Cyclobutenes as Isolable Intermediates in the Gold(I)-Catalysed Cycloisomerisation of 1,8-Enynes

Stereoselective Approach to the Core Structure of (+)-Phainanoid A via Strategically Engineered Cascade Polyene Cyclization

Stereoselective synthesis of 3b and its cascade polyene cyclization to 18b have been described. Acyclic polyene 3b was prepared from allyl bromide 4 and 1,3-dithiane 5, and intermediates 4 and 5 were synthesized from the commercially available geraniol (6) and cyclopenten-2-one (8), respectively, using enantioselective reduction of ketone, Johnson-Claisen rearrangement, and the Suzuki reaction as key steps. Au(I)-mediated diastereoselective polyene cyclization of 3b efficiently afforded tetracyclic compound 18b.

Photochemical α-selective radical ring-opening reactions of 1,3-disubstituted acyl bicyclobutanes with alkyl halides: modular access to functionalized cyclobutenes

Although ring-opening reactions of bicyclobutanes bearing electron-withdrawing groups, typically with β-selectivity, have evolved as a powerful platform for synthesis of cyclobutanes, their application in the synthesis of cyclobutenes remains underdeveloped. Here, a novel visible light induced α-selective radical ring-opening reaction of 1,3-disubstituted acyl bicyclobutanes with alkyl radical precursors for the synthesis of functionalized cyclobutenes is described. In particular, primary, secondary, and tertiary alkyl halides are all suitable substrates for this photocatalytic transformation, providing ready access to cyclobutenes with a single all-carbon quaternary center, or with two contiguous centers under mild reaction conditions.

Gold(I)-Catalyzed 1,6-Enyne Single-Cleavage Rearrangements: The Complete Picture

We identify the factors that rule the selectivity in single-cleavage skeletal rearrangements promoted by gold(I) catalysts. We find that stereoconvergence is enabled by a rotational equilibrium when electron-rich substituents are used. The anomalous Z-selective skeletal rearrangement is found to be due to electronic factors, whereas endo-selectivity depends on both steric and electronic factors.

Synthetic Strategy for Construction of Highly Congested Tetracyclic Core (6-5-7-4) of Harziane Diterpenoids

The structurally intriguing tetracyclic core of complex harziane diterpenoid was constructed in 14 steps from commercially available 3-ethoxycyclohex-2-en-1-one. The key steps were a Mn/Cu-mediated oxidative 1,3-dicarbonyl radical cascade cyclization reaction, which diastereoselectively formed the core of dimethylbicyclo[3.2.1]octane structure, and a Au-catalyzed diastereoselective formal [2 + 2] cycloaddition for construction of the harziane diterpenoid tetracyclic framework. The developed method paves the way for achieving total synthesis of this type of complex natural product.

Uncommon Four-Membered Building Blocks - Cyclobutenes, Azetines and Thietes

Strained ring systems have gained considerable importance over the last few years for their implication in natural product syntheses or in drug discovery programs. We present herein a recollection of our work on the construction and functionalization of unsaturated four-membered carbo- and heterocycles in the context of the literature, as well as their applications in further reactions.

[Development of Gold-catalyzed Reaction Utilizing Electron Acceptability of Z-type Ligand]

The interaction between transition metals and ligands is important for catalytic reactions. The ligands are largely dominated by the covalent X-type (hydride, alkyl and halogen) and/or dative L-type ligands (e.g., P, N, CO, olefin, etc.). Therefore, the interaction of the Z-type ligands (B, Al and Si, etc.) with transition metals is emerging as a new concept for the reactivity of the metal center. Recently, we developed the synthesis of the gold complex Au(DPB)X (DPB=diphosphine-borane) featuring the Z-type ligand, and their catalytic reaction. The gold catalysts showed a high activity compared to the general catalysts (without Z-ligand) for the various cyclization reactions due to the electron-withdrawing effect of the Z-ligand on the coordinating gold center. In this review, first the structure analysis of the synthesized Au→Z complex is introduced in detail, and second, the catalytic reactions based on the alkyne activation are described.

Selective Synthesis of Phenanthrenes and Dihydrophenanthrenes via Gold-Catalyzed Cycloisomerization of Biphenyl Embedded Trienynes

Readily available o'-alkenyl-o-alkynylbiaryls, a particular type of 1,7-enynes, undergo a selective cycloisomerization reaction in the presence of a gold(I) catalyst to give interesting phenanthrene and dihydrophenanthrene derivatives in high yields. The solvent used provokes a switch in the evolution of the gold intermediate and plays a key role in the reaction outcome.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.

