Pentannulation Reaction by Tandem Gold(I)-Catalyzed Propargyl Claisen Rearrangement/Nazarov Cyclization of Enynyl Vinyl Ethers

Gold(I)-Catalyzed Rautenstrauch/Hetero-Diels-Alder/Retro-aza-Michael Cascade Reaction for the Synthesis of α-Hydrazineyl-2-cyclopentenones

A one-pot synthesis of ring-fused, α-hydrazineyl-2-cyclopentenone derivatives is achieved by a gold(I)-catalyzed Rautenstrauch/hetero Diels-Alder/ring opening tandem reaction of suitable propargyl esters. By mixing the latter with a dialkylazodicarboxylate in the presence of a gold(I) catalyst, the 1,2-acyloxy migration/cyclization process (Rautenstrauch reaction) leads to cyclopentadienyl ester intermediates which are trapped by the heterodienophile present in situ. This provides strained intermediates which spontaneously undergo highly regioselective ring opening by a retro aza-Michael reaction promoted by the gold(I) catalyst, eventually yielding the target compounds. Six- and seven-membered ring-fused cyclopentenones bearing a pendant α-hydrazineyl moiety can be obtained in moderate to excellent yield (50-98%) by this approach, with a minimal erosion of the initial optical purity when using enantioenriched substrates.

Ni-Catalyzed 1,5-Sigmatropic Ester Shift on Cyclopentadiene Rings: Regioselective Conversion of 5,5-Disubstituted Cyclopentadienes to CH2-Cyclopentadienes

Described herein is a nickel(II)-catalyzed regioselective rearrangement of 5,5-disubstituted cyclopentadienes to fully functionalized CH2-cyclopentadienes via successive 1,5-sigmatropic shifts of the ester group on the quaternary carbon and hydrogen under mild basic conditions. The obtained CH2-cyclopentadienes were also readily applied in the preparation of highly functionalized dibenzo[e,g]azulene derivatives in two steps.

Synthesis of isolable β-chloroenamines from N-alkoxylactams with organometallic reagents

An efficient approach to access isolable β-chloroenamines via nucleophilic addition/dehydration of α-chloro N-alkoxylactam with organolithium and Grignard reagents is reported. This approach is amenable to the synthesis of β-chloroenamines by incorporating various C(sp) and C(sp²) units, such as alkyne, aryl, and heteroaryl moieties. The sequential reaction has a broad substrate scope and can be carried out for a scalable synthesis of β-chloroenamines. Control experiments suggested that both chloro and alkoxy groups act as inductive electron-withdrawing substituents to improve the stability of the enamines.

Recent progress in the homogeneous gold-catalysed cycloisomerisation reactions

This review focuses on recent advancements in the efficacy of gold catalysts for the cycloisomerisation of ynamides, diynes, and 1,n-enynes to build complex molecules, with critical insight into their mechanism and reaction scope.

Gold(I)-Catalyzed Cycloisomerization/Hetero-Diels-Alder Reaction/Ring Opening Cascade to Functionalized Cyclopentadienes

Six- and seven-membered ring-fused, functionalized cyclopentadienes can be obtained in moderate to excellent yields by a cascade process entailing the Au(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization of propargyl vinyl ethers, the hetero-Diels-Alder reaction with dialkylazodicarboxylates, and the spontaneous conversion of cycloaddition products into cyclopentadienes by a highly regioselective cleavage of a C-N bond. Depending on the treatment of the crude reaction mixtures, two types of products can be obtained: cyclopentadienes with pendant hydrazine and aldehyde moieties that intramolecularly react to form hemiaminals are obtained in 43-52% overall yields when the crude reaction mixtures are left over K₂CO₃ in a DCM solution. Instead, by reducing in situ the aldehyde group just after addition of the heterodienophile, the regioselective C-N bond cleavage generates the corresponding cyclopentadienes bearing a hydrazine and an alcohol appendage in excellent yields (66-82%) over four steps, all in one pot. Two examples from the latter class of compounds were also converted into ring-fused, functionalized cyclopentadienes, bearing a protected amino group, by the selective N-N cleavage of the hydrazine moiety.

