Diels–Alder Reaction of Tropones with Arynes: Synthesis of Functionalized Benzobicyclo[3.2.2]nonatrienones

Substrate-Controlled [8 + 3] Cycloaddition of Tropsulfimides and Tropones with Zwitterionic Allenyl Palladium Species Derived from Vinylidenecyclopropane-diesters

A palladium-catalyzed regioselective [8 + 3] cycloaddition of tropsulfimides and tropones with vinylidenecyclopropane-diesters (VDCP-diesters) has been disclosed in this paper, affording decahydro-1H-cyclohepta[b]pyridine derivatives bearing an allene moiety or decahydro-1H-cyclohepta[b]pyran derivatives having a conjugated diene unit in moderate to good yields. The reactions proceed through a zwitterionic allenyl palladium species derived from VDCP-diesters. The substrate scopes have been investigated and the plausible reaction mechanisms have also been proposed according to the previous work, the first captured zwitterionic Pd-allenyl intermediate, and control experiments.

Total Syntheses of the Tropolone-Containing Sesquiterpene Olaximbriside A and Its Decarbonylated Counterpart Olaximbriside B

The racemic modification of the α-tropolone-containing sesquiterpene olaximbriside A [viz. (±)-4)] has been prepared over 12 steps from the readily accessible decalin derivative 12. The last two of these steps involve a fully regiocontrolled substitution reaction of bromotropone 24. The aromatization of a stereoisomeric and co-produced form of compound 12 has provided (±)-olaximbriside B [(±)-5)].

Discovery of colchicine aryne cycloadduct as a potent molecule for the abrogation of epithelial to mesenchymal transition via modulating cell cycle regulatory CDK-2 and CDK-4 kinases in breast cancer cells

In this study, we synthesized a new-generation library of colchicine derivatives via cycloaddition of colchicine utilizing position C-8 and C-12 diene system regioselectivity with aryne precursor to generate a small, focused library of derivatives. We assessed their anticancer activity against various cancer cell lines like MCF-7, MDA-MB-231, MDA-MB-453, and PC-3. Normal human embryonic kidney cell line HEK-293 was used to determine the toxicity. Among these derivatives, silicon-tethered compound B-4a demonstrated the highest potency against breast cancer cells. Subsequent mechanistic studies revealed that B-4a effectively modulates cell cycle regulatory kinases (CDK-2 and CDK-4) and their associated cyclins (cyclin-B1, cyclin-D1), inducing apoptosis. Additionally, B-4a displayed a noteworthy impact on tubulin polymerization, compared to positive control flavopiridol hydrochloride in a dose-dependent manner, and significantly disrupted the vimentin cytoskeleton, contributing to G1 arrest in breast cancer cells. Moreover, B-4a exhibited substantial anti-metastatic properties by inhibiting breast cancer cell migration and invasion. These effects are attributed to the down-regulation of major epithelial to mesenchymal transition (EMT) factors, including vimentin and Twist-1, and the upregulation of the epithelial marker E-cadherin in an apoptosis-dependent manner.

Exploring Heterotropones and Examining Their Propensity to Undergo [4 + 2] Cycloadditions

An efficient strategy to obtain a broad array of chiral and achiral heterotropones and their corresponding [4 + 2] cycloadducts is disclosed. This strategy enables access to unique heterotropones and intricate bicyclic structures in high yields and diastereoselectivities through a simple procedure and from easily accessible starting materials.

Quantum Chemical Study of the Cycloaddition Reaction of Tropone with 1,1-Diethoxyethene Catalyzed by B(C6F5)3 or BPh3

Cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by Lewis acid (LA), B(C6F5)3 or BPh3, was examined by using ωB97X-D-level density functional theory (DFT) calculations. In the absence of LA, the reaction proceeds in a stepwise fashion to form two chemical bonds, first between the C2 atom in tropone and the C2 atom in ethene and then between the C5 atom in the former and the C1 atom in the latter. When B(C6F5)3 is attached to the O atom in tropone, the C5 atom in tropone is attacked preferentially by the C1 atom in ethene in the second stage. The attack of the O atom in tropone is shown to be less likely; thus, the [4 + 2] addition is favored in the B(C6F5)3-catalyzed reaction. In contrast, the attack of the O atom in the BPh3-attached tropone to the C1 atom in ethene is preferred over the attack of the C5 atom, indicating that the [8 + 2] cycloaddition instead of the [4 + 2] cycloaddition proceeds in the BPh3-catalyzed reaction. Whether the C1 atom in ethene is attacked by C5 or by O in the second bond formation step is shown in this study to be governed mainly by the nucleophilicity of σ-lone pair electrons of the carbonyl O atom of tropone in the presence of LA. These results are consistent with the experiments reported by Li and Yamamoto.

