Synthesis of Rumphellaone A and Hushinone by a Gold-Catalyzed [2 + 2] Cycloaddition

Enantioselective Hydrofunctionalization of Cyclobutenones: Total Synthesis of gem-Dimethylcyclobutane Natural Products

Cyclobutanes with a gem-dimethyl group are common motifs in natural products. However, strategies for constructing enantioenriched gem-dimethyl cyclobutanes are still underdeveloped. Herein, we report an enantioselective approach to synthesize a broad group of chiral 2,3-disubstituted cyclobutanones through sequential 1,4-conjugate addition/trapping/cross-coupling of readily available cyclobutenones. The intermediate 2-bromocyclobutanone provides a valuable synthetic handle for further coupling transformations. In addition, this strategy was successfully utilized to synthesize gem-dimethyl cyclobutane-containing natural products, including (+)-β-caryophyllene, (-)-raikovenal, (-)-1β,9αH-5-linoleoyloxy-4,5-secocaryophyllen-4-one, and (-)-rumphellanones A-C.

Stereo- and regiocontrol in intermolecular [2+2] cycloadditions between diarylketenes and allenamides to access substituted α-methylenecyclobutanones

The development of intermolecular [2+2] cycloadditions between allenamides and diarylketenes is described. α-Aryldiazo arylketones are employed as ketene precursors that react smoothly with allenamides in the presence of a P(C6F5)3 promoter. High diastereoselectivity (dr > 20 : 1) with endo-regioselectivity can be achieved when two aryl groups of ketenes have opposite electronic properties. The role of P(C6F5)3 is to increase the reaction yields, but also enhancing stereoselectivity.

New-Generation Ligand Design for the Gold-Catalyzed Asymmetric Activation of Alkynes

Gold(I) catalysts are ideal for the activation of alkynes under very mild conditions. However, unlike allenes or alkenes, the triple bond of alkynes cannot be prochiral. In addition, the linear coordination displayed by gold(I) complexes places the chiral ligand far away from the substrate resulting in an inefficient transfer of chiral information. This poses a significant challenge for the achievement of high enantiocontrol in gold(I)-catalyzed reactions of alkynes. Although considerable progress on enantioselective gold(I)-catalyzed transformations has recently been achieved, the asymmetric activation of non-prochiral alkyne-containing small molecules still represents a great challenge. Herein we summarize recent advances in intra- and intermolecular enantioselective gold(I)-catalyzed reactions involving alkynes, discussing new chiral ligand designs that lie at the basis of these developments. We also focus on the mode of action of these catalysts, their possible limitations towards a next-generation of more efficient ligand designs. Finally, square planar chiral gold(III) complexes, which offer an alternative to chiral gold(I) complexes, are also discussed.

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.

Novel Caryophyllane-Related Sesquiterpenoids with Anti-Inflammatory Activity from Rumphella antipathes (Linnaeus, 1758)

Two previously undescribed caryophyllane-related sesquiterpenoids, antipacids A (1) and B (2), with a novel bicyclo[5.2.0] core skeleton, and known compound clovane-2β,9α-diol (3), along with rumphellolide L (4), an esterified product of 1 and 3, were isolated from the organic extract of octocoral Rumphella antipathes. Their structures, including the absolute configurations were elucidated by spectroscopic and chemical experiments. In vivo anti-inflammatory activity analysis indicated that antipacid B (2) inhibited the generation of superoxide anions and the release of elastase by human neutrophils, with IC₅₀ values of 11.22 and 23.53 μM, respectively, while rumphellolide L (4) suppressed the release of elastase with an IC₅₀ value of 7.63 μM.

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.

A Concise Bioinspired Semisynthesis of Rumphellaones A-C and Their C-8 Epimers from β-Caryophyllene

The first semisynthetic route toward rumphellaones B (2) and C (3) and their C-8 epimers as well as the shortest synthesis of rumphellaone A (1) and its C-8 epimer from the most accessible sesquiterpene, β-caryophyllene (4), is presented. Synthetic routes involved caryophyllonic acid as a key intermediate, which was converted to rumphellaone A (and epimer) via acid-catalyzed lactonization and rumphellaone C (and epimer) using one-pot epoxidation-lactonization. Rumphellaone B (2) and its epimer were obtained from rumphellaone A (1) and its epimer, respectively, using Saegusa-Ito oxidation. The absolute configuration at C-8 was confirmed by single-crystal X-ray analysis of rumphellaone B (2) and an acylated derivative of rumphellaone C.