Brønsted acid promoted substrate-dependent regiodivergent alkynylcyclopropane–cyclopentadiene rearrangement assisted by the internal carbonyl group

Facile Brønsted acid promoted alkynylcyclopropane–cyclopentadiene rearrangement, avoiding the five-membered cycloallene intermediate assisted by the internal carbonyl groups, has been developed.

Pentannulation of N-heterocycles by a tandem gold-catalyzed [3,3]-rearrangement/Nazarov reaction of propargyl ester derivatives: a computational study on the crucial role of the nitrogen atom

The tandem gold(I)-catalyzed rearrangement/Nazarov reaction of enynyl acetates in which the double bond is embedded in a piperidine ring was computationally and experimentally studied. The theoretical calculations predict that the position of the propargylic acetate substituent has a great impact on the reactivity. In contrast to our previous successful cyclization of the 2-substituted substrates, where the nitrogen favors the formation of the cyclized final product, the substitution at position 3 was computed to have a deleterious effect on the electronic properties of the molecules, increasing the activation barriers of the Nazarov reaction. The sluggish reactivity of 3-substituted piperidines predicted by the calculations was further confirmed by the results obtained with some designed substrates.

One-Pot Access to 1,7a-Dihydro-1,3a-ethano-indene and 1,8a-Dihydro-1,3a-ethano-azulene Skeletons by a Sequential Gold(I)-Catalyzed Propargyl Claisen Rearrangement/Nazarov Cyclization/[4+2] Cycloaddition Reaction

An efficient synthetic approach to the tricyclic 1,7a-dihydro-1,3a-ethano-indene and 1,8a-dihydro-1,3a-ethano-azulene skeletons from suitable propargyl vinyl ethers is based on a one-pot, multistep process entailing a gold(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization, a [4+2] cycloaddition of the formed six- or seven-membered ring-fused cyclopentadiene system, and a final protection step for the easy isolation and purification of the products by chromatography.

Nucleophile-assisted cyclization of β-propargylamino acrylic compounds catalyzed by gold(i): a rapid construction of multisubstituted tetrahydropyridines and their fused derivatives

A variety of medicinally important multisubstituted tetrahydropyridines, 4-aryl piperidines, isoquinolines and fused pyridines were prepared in 1–3 step sequences via nucleophile-assisted Au(i)-catalyzed cyclizations of easily available azaenynes.

Ionization of gold (γ-methoxy)vinyl complexes generates reactive gold vinyl carbene complexes

Cationic gold vinyl carbene/allylic cation complexes of the form (E)-[(L)AuC(H)C(H)CAr₂]⁺ OTf^- {L = IPr, Ar = Ph [(E)-5a], L = IPr, Ar = 4-C₆H₄OMe [(E)-5b], L = P(t-Bu)₂ o-biphenyl, Ar = 4-C₆H₄OMe [(E)-5c]} were generated in solution via Lewis acid-mediated ionization of the corresponding gold (γ-methoxy)vinyl complexes (E)-(L)AuC(H)C(H)C(OMe)Ar₂ at or below -95 °C. Complexes (E)-5b and (E)-5c were fully characterized in solution employing multinuclear NMR spectroscopy, which established the predominant contribution of the aurated allylic cation resonance structure and the significant distribution of positive charge into the γ-anisyl rings. Complex (E)-5b reacted rapidly at -95 °C with neutral two-electron, hydride, and oxygen atom donors exclusively at the C1 position of the vinyl carbene moiety and with p-methoxystyrene to form the corresponding vinylcyclopropane. In the absence of nucleophile (E)-5a decomposed predominantly via intermolecular carbene dimerization whereas formation of 1-aryl-5-methoxy indene upon ionization of (Z)-(IPr)AuC(H)C(H)C(OMe)(4-C₆H₄OMe)₂ [(Z)-6b] implicated an intramolecular Friedel-Crafts or electrocyclic Nazarov pathway for the decomposition of the unobserved vinyl carbene complex (Z)-[(IPr)AuC(H)C(H)C(4-C₆H₄OMe)₂]⁺ OTf^- [(Z)-5b].

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.