Aryne Annulations for the Synthesis of Carbocycles and Heterocycles: An Updated Review

Carbocycles and heterocycles are the most relevant structural moieties, which find boundless applications in interdisciplinary areas. In this regard, the interesting chemistry of aryne is highly embraced by the researchers for the synthesis of structurally different polycyclic systems i.e. carbocycles and heterocycles. In this review, we intended to provide a comprehensive overview on annulative approach considering Kobayashi's aryne intermediate as the key reactive species with diverse nucleo-/electrophilic partners. The review highlights the different aryne annulation reactions adopted for the synthesis of carbocycles and heterocycles (covering articles mainly from middle of 20’s), showcasing their viability towards broad range of substrates bearing diverse functionalities.

One-pot synthesis of 3-substituted-3-hydroxyindolin-2-ones: three component coupling of N-protected isatin, aryne precursor and 1,3-cyclodione under metal-free conditions

A transition-metal free synthetic strategy for the direct synthesis of 3-substituted-3-hydroxy-indolin-2-ones via three component coupling of N-protected isatin, aryne precursor, and 1,3-cyclodione.

Tandem [5 + 1]/[8 + 2] cycloaddition reactions involving phosphiranes and tropones: facile access to 6,5,7-fused tricyclic skeletons

Tandem [5 + 1] carbonyl cyclization/[8 + 2] cycloaddition reactions of phosphiranes and tropones were developed as a straightforward method to access 6,5,7-fused tricyclic scaffolds.

Three-Component, Diastereoselective [6 + 3] Annulation of Tropone, Imino Esters, and Arynes

A transition-metal-free, three-component, and diastereoselective [6 + 3] annulation reaction employing tropone, imino esters, and arynes allowing the synthesis of bridged azabicyclo[4.3.1]decadienes is demonstrated. The key nitrogen ylides for the [6 + 3] annulation were generated by the addition of imino esters to the arynes followed by a proton transfer. The nitrogen ylides undergo a regioselective addition to tropone to furnish the desired products in moderate to good yields with good functional group tolerance under mild conditions.

Atom-Economic Synthesis of Highly Functionalized Bridged Ring Systems Initiated by Ring Expansion of Indene-1,3-dione

An atom economic procedure for the regioselective synthesis of bridged seven-membered-ring compounds from simple reactants such as ynones and indene-1,3-dione has been developed. This process was realized through the one-pot reactions of ring-expansion of indene-1,3-dione with alkynyl ketones and successive formal [4+2] cycloaddition. The Michael addition reaction is the key for the regioselectivity of the formal [4+2] cycloaddition.

Recyclable heterogeneous gold(I)-catalyzed oxidative ring expansion of alkynyl quinols: a practical access to tropone and its analogues

The heterogeneous gold(i)-catalyzed oxidative ring expansion of alkynyl quinols has been achieved by using a benzyldiphenylphosphine-modified MCM-41-immobilized gold(i) complex [MCM-41-BnPh2P-AuNTf2] as the catalyst and 8-methylquinoline N-oxide as the oxidant under mild reaction conditions, yielding a variety of functionalized tropone derivatives in good to excellent yields. Extension of this methodology allows for facile construction of other seven- or six-membered ring systems including dibenzotropones, dibenzooxepines, phenanthrenes, and quinolin-2(1H)-ones. This new heterogeneous gold(i) complex can be readily recovered through a simple filtration process and recycled at least eight times without any apparent decrease in catalytic efficiency.

Uncovering the Neglected Similarities of Arynes and Donor-Acceptor Cyclopropanes

Arynes and donor-acceptor (D-A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2-disubstitution, whereas D-A cyclopropanes as polarized systems lead to 1,3-bisfunctionalization thereby showing striking similarities. Transformations with 1,2- and 1,3-dipoles afford cyclic structures. With arynes, emerging four-membered rings as intermediates might react further, whereas the analogous five-membered rings obtained from D-A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D-A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.

Copper-Catalyzed [4+2] Cycloaddition of 9 H-Cyclohepta[ b]pyridine-9-one and Electron-Rich Alkenes

9 H-Cyclohepta[ b]pyridin-9-one was used as a diene cycloaddition partner to construct [3.2.2] bicycles in a copper-catalyzed [4+2] cycloaddition. Oxygen- and nitrogen-substituted terminal, disubstituted, trisubstituted, and cyclic alkenes reacted to afford the cycloadducts as single constitutional isomers in 48-80% yields and diastereomeric ratios up to 5.6:1.

Aryne insertion into the PO bond: one-pot synthesis of quaternary phosphonium triflates

A novel transition-metal free synthetic strategy for the direct synthesis of quaternary phosphonium triflates via insertion of aryne into phosphine oxide.

