascade Furan-Participated Diels-Alder Cycloaddition Reactions

The Pd-catalyzed reaction of 1-(2-iodophenyl)-3-arylprop-2-yn-1-ones with benzene-tethered furan-ynes produces novel fused hexacyclic indenone-furan scaffolds, while the reaction with propargyl furfuryl ethers affords the pentacyclic indenone-spirodifuran system. The thiophene-containing alkynes also proceed by similar transformations. A mechanism involving the Pd-catalyzed coupling and propargylic Alder-ene reaction to generate the indenone-allene intermediate, followed by an intramolecular furan/thiophene-participated inverse electron-demand Diels-Alder reaction, is proposed.

Unveiling the Chemistry and Synthetic Potential of Catalytic Cycloaddition Reaction of Allenes: A Review

Allenes with two carbon-carbon double bonds belong to a unique class of unsaturated hydrocarbons. The central carbon atom of allene is sp hybridized and forms two σ-bonds and two π-bonds with two terminal sp² hybridized carbon atoms. The chemistry of allenes has been well documented over the last decades. They are more reactive than alkenes due to higher strain and exhibit significant axial chirality, thus playing a vital role in asymmetric synthesis. Over a variety of organic transformations, allenes specifically undergo classical metal catalyzed cycloaddition reactions to obtain chemo-, regio- and stereoselective cycloadducts. This review briefly describes different types of annulations including [2+2], [2+2+1], [3+2], [2+2+2], [4+2], [5+2], [6+2] cycloadditions using titanium, cobalt, rhodium, nickel, palladium, platinum, gold and phosphine catalyzed reactions along with a mechanistic study of some highlighted protocols. The synthetic applications of these reactions towards the synthesis of natural products such as aristeromycin, ent-[3]-ladderanol, waihoensene(-)-vindoline and (+)-4-epi-vindoline have also been described.

Gold-Catalyzed Spirocyclization of Furan-ynones and Unexpected Skeleton Rearrangement of the Resulting Spirohydrofurans

A gold-catalyzed cyclization of aniline-tethered furan-ynones has been developed. The reaction proceeds via trapping of the resulting stabilized cationic intermediate with an amide group leading to polycycles featured with a spiro-cyclohexadienone-hydrofuran framework with high efficiency. The resulting N-alkyl products undergo photorearrangements to afford the ring-enlarged benzo[b]azepine derivatives or iron-promoted novel rearrangement to diketone-containing spirocycles involving multiple C-X bond cleavages and formations.

Intramolecular iron-catalyzed transannulation of furans with O-acetyl oximes: synthesis of functionalized pyrroles

Intramolecular iron(iii)-catalyzed reaction of furyl-tethered O-acetyl oximes yields substituted pyrroles through electrophilic 5-exo-trig ipso-addition of nitrogen to the furan ring.

Gold Catalysis and Furans: A Powerful Match for Synthetic Connections

This review summarizes the advances made on the synthesis and functionalization of furans via gold catalysis during the period between 2016 and 2020. A separate section is dedicated to the tandem gold-catalyzed synthesis and functionalization of furans.

