Transition-Metal Catalyzed Stereoselective Desymmetrization of Prochiral Cyclohexadienones

Ag(I)-catalyzed diastereoselective oxidative cyclopropanation of prochiral alkyne-tethered 1,3-dicarbonitriles

Herein, we developed a highly diastereoselective silver-catalyzed intramolecular cyclopropanation of prochiral alkyne-tethered 1,3-dicarbonitriles using perchloric acid as an effective oxidizing agent. This method facilitates the construction of densely functionalized complex [6.6.5.3] frameworks having three all-carbon quaternary stereocenters in high yields. The significance of the reaction was demonstrated by a gram-scale reaction and post-synthetic modifications of the product.

Anion-Relay Double Aza-Michael-Michael Cascades to Enone-Tethered Cyclohexadienones: Access to an Intricate Bridged Ring System

An anion-relay double aza-Michael-Michael addition strategy has been reported for the synthesis of intricate scaffolds from enone-tethered cyclohexadienones and primary amines. This method discloses the base-catalyzed synthesis of highly valued bridged aza-tricyclic frameworks with a high level of product selectivity and stereoselectivity. Gram scale synthesis and synthetic transformation were shown to afford structurally diverse bridged aza-polycyclic amines. Control experiments and the kinetic profile were studied to determine a plausible reaction mechanism.

Ruthenium and Iodine Anion Cocatalyzed Cascade Dihalogenation and Cyclization of Internal Alkyne-Tethered Cyclohexadienones with 1,2-Dihaloethanes

We have established an efficient ruthenium(II) and iodine anion cocatalyzed dihalogenation and cascade cyclization of internal alkyne-tethered cyclohexadienones, which stereoselectively afforded numerous dihalogenation products with a bioactive hydrobenzofuran skeleton in high yields under mild conditions. In this transformation, the reaction pathway was determined by the concentration of electrophilic iodine reagent, which also provided a strategy for control of the reaction selectivity. Furthermore, this method features the use of 1,2-dihaloroethane as the halogen source via iodine anion catalyst.

Strategic enzymatic enantioselective desymmetrization of prochiral cyclohexa-2,5-dienones

Asymmetric desymmetrization through the selective reduction of one double bond of prochiral 2,5-cyclohexadienones is highly challenging. A novel method has been developed for synthesizing chiral cyclohexenones by employing an ene-reductase (Bacillus subtilis YqjM) enzyme that belongs to the OYE family. Our strategy demonstrates high substrate scope and enantioselectivity towards substrates containing all-carbon as well as heteroatom (O, N)-containing quaternary centers. The mechanistic studies (kH/D = ∼1.8) indicate that hydride transfer is probably the rate-limiting step. Mutation of several active site residues did not affect the stereochemical outcomes. This work provides a convenient way of synthesizing various enantioselective γ,γ-disubstituted cyclohexanones using enzymes.

Synthesis of Tricyclic Fused 6-5-4 Carbocycles via Acid-Promoted Cascade Intramolecular Cyclization of Allenylsilane-Tethered Cyclohexadienones

The reaction of 2,5-cyclohexadienones with methylene-tethered allenylsilane in the presence of Lewis or Brønsted acids leads to a cascade of intramolecular cyclization, yielding stereoselective tricyclic fused 6-5-4 carbocycles featuring a silyl-methylenecyclobutane ring. This transformation is notable for the diastereoselective asymmetric desymmetrization of prochiral dienones, attributed to the axial chirality of allene.

Rhodium(I)/Chiral Diene Complexes Catalyzed Asymmetric Desymmetrization of Alkynyl-Tethered 2,5-Cyclohexadienones Through an Arylative Cyclization Cascade

Cis-hydrobenzofurans, cis-hydroindoles, and cis-hydrindanes, privileged structural motifs found in numerous biologically active natural and synthetic compounds, are efficiently prepared by a Rh(I)-catalyzed cascade syn-arylation/1,4-addition protocol. This approach starts with the regioselective syn-arylation of the alkyne tethered to 2,5-hexadienone moieties, using a chiral Rh(I) catalyst generated in situ from a chiral bicyclo[2.2.1]hepatadiene ligand L4f. By forging two new carbon-carbon bonds and introducing two chiral centers, the resulting alkenylrhodium species undergoes desymmetrization via an intramolecular 1,4-addition reaction, delivering annulated products with high yields and enantioselectivities.

Bifunctional Iminophosphorane Catalyzed Amide Enolization for Enantioselective Cyclohexadienone Desymmetrization

The organocatalytic enolization of 2-arylacetamides, followed by an enantioselective intramolecular conjugate addition to tethered 2,5-cyclohexadienones, yielding 3D fused N-heterocycles, is described. The transformation represents the first strong activating group-free activation of carboxamides via α-C-H deprotonation in a metal-free, catalytic, and enantioselective reaction, and is achieved by employing a bifunctional iminophosphorane (BIMP) superbase.