Highly Strained Organogold Complexes and Their Gold- or Rhodium-Catalyzed Isomerizations

The synthesis and characterization of (NHC)(tricyclo[4.1.0.0^2,7 ]hept-1-yl)gold(I) compounds is reported. Several of these organometallic compounds could be obtained by transmetalation from the organolithium precursor to the corresponding (NHC)gold(I) chlorides. These are by far the most strained stable organogold compounds reported so far. With these new compounds we were able to efficiently and selectively accomplish the first gold-catalyzed rearrangement of organogold compounds, even better yields were obtained by the first rhodium-catalyzed rearrangement of these organogold species. (E/Z)-Mixtures of (NHC)(cyclohex-2-en-1-ylidenemethyl)gold(I) were obtained, none of the other known rearrangement products of tricycloheptanes. Additional insight into the chemical and physical properties of the strained compounds was obtained by spectroscopic and computational studies.

Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes: The Complete Mechanistic Picture

The intermolecular gold(I)-catalyzed reaction between arylalkynes and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which takes place stepwise, first by formation of cyclopropyl gold(I) carbenes, followed by a ring expansion. However, 1,3-butadienes are also formed in the case of ortho-substituted arylalkynes by a metathesis-type process. The corresponding reaction of alkenes with aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed reaction of alkynes with alkenes is proposed on the basis of density functional theory calculations, which shows that the two pathways leading to cyclobutenes or dienes are very close in energy. The key intermediates are cyclopropyl gold(I) carbenes, which have been independently generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa[a]naphthalenes.

Divergent reaction pathways in gold-catalyzed cycloisomerization of 1,5-enynes containing a cyclopropane ring: dramatic ortho substituent and temperature effects

A gold(i)-catalyzed cycloisomerization of easily available 1,5-enynes containing a cyclopropane ring has been developed, efficiently providing cyclobutane-fused 1,4-cyclohexadiene, tricyclic cyclobutene, biscyclopropane and 1,3-cyclohexadiene derivatives in moderate to excellent yields. When the phenyl group was not ortho substituted, 1,4-cyclohexadienes could be produced. With an ortho substituent, three different products could be selectively synthesized by control of the temperature and the used gold(i) catalyst. The 1,5-enyne substrate first undergoes a classical enyne cycloisomerization to form a tricyclic cyclobutene key intermediate, which undergoes subsequent transformation to produce the desired products. A plausible reaction mechanism was proposed according to deuterium labeling experiments and intermediate trapping experiments, as well as DFT calculations. In our current reaction, the ortho substituent on the phenyl group controls the reaction outcome and the ortho substituent effect was found to originate from steric and electronic factors.

Gold-Catalyzed Reactions via Cyclopropyl Gold Carbene-like Intermediates

Cycloisomerizations of 1,n-enynes catalyzed by gold(I) proceed via electrophilic species with a highly distorted cyclopropyl gold(I) carbene-like structure, which can react with different nucleophiles to form a wide variety of products by attack at the cyclopropane or the carbene carbons. Particularly important are reactions in which the gold(I) carbene reacts with alkenes to form cyclopropanes either intra- or intermolecularly. In the absence of nucleophiles, 1,n-enynes lead to a variety of cycloisomerized products including those resulting from skeletal rearrangements. Reactions proceeding through cyclopropyl gold(I) carbene-like intermediates are ideally suited for the bioinspired synthesis of terpenoid natural products by the selective activation of the alkyne in highly functionalized enynes or polyenynes.

Air-stable cationic gold(I) catalyst featuring a Z-type ligand: promoting enyne cyclizations

An air-stable cationic Au(I) complex featuring a Z-type ligand (boron atom) as a σ-acceptor was developed for elucidating the effect of B on catalytic reactions. An enyne cyclization in the presence of either [Au→B](+) or [Au](+) showed that [Au→B](+) promotes the reactivity, which enabled the effective construction of not only five- and six-membered rings, but also seven-membered rings.

Gold-catalyzed cycloisomerization of 1,6-diyne esters to 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes

A synthetic method to chemoselectively prepare 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes efficiently by gold-catalyzed cycloisomerization of 1,6-diyne esters is described. These three product classes were accessed divergently by taking advantage of the electronic and steric differences between a phosphine and NHC (NHC = N-heterocyclic carbene) ligand in the respective gold(I) complexes and that of gold(III) complex combined with substrate substitution patterns and optimized reaction conditions. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, substrates with a pendant aryl group at the acetate alkynyl position were found to undergo preferential 1,3-acyloxy migration/5-exo-dig cyclization/Friedel-Crafts reaction to give 1H-cyclopenta[b]naphthalenes. In contrast, the analogous reactions with PicAuCl2 (Pic =2-picolinate) were found to proceed by selective 1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to afford cis-cyclopenten-2-yl δ-diketones. Changing the catalyst to [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) and the acetate alkynyl position from an aryl to vinyl substituent in the starting ester led to 1,3-acyloxy migration/5-exo-dig cyclization/Prins-type [2 + 2]-cycloaddition to provide bicyclo[3.2.0]hepta-1,5-dienes.

