Enantioselective synthesis of spiro cyclopentane-1,3'-indoles and 2,3,4,9-tetrahydro-1H-carbazoles by iridium-catalyzed allylic dearomatization and stereospecific migration

Rhodium-Catalyzed Enantioselective Cyclization of 3-Allenyl-indoles: Access to Functionalized Tetrahydrocarbazoles

A highly selective rhodium-catalyzed cyclization of tethered 3-allenylindoles is reported. In a smooth reaction, 1-vinyltetrahydrocarbazoles are obtained in excellent yields and enantioselectivities. Aside from a great functional group tolerance, this method requires neither the Schlenk technique nor the use of anhydrous solvents. Preliminary mechanistic investigations proved that the reaction proceeds via an intermediary formed spiroindolenine which rapidly undergoes an acid-catalyzed stereospecific migration.

Palladium(0)-Catalyzed Intermolecular Asymmetric Cascade Dearomatization Reaction of Indoles with Propargyl Carbonate

An intermolecular asymmetric cascade dearomatization reaction of indole derivatives with propargyl carbonate was developed. The challenges associated with both the chemoselectivity between the carbon and nitrogen nucleophile and the enantioselective control during the formation of an all-carbon quaternary stereogenic center were well addressed by a Pd catalytic system derived from the Feringa ligand. A series of enantioenriched multiply substituted fused indolenines were provided in good yields (71-86 %) with excellent enantioselectivity (91-96 % ee) and chemoselectivity (3/4>19:1 in most cases).

Catalytic asymmetric synthesis of tetrahydrocarbazoles

This feature article summarises recent advances in catalytic asymmetric synthesis of THCs, with emphases on reaction type and reaction mechanism.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Enantioselective C2-Alkylation of Indoles through a Redox-Relay Heck Reaction of 2-Indole Triflates

A palladium-catalyzed enantioselective redox-relay Heck reaction of 2-indole triflates and disubstituted alkenes is reported. This process combines readily available indole triflates with a variety of alkenes to afford a range of indole derivatives bearing a stereocenter adjacent to C2. Enantioselectivity is achieved through use of a simple pyridine-oxazoline ligand. Tuning the electronics of the indole, through judicious choice of N-protecting group, is required to ensure selective β-hydride elimination away from the indole core. Utility of this method is highlighted in a modular formal synthesis of an S1P₁ agonist precursor developed by Merck.

Dearomatizing Spiroannulation Reagents: Direct Access to Spirocycles from Indoles and Dihalides

Unfunctionalized indoles can be directly converted into 3,3'-spirocyclic indolenines and indolines upon reaction with electrophilic dihalides in the presence of t-BuOK/BEt₃. This double C-C bond forming reaction, which simultaneously generates a quaternary spirocyclic center, typically proceeds in high yield and has good functional group tolerance. In contrast to existing dearomatizing spirocyclization approaches, there is no need to prepare a prefunctionalized aromatic precursor, enabling faster access to valuable spirocyclic products from simple, commercially available aromatics in one step.

Asymmetric fluorinative dearomatization of tryptamine derivatives

An asymmetric fluorinative dearomatization reaction of tryptamine derivatives was developed by using a chiral anion phase transfer catalyst (PTC) system, and the preliminary results of the reaction mechanistic study were achieved. This method is characterized by a simple operation, facile introduction of a fluorine atom in a highly enantioselective manner and construction of two contiguous quaternary stereogenic centers.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles

An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

Synthesis of spiroindolenines by intramolecular ipso-iodocyclization of indol ynones

A high-yielding fast spirocyclization of easily available indol ynones has been developed by applying N-iodosuccinimide.

Pd(ii)-Catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with alkynes via a C–H activation/dearomatization approach

A Pd(ii)-catalyzed [3 + 2] spiroannulation of α-aryl-β-naphthols with internal alkynes has been developed by relying on a C–H activation/arene dearomatization approach.

Organocatalytic asymmetric synthesis of tetrahydrocarbazoles via an inverse-electron-demand Diels–Alder reaction of 2,3-indole-dienes with enals

A novel 2,3-indole-diene synthon was designed and first applied to an inverse-electron-demand Diels–Alder reaction to construct tetrahydrocarbazoles with excellent stereoselectivities.

Iridium-Catalyzed Asymmetric Allylic Dearomatization by a Desymmetrization Strategy

A desymmetrization strategy was developed involving iridium-catalyzed allylic dearomatization of indoles. The six-membered-ring spiroindolenines contain three contiguous stereogenic centers, including an all-carbon quaternary center, and were obtained in up to 99 % yield with 99 % ee and >95:5 d.r. When treated with a catalytic amount of tosylic acid, six-membered spiroindolenine undergoes an unprecedented six-to-seven-membered ring expansion, affording the corresponding hexahydroazepino[4,5-b]indole.

Chiral Phosphoric Acid Catalyzed Intramolecular Dearomative Michael Addition of Indoles to Enones

An enantioselective intramolecular dearomative Michael addition of indolyl enones is presented. In the presence of catalytic amount of chiral phosphoric acid, various enantioenriched spiro-indolenines bearing a quaternary stereogenic center were obtained with good yields and enantioselectivity (up to 97% ee) under mild reaction conditions.

Temperature switchable Brønsted acid-promoted selective syntheses of spiro-indolenines and quinolines

A high-yielding, temperature switchable divergent approach towards the synthesis of either spiro-indolenines or quinolines is described, starting from easily available indolyl ynones.

“A Jack of Trio”-robust one-pot metal free oxidative amination, azidation and peroxidation of phenols

Facile generation of quaternary carbon centres has been realised in the presence of phenyl trimethyl ammonium tribromide (PTAB) mediated oxidative dearomatisation.

Recent progress in transition-metal-catalyzed enantioselective indole functionalizations

Recent progress on the transition-metal-catalyzed enantioselective functionalization reaction of indoles is reviewed, which is mainly focused on asymmetric indole alkylations, arylations, cycloaddition reactions, and other reactions.

Iridium-Catalyzed Regio- and Enantioselective Allylic Substitution of Trisubstituted Allylic Electrophiles

The first Ir-catalyzed enantioselective allylation of trisubstituted allylic electrophiles has been developed. Through modification of the leaving group of allylic electrophiles, we found that trisubstituted allylic phosphates are suitable electrophiles for asymmetric allylation. The reaction of allylic phosphates with enol silanes derived from dioxinones gave allylated products in good yields with high enantioselectivities.

Pd-catalyzed cascade allylic alkylation and dearomatization reactions of indoles with vinyloxirane

We have developed Pd-catalyzed intermolecular Friedel-Crafts-type allylic alkylation and allylic dearomatization reactions of substituted indoles bearing a nucleophilic group with vinyloxirane, providing an efficient method to synthesize structurally diverse tetrahydrocarboline and spiroindolenine derivatives under mild conditions.