Combined Lewis Acid Catalyzed Diastereoselective Halogenative Cascade Annulation of Enone-Tethered Cyclohexadienones

The cascade difunctionalization of α,β-unsaturated carbonyls by nucleophilic halogenation followed by enolate trapping with other electrophiles is highly challenging in synthetic organic chemistry. Herein, we report a chemo- and diastereoselective cascade annulation of enone-tethered cyclohexadienones by using an unconventional combined Lewis acid catalyzed halogenation reaction in the presence of an In(III)-catalyst and trimethylsilyl halide. The reaction provides highly functionalized bicyclic enones with four contiguous stereocenters.

Enantioselective Dearomatization of Substituted Phenols via Organocatalyzed Electrophilic Amination

Highly efficient and stereoselective dearomatization of substituted phenols was achieved via chiral phosphoric acid-catalyzed electrophilic para-amination with commercially available azodicarboxylates. This protocol readily afforded a series of chiral 2,5-cyclohexadienones bearing 4-aza-quaternary stereocenters with excellent yields and enantioselectivities (≤99% yield and >99% ee). Easy scale-up of this reaction to a gram scale and diverse derivatizations of the chiral products into α-tertiary amines and α-tertiary heterocycles derivatives well demonstrated the potential of this method.

Copper-Catalyzed Enantioselective Desymmetric Protosilylation of Prochiral Diynes: Access to Optically Functionalized Tertiary Alcohols

In this protocol, a copper-catalyzed desymmetric protosilylation of prochiral diynes was developed. The corresponding products were obtained in moderate to high yields and enantiomeric ratios. This approach provides a simple method for synthesizing functionalized chiral tertiary alcohols in the presence of a chiral pyridine-bisimidazoline (Pybim) ligand.

Catalyst-Controlled Diastereoselective Synthesis of Bridged [3.3.1] Bis(Indolyl)-Oxanes and Oxepanes via Desymmetrization of Bis(Indolyl)-Cyclohexadienones

A catalyst-controlled divergent synthesis of bridged [3.3.1] bis(indolyl)-oxanes and cis-[6.7] fused bis(indolyl) oxepanes via diastereoselective desymmetrization of bis(indolyl)-cyclohexadienones is presented for the first time. The reaction is highly atom- and step-economic, furnishing sp³-rich functionalized bis(indolyl) derivatives in good to excellent yields with wide substrate scope. The reaction proceeds through Friedel-Crafts alkylation followed by catalyst-controlled selective C-C bond formation/rearrangement. Gram scale synthesis and synthetic utility to generate bis(indolyl) alkaloid-like molecular diversity were also illustrated.

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

Site-selective and stereoselective transformations on p-quinols & p-quinamines

The intermolecular transformation of simple substrates into highly functionalized scaffolds with multiple stereogenic centers is an attractive strategy in modern organic synthesis. Prochiral 2,5-cyclohexadienones, being stable and easily accessible, are privileged key building blocks for the synthesis of complex molecules and bioactive natural products. In particular, p-quinols and p-quinamines are important subclasses of cyclohexadienones, having both nucleophilic and electrophilic sites, and can undergo various intermolecular cascade annulations via formal cycloadditions and other transformations. This article highlights the recent developments of intermolecular transformations on p-quinols and p-quinamines along with plausible reaction mechanisms. We hope that this review will inspire the readers to explore the new potential applications of these unique prochiral molecules.

Solvent-Mediated Enantioselective Rauhut-Currier Cyclization via Iminium and Enamine Activation

In this work, we have developed an unconventional and highly enantioselective solvent-promoted Rauhut-Currier cyclization of enal-tethered cyclohexadienone by exploiting the reactivity of a simple Jørgensen-Hayashi catalyst through the merging of iminium and enamine activation. This asymmetric desymmetrization reaction has broad substrate scope in good yields with high to excellent enantioselectivity. DFT calculations suggest that the elimination of the alkoxy group is the rate-limiting step and that it proceeds through proton abstraction by solvent instead of a direct 1,3-proton shift.

Diphenyl Ditelluride: An Unconventional Reducing Agent in the Sulfonylative Cascade of Alkynyl Cyclohexadienones

Herein, we report a reductive hydrazo-sulfonylative difunctionalization cascade of alkynyl cyclohexadienones employing PhTeTePh as an uncommon reducing agent. Diphenyl ditelluride is a commercially available solid with a good solubility profile in most organic solvents, and this is the first report illustrating it as a reducing agent. The protocol afforded a variety of difunctionalized dihydrochromenones and dihydrobenzofuranones in good yields under relatively mild conditions. The reactions were scalable, and mechanistic studies were conducted to probe the reaction mechanism. Additionally, photophysical studies of the products were carried out, which revealed that they had significant absorption (400-450 nm) and emission (520-570 nm) in the visible region.