Recent advances in the synthesis of gem-dimethylcyclobutane natural products

Covering: January 2000 to July 2018 gem-Dimethylcyclobutanes are a common motif found in a multitude of natural products, and thus these structures have captivated synthetic chemists for years. However, until the turn of the century, most synthetic efforts relied upon the use of widely available terpenes, such as pinene or caryophyllene, that already contain the gem-dimethylcyclobutane motif. This approach limits the scope of molecules that can be accessed readily. This review highlights recent syntheses in which the gem-dimethylcyclobutane is assembled via de novo approaches. An outlook on the future of this research area is also provided.

Coligand driven diverse organometallation in benzothiazolyl-hydrazone derivatized pyrene: ortho vs. peri C–H activation

Divergent aromatic C–H activation at both ortho and peri positions in polyaromatic hydrocarbons has been successfully accomplished by suitable variation of the coligand.

Synthesis of tricyclo[7.2.1.09,10]dodecan-11-one core ring systems of norditerpenoid alkaloids and racemulosine

A synthetic strategy involving [2 + 2] enyne cycloaddition and pinacol rearrangement to construct common BCD ring systems of norditerpenoid alkaloids and racemulosine was developed.

Metal-free alkene carbooxygenation following tandem intramolecular alkoxylation/Claisen rearrangement: stereocontrolled access to bridged [4.2.1] lactones

Alkene carbooxygenation has attracted considerable attention over the past few decades as this approach provides an efficient access to various oxygen-containing molecules, especially the valuable O-heterocycles. However, examples of catalytic alkene carbooxygenation via a direct C-O cleavage are quite scarce, and the C-O cleavage in these cases is invariably initiated by transition metal-catalyzed oxidative addition. We report here a novel Brønsted acid-catalyzed intramolecular alkoxylation-initiated tandem sequence, which represents the first metal-free intramolecular alkoxylation/Claisen rearrangement. Significantly, an unprecedented Brønsted acid-catalyzed intramolecular alkene insertion into the C-O bond via a carbocation pathway was discovered. This method allows the stereocontrolled synthesis of valuable indole-fused bridged [4.2.1] lactones, providing ready access to biologically relevant scaffolds in a single synthetic step from an acyclic precursor. Moreover, such an asymmetric cascade cyclization has also been realized by employing a traceless chiral directing group. Control experiments favor the feasibility of a carbocation pathway for the process. In addition, biological tests showed that some of these newly synthesized indole-fused lactones exhibited their bioactivity as antitumor agents against different breast cancer cells, melanoma cells, and esophageal cancer cells.

A modular approach to prepare enantioenriched cyclobutanes: synthesis of (+)-rumphellaone A

A modular synthesis of enantioenriched polyfunctionalized cyclobutanes was developed that features an 8-aminoquinolinamide directed C-H arylation reaction. The C-H arylation products were derivatized through subsequent decarboxylative coupling processes. This synthetic strategy enabled a 9-step enantioselective total synthesis of the antiproliferative meroterpenoid (+)-rumphellaone A.

Synthesis of (-)-Hebelophyllene E: An Entry to Geminal Dimethyl-Cyclobutanes by [2+2] Cycloaddition of Alkenes and Allenoates

The first synthesis of hebelophyllene E is presented, along with assignment of its previously unknown relative configuration through synthesis of epi-ent-hebelophyllene E. Development of a catalytic enantioselective [2+2] cycloaddition of alkenes and allenoates provides access to the required chiral geminal dimethylcyclobutanes. Key to its success is the identification of a novel oxazaborolidine catalyst which promotes the cycloaddition in high enantioselectivities with good functional-group tolerance (9 examples, up to 97:3 e.r.). Thus, a late-stage cycloaddition using a fully functionalized alkene, followed by a diastereoselective reduction allows a concise entry to this class of natural products.