1 Introduction

2 Gold-Catalyzed Synthesis of Furans

2.1 Cycloisomerizations of Alkynyl and Cumulenyl Alcohols

2.2 Cycloisomerizations of Alkynyl and Allenyl Ketones

2.3 Reactions with External Oxidants

2.4 Miscellaneous

3 Gold-Catalyzed Functionalization of Furans

3.1 Cycloadditions

3.2 Furan Ring Decorations

3.3 Reactions Involving Furan Ring Opening

4 Gold-Catalyzed Tandem Synthesis and Functionalization of Furans­

4.1 Cycloisomerizations Followed by Gold-Catalyzed Cycloaddition

4.2 Cycloisomerizations to a Gold 1,3- or 1,4-Dipole and Intermolecular Annulation

4.3 Cycloisomerizations to a Gold Carbene and Intermolecular Trapping

5 Conclusion

Gold(I)-Catalyzed Haloalkynylation of Aryl Alkynes: Two Pathways, One Goal

Haloalkynylation reactions provide an efficient method for the simultaneous introduction of a halogen atom and an acetylenic unit. For the first time, we report a gold(I)‐catalyzed haloalkynylation of aryl alkynes that delivers exclusively the cis addition product. This method enables the simple synthesis of conjugated and halogenated enynes in yields of up to 90 %. Notably, quantum chemical calculations reveal an exceptional interplay between the place of the attack at the chloroacetylene: No matter which C−C bond is formed, the same enyne product is always formed. This is only possible through rearrangement of the corresponding skeleton. Hereby, one reaction pathway proceeds via a chloronium ion with a subsequent aryl shift; in the second case the corresponding vinyl cation is stabilized by a 1,3‐chlorine shift. ¹³C‐labeling experiments confirmed that the reaction proceeds through both reaction pathways.

Intramolecular azavinyl carbene-triggered rearrangement of furans

An intramolecular rhodium-catalyzed reaction of 1-tosyl-1,2,3-triazoles with furans has been explored. The tosylimino functionality was found to play a significant chemical role participating in the subsequent domino-transformations of a key reaction intermediate. One of the reaction pathways leads to valuable 2-formyl- and 2-acetylpyridine building blocks, which could be obtained in one-pot starting from easily accessible (furan-2-ylmethyl)propargyl amines. An original method for the synthesis of highly functionalized indolizines from the obtained pyridines has been proposed.

Strain-Activated Diels-Alder Trapping of 1,2-Cyclohexadienes: Intramolecular Capture by Pendent Furans

Intramolecular [4 + 2] cycloaddition reactions of substituted 1,2-cyclohexadienes with pendent furans enables the synthesis of complex tetracyclic scaffolds in a single step under mild conditions. All Diels-Alder cycloadducts were obtained as single diastereomers, assigned as the endo isomer. Substrates were easily assembled via Stork-Danheiser alkylation of 3-ethoxy-2-bromocyclohex-2-enone to accommodate a range of tethers and furan traps. Cleavage of enol acetate moieties resulted in room-temperature Diels-Alder cycloreversion to tethered furyl cyclohexenones.

Gold-Catalyzed Homogeneous (Cyclo)Isomerization Reactions

Gold is currently one of the most used metals in organometallic catalysis. The ability of gold to activate unsaturated groups in different modes, together with its tolerance to a wide range of functional groups and reaction conditions, turns gold-based complexes into efficient and highly sought after catalysts. Natural products and relevant compounds with biological and pharmaceutical activity are often characterized by complex molecular structures. (Cyclo)isomerization reactions are often a useful strategy for the generation of this molecular complexity from synthetically accessible reactants. In this review, we collect the most recent contributions in which gold(I)- and/or gold(III)-catalysts mediate intramolecular (cyclo)isomerization transformations of unsaturated species, which commonly feature allene or alkyne motifs, and organize them depending on the substrate and the reaction type.

Ligand-Effect in Gold(I)-Catalyzed Rautenstrauch Rearrangement: Regio- and Stereoselective Synthesis of Bicyclo[3.2.1]octa-3,6-dienes through Cyclodimerization of 1-Ethynyl-2-propenyl Esters

Gold(I) complexes bearing sterically demanding phosphine ligands such as ^tBuXphos catalyze the cascade Rautenstrauch rearrangement/[4 + 3] cycloaddition of 1-ethynyl-2-propenyl esters. The reaction provides an efficient and straightforward route to bicyclo[3.2.1]octa-3,6-dienes with high regio- and stereoselectivity. The formation of the [4 + 3] cycloadducts likely proceeds through the cycloaddition of a gold(I) carbenoid/gold-stabilized allyl cation intermediate with cyclopentadiene arising from Rautenstrauch rearrangement.