Gold-catalyzed rearrangements and beyond

Cycloisomerizations of enynes are probably the most representative carbon-carbon bond forming reactions catalyzed by electrophilic metal complexes. These transformations are synthetically useful because chemists can use them to build complex architectures under mild conditions from readily assembled starting materials. However, these transformations can have complex mechanisms. In general, gold(I) activates alkynes in the presence of any other unsaturated functional group by forming an (η(2)-alkyne)-gold complex. This species reacts readily with nucleophiles, including electron-rich alkenes. In this case, the reaction forms cyclopropyl gold(I) carbene-like intermediates. These can come from different pathways depending on the substitution pattern of the alkyne and the alkene. In the absence of external nucleophiles, 1,n-enynes can form products of skeletal rearrangement in fully intramolecular reactions, which are mechanistically very different from metathesis reactions initiated by the [2 + 2] cycloaddition of a Grubbs-type carbene or other related metal carbenes. In this Account, we discuss how cycloisomerization and addition reactions of substituted enynes, as well as intermolecular reactions between alkynes and alkenes, are best interpreted as proceeding through discrete cationic intermediates in which gold(I) plays a significant role in the stabilization of the positive charge. The most important intermediates are highly delocalized cationic species that some chemists describe as cyclopropyl gold(I) carbenes or gold(I)-stabilized cyclopropylmethyl/cyclobutyl/homoallyl carbocations. However, we prefer the cyclopropyl gold(I) carbene formulation for its simplicity and mnemonic value, highlighting the tendency of these intermediates to undergo cyclopropanation reactions with alkenes. We can add a variety of hetero- and carbonucleophiles to the enynes in the presence of gold(I) in intra- or intermolecular reactions, leading to the corresponding adducts with high stereoselectivity through stereospecific anti-additions. We have also developed stereospecific syn-additions, which probably occur through similar intermediates. The attack of carbonyl groups at the cyclopropyl carbons of the intermediate cyclopropyl gold(I) carbenes initiates a particularly interesting group of reactions. These trigger a cascade transformation that can lead to the formation of two C-C and one C-O bonds. In the fully intramolecular process, this stereospecific transformation has been applied for the synthesis of natural sesquiterpenoids such as (+)-orientalol F and (-)-englerin A. Intra- and intermolecular trapping of cyclopropyl gold(I) carbenes with alkenes leads to the formation of cyclopropanes with significant increase in the molecular complexity, particularly in cases in which this process combines with the migration of propargylic alkoxy and related OR groups. We have recently shown this in the stereoselective total synthesis of the antiviral sesquiterpene (+)-schisanwilsonene by a cyclization/1,5-acetoxy migration/intermolecular cyclopropanation. In this synthesis, the cyclization/1,5-acetoxy migration is faster than the alternative 1,2-acyloxy migration that would result in racemization.

Intriguing mechanistic labyrinths in gold(I) catalysis

Many mechanistically intriguing reactions have been developed in the last decade using gold(I) as catalyst. Here we review the main mechanistic proposals in gold-catalysed activation of alkynes and allenes, in which this metal plays a central role by stabilising a variety of complex cationic intermediates.

Aerobic oxygenative cleavage of electron deficient C–C triple bonds in the gold-catalyzed cyclization of 1,6-enynes

Gold-catalyzed aerobic oxygenative cleavage of triple bonds that occurs under the ambient pressure of air and at room temperature is reported; radical inhibition tests suggest that oxygenation occurs via a gold-bound metalloradical intermediate.

Nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes

A nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes has been developed. A variety of electron-deficient alkenes as well as electronically neutral norbornene and 1-decene were applicable to this reaction. The use of conjugated enynes circumvented possible side rections, such as oligomerizations and cyclotrimerizations. The isolation of reaction intermediate complexes revealed that the η(3)-butadienyl coordination is the key for the selective formation of cyclobutene.

The role of cyclobutenes in gold(I)-catalysed skeletal rearrangement of 1,6-enynes

1,6-Enynes with electron-donating substituents at the alkyne undergo gold(I)-catalysed single cleavage skeletal rearrangement, whereas substrates with electron-withdrawing substituents evolve selectively to double cleavage rearrangement. Theoretical calculations provide a qualitative rationale for these effects, and suggest that bicyclo[3.2.0]hept-5-enes are involved as intermediates. We provide the first X-ray structural evidence for the formation of a product of this class in a cycloisomerisation of a 1,6-enyne.