Gold- and silver-catalyzed intramolecular annulation and rearrangement of aniline-linked 1,6-enynes containing methylenecyclopropanes

A new synthetic approach for the construction of 1,2-dihydroquinolines having a methylenecyclopropane moiety or a cyclobutene moiety has been developed from gold- and silver-catalyzed annulation of N-tethered 1,6-enynes containing methylenecyclopropanes.

Role of σ,π-Digold(I) Alkyne Complexes in Reactions of Enynes

Gold(I) acetylide and σ,π-digold(I) alkyne complexes derived from one prototypical 1,6-enyne and from 7-ethynyl-1,3,5-cycloheptatriene have been prepared and structurally characterized. Their possible role in gold(I)-catalyzed cycloisomerizations has been studied by experiment and by DFT calculations. Gold(I) acetylides are totally unproductive complexes in the absence of Brønsted acids. Similarly, no cyclizations were observed by heating σ,π-digold(I) alkyne digold(I) at least up to 130 °C. Theoretical studies provide a rationale for the much lower reactivity of digold species in reactions of enynes.

Enantioselective Synthesis of Cyclobutenes by Intermolecular [2+2] Cycloaddition with Non-C2 Symmetric Digold Catalysts

The enantioselective intermolecular gold(I)-catalyzed [2+2] cycloaddition of terminal alkynes and alkenes has been achieved using non-C2-chiral Josiphos digold(I) complexes as catalysts, by the formation of the monocationic complex. This new approach has been applied to the enantioselective total synthesis of rumphellaone A.

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.

Broad scope gold(i)-catalysed polyenyne cyclisations for the formation of up to four carbon–carbon bonds

The polycyclisation of polyeneynes catalyzed by gold(i) has been extended for the first time to the simultaneous formation of up to four carbon–carbon bonds, leading to steroid-like molecules with high stereoselectivity in a single step with low catalyst loadings.

Gold-Catalyzed Vinyl Ether Hydroalkynylation: An Alternative Pathway for the Gold-Catalyzed Intermolecular Reaction of Alkenes and Alkynes

In this report, the gold-catalyzed intermolecular reaction of vinyl ethers and terminal alkynes is investigated. Utilizing a triazole gold catalyst lessens gold decomposition in the presence of the vinyl ether and affords an alkynylation product instead of the [2 + 2] product. This protocol has been expanded to include glycal substrates, which undergo a one-pot alkynylation-Ferrier reaction to produce functionalized sugars in moderate to excellent yields with high diastereoselectivity.

Intramolecular [2 + 2] and [4 + 2] Cycloaddition Reactions of Cinnamylamides of Ethenetricarboxylate in Sequential Processes

Intramolecular [2 + 2] and [4 + 2] cycloaddition reactions of cinnamylamides of ethenetricarboxylate in sequential processes have been studied. Reaction of 1,1-diethyl 2-hydrogen ethenetricarboxylate and trans-cinnamylamines in the presence of EDCI/HOBt/Et₃N led to pyrrolidine products in one pot, via intramolecular [2 + 2], [4 + 2], and some other cyclizations. The types of the products depend on the substituents on the benzene ring and the reaction conditions. Reaction of cinnamylamines without substituents on the benzene ring and with halogens and OMe on the para position at room temperature gave cyclobutane-fused pyrrolidines as major products via [2 + 2] cycloaddition. The reaction at 80 °C in 1,2-dichloroethane gave δ-lactone fused pyrrolidines as major products, probably via ring-opening of the cyclobutanes. Interestingly, reaction of 1,1-diethyl 2-hydrogen ethenetricarboxylate and cinnamylamines bearing electron-withdrawing groups such as NO₂, CN, CO₂Me, CO₂Et, and CF₃ on ortho and para positions in the presence of EDCI/HOBt/Et₃N at room temperature or at 60-80 °C gave tetrahydrobenz[f]isoindolines via [4 + 2] cycloaddition as major products. DFT studies have been performed to explained the observed [2 + 2]/[4 + 2] selectivity.

A Au(i)-catalyzed hydrogen bond-directed tandem strategy to synthesize indeno-chromen-4-one and indeno-quinolin-4-one derivatives

A gold-catalyzed hydrogen bond-directed tandem cyclization strategy to synthesize indeno-chromen-4-one and indeno-quinolin-4-one derivatives has been developed.