Decarbethoxylative Arylation Employing Arynes: A Metal-Free Pathway to Arylfluoroamides

An efficient, metal-free decarbethoxylative arylation protocol for the synthesis of α-aryl-α-fluoroamides from fluoromalonamates, under ambient reaction conditions using aryne as an electrophilic arylating agent, is reported. This decarbethoxylative arylation proceeds under mild conditions and provides a practical and effective entry to a wide range of α-aryl-α-fluoroacetamides. Interestingly, the use of the tert-butyl ester of fluoromalonamate prevented the otherwise rapid decarboxylation step, affording the arylated fluoromalonamate in moderate yield.

Employing Arynes in Diels-Alder Reactions and Transition-Metal-Free Multicomponent Coupling and Arylation Reactions

Arynes are highly reactive intermediates having several applications in organic synthesis for the construction of various ortho-disubstituted arenes. Traditionally, arynes are generated in solution from haloarenes under strongly basic conditions. However, the scopes of many of the aryne reactions are limited because of the harsh conditions used for their generation. The renaissance of interest in aryne chemistry is mainly due to the mild conditions for their generation by the fluoride-induced 1,2-elimination of 2-(trimethylsilyl)aryl triflates. This Account is focused on the Diels-Alder reaction of arynes and their transition-metal-free application in multicomponent couplings as well as arylation reactions. The Diels-Alder reaction of arynes is a powerful tool for constructing benzo-fused carbocycles and heterocycles. In 2012, we developed an efficient, broad-scope, and scalable Diels-Alder reaction of pentafulvenes with arynes affording benzonorbornadiene derivatives. Subsequently, we accomplished the Diels-Alder reaction of arynes with dienes such as 1,2-benzoquinones and tropones. Moreover, we uncovered a transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes by the reaction of arynes with styrenes that proceeds via a Diels-Alder/ene-reaction cascade. In addition, we demonstrated the reaction of arynes with indene/benzofurans, which proceeds via a tandem [4 + 2]/[2 + 2] sequence. Multicomponent coupling (MCC) involving arynes mainly comprises the initial addition of a nucleophile to the aryne followed by interception of the aryl anion intermediate with an electrophile (provided the nucleophilic and electrophilic moieties do not belong to the same molecule). We have disclosed aryne MCCs initiated by N-heterocycles such as (iso)quinoline, pyridine, and aziridines. When (iso)quinoline is used as the nucleophilic trigger and N-substituted isatin as the third component, the reaction affords spirooxazino(iso)quinolines via 1,4-dipolar intermediates. Unexpectedly, using pyridine affords indolin-2-ones, where the reaction proceeds via the pyridylidene intermediate. Additionally, we developed the phosphine-triggered aryne MCCs for the synthesis of functionalized benzooxaphospholes. In another phase of our work, we studied the synthetic utility of CO2 as a one-carbon synthon in aryne MCCs for the synthesis of phthalimides. Engaging arynes as an aryl source is one of the transition-metal-free methods for arylation reactions. We have demonstrated the N-arylation of aromatic tertiary amines and O-arylation of aliphatic alcohols using arynes. It is anticipated that the chemistry of arynes will continue to prosper and will lead to surprising developments for the synthesis of various 1,2-disubstituted arenes of molecular complexity and structural diversity. Future challenges in this area include the utility of arynes in enantioselective transformations and the synthesis and reactions of exotic heterocyclic arynes.

Construction of benzo[a]carbazole derivatives via Diels–Alder reaction of arynes with vinylindoles

A protocol for high-efficiency and versatile Diels–Alder reaction of vinylindoles with arynes has been developed. Various functionalized benzo[a]carbazoles were afforded in good to perfect yieldsvia[4 + 2] cycloaddition/aromatization.

Ni(NHC)]-catalyzed cycloaddition of diynes and tropone: apparent enone cycloaddition involving an 8π insertion

A Ni/N-heterocyclic carbene catalyst couples diynes to the C(α)-C(β) double bond of tropone, a type of reaction that is unprecedented for metal-catalyzed cycloadditions with aromatic tropone. Many different diynes were efficiently coupled to afford [5-6-7] fused tricyclic products, while [5-7-6] fused tricyclic compounds were obtained as minor byproducts in a few cases. The reaction has broad substrate scope and tolerates a wide range of functional groups, and excellent regioselectivity is found with unsymmetrical diynes. Theoretical calculations show that the apparent enone cycloaddition occurs through a distinctive 8π insertion of tropone. The initial intramolecular oxidative cyclization of diyne produces the nickelacyclopentadiene intermediate. This intermediate undergoes an 8π insertion of tropone, and subsequent reductive elimination generates the [5-6-7] fused tricyclic product. This initial product undergoes two competing isomerizations, leading to the observed [5-6-7] and [5-7-6] fused tricyclic products.