Highly electrophilic silver carbenes

Catalytic carbene transfer reactions are fundamental transformations in modern organic synthesis, which enable direct access to diverse structurally complex molecules. Despite diazo precursors playing a crucial role in catalytic carbene transfer reactions, most reported methodologies take into account only diazoacetates or related compounds. This is primarily because diazoalkanes, unless they contain a resonance stabilizing group, are more susceptible to violent exothermic decomposition. In this feature article, we present an alternative approach to carbene-transfer reactions based on the formation of highly electrophilic silver carbenes from N-sulfonylhydrazones, where the high electrophilicity of silver carbenes stems from the weak interaction between silver and the carbenic carbon. These precursors are readily accessible, stable, and environmentally sustainable. Using the strategy that employs highly electrophilic silver carbenes, it is possible to develop novel intermolecular transformations involving non-stabilized carbenes, including C(sp³)-H insertion, C(sp³)-C(O) insertion, cycloaddition, and defluorinative functionalization. The silver-catalyzed carbene transfer reactions described here have high efficiency, unusual reactivity, exceptional selectivity, and a reaction pathway that differs from typical transition metal-catalyzed reactions. Our research provided fundamental insight into silver carbene chemistry, and we hope to apply this mode of catalysis to other more general transformations, including asymmetric transformations.

Palladium(0)-Catalyzed Remote Decarboxylative Allylation and Base-Mediated 1,3-Migration

Herein we have reported the palladium(0)-catalyzed decarboxylative oxa-Michael addition/remote α-allylation/1,3- migration of prochiral allyl carbonate-tethered cyclohexadienones in good yields. This unconventional intramolecular rearrangement is triggered by the base-mediated retro-Michael ring-opening reaction (β-elimination) and subsequent syn-selective oxa-Michael addition on the less substituted enone functionality. The generality of tandem decarboxylative allylation was examined with various substrates and in the gram-scale reaction.

A metal-free four-component sulfonylation, Giese cyclization, selenylation cascade via insertion of sulfur dioxide

We hereby report a highly regio- and diastereoselective arylsulfonylation-radical cyclization-selenylation cascade of alkynyl cyclohexadienones for the facile synthesis of highly functionalized dihydrochromenones. The protocol utilizes aryldiazonium salts as aryl partners and DABSO as a benign SO₂ source and also as a redox mediator. Additionally, we also developed a visible light mediated protocol wherein diaryliodonium salts were used as the aryl partners at room temperature.

Silver-Catalyzed Asymmetric Desymmetrization of Cyclohexadienones via Van Leusen Pyrrole Synthesis

A catalytic asymmetric desymmetrization of cyclohexadienones was accomplished via Van Leusen pyrrole synthesis in the presence of a chiral silver catalyst. The ready access to chiral-fused pyrrole derivatives is attributed to the identification of a suitable isocyanomethyl sulfone surrogate, NasMIC. The current Ag(I)-chiral phosphino-carboamide ligand system can be extended to the kinetic resolution of racemic cyclohexadienones, capitalizing on the differentiated reactivity of chiral quaternary complexes from AgSbF₆, chiral ligand (L*), cyclohexadienone, and NasMIC.

Rhodium-Catalyzed Desymmetric Arylation of γ,γ-Disubsituted Cyclohexadienones: Asymmetric Synthesis of Chiral All-Carbon Quaternary Centers

The desymmetric arylation of prochiral cyclohexadienones with ArZnCl in the presence of an (R)-segphos-rhodium catalyst gave high yields of the corresponding cyclohexenones, which contain a chiral arylated carbon center at the β-position and a chiral all-carbon quaternary center at the γ-position, with high diastereo- and enantioselectivities. This catalytic system was also applied to the arylation of spirocarbocyclic cyclohexadienones and afforded the corresponding cyclohexenones bearing a chiral spiro quaternary carbon with high dr and ee.

Steric- and Electronic-Controlled Intramolecular [2 + 2]-Cycloaddition of Cyclohexadienone-Containing 1,7-Enynes

Herein we have developed the silver-catalyzed electronic- and steric-controlled intramolecular formal [2 + 2]-cycloaddition of alkyne-tethered cyclohexadienones. Substrates with electron-rich alkynes and a less hindered quaternary carbon center afford tricyclic fused cyclobutenes through 1,7-enyne cyclization. In contrast, the formation of dihydrofurans was observed from electron-deficient alkynes via proton abstraction/C-O bond cleavage. The synthetic potential of this method was also broadened with a gram-scale reaction and various transformations on cyclobutene.