Axially Chiral Aryl‐Alkene‐Indole Framework: A Nascent Member of the Atropisomeric Family and Its Catalytic Asymmetric Construction

A new class of axially chiral aryl‐alkene‐indole frameworks have been designed, and the first catalytic asymmetric construction of such scaffolds has been established by the strategy of organocatalytic (Z/E)‐selective and enantioselective (4+3) cyclization of 3‐alkynyl‐2‐indolylmethanols with 2‐naphthols or phenols (all >95 : 5 E/Z, up to 98% yield, 97% ee). This reaction also represents the first catalytic asymmetric construction of axially chiral alkene‐heteroaryl scaffolds, which will add a new member to the atropisomeric family. This approach has not only confronted the great challenges in constructing axially chiral alkene‐heteroaryl scaffolds but also provided a powerful strategy for the enantioselective construction of axially chiral aryl‐alkene‐indole frameworks.     

Palladium‐Catalyzed Asymmetric [4+3] Cyclization of Trimethylenemethane: Regio‐, Diastereo‐, and Enantioselective Construction of Benzofuro[3,2‐b]azepine Skeletons

The palladium‐catalyzed asymmetric [4+3] cyclization of trimethylenemethane donors with benzofuran‐derived azadienes furnishes chiral benzofuro[3,2‐b]azepine frameworks in high yields (up to 98 %) with exclusive regioselectivities and excellent stereoselectivities (up to >20:1 d.r., >99 % ee). This catalytic asymmetric [4+3] cyclization of Pd‐trimethylenemethane can enrich the arsenal of Pd‐TMM reactions in organic synthesis. In addition, this strategy provides an alternative approach to chiral azepines by a transition‐metal‐catalyzed asymmetric [4+3] cyclization.     

Design and Catalytic Asymmetric Construction of Axially Chiral 3,3′‐Bisindole Skeletons

The first catalytic asymmetric construction of 3,3′‐bisindole skeletons bearing both axial and central chirality has been established by organocatalytic asymmetric addition reactions of 2‐substituted 3,3′‐bisindoles with 3‐indolylmethanols (up to 98 % yield, all >95:5 d.r., >99 % ee). This reaction also represents the first highly enantioselective construction of axially chiral 3,3′‐bisindole skeletons, and utilizes the strategy of introducing a bulky group to the ortho‐position of prochiral 3,3′‐bisindoles. This reaction not only provides a good example for simultaneously controlling axial and central chirality in one operation, but also serves as a new strategy for catalytic enantioselective construction of axially chiral 3,3′‐bisindole backbones from prochiral substrates.     

Catalytic Enantioselective and Regioselective [3+3] Cycloadditions Using 2‐Indolylmethanols as 3 C Building Blocks

The first catalytic asymmetric cycloaddition using 2‐indolylmethanols as 3C building blocks has been established by a chiral phosphoric acid‐catalyzed enantioselective and regioselective [3+3] cycloaddition of 2‐indolylmethanols with azomethine ylides, which constructed biologically important tetrahydro‐γ‐carboline frameworks in high yields and excellent enantioselectivities (up to 83 % yield, 99:1 e.r.). This reaction not only represents the first application of 2‐indolylmethanols as 3C building blocks in catalytic asymmetric cycloadditions, but also has established an abnormal regioselectivity in indolylmethanol‐involved transformations.     

Design and Enantioselective Construction of Axially Chiral Naphthyl‐Indole Skeletons

The first enantioselective construction of a new class of axially chiral naphthyl‐indole skeletons has been established by organocatalytic asymmetric coupling reactions of 2‐naphthols with 2‐indolylmethanols (up to 99 % yield, 97:3 e.r.). This approach not only affords a new type of axially chiral heterobiaryl backbone, but also provides a new catalytic enantioselective strategy for constructing axially chiral biaryl scaffolds by making use of the C3‐electrophilicity of 2‐indolylmethanols.     

Construction of Novel Tetrahydro‐β‐carboline‐1‐thione Spirooxindoles by Brønsted Acid Mediated Formal [3+3] Cyclization of 3‐Indolylmethanols with 3‐Isothiocyanato Oxindoles

p‐Toluenesulfonic acid mediated formal [3+3] cyclization of 3‐indolylmethanols with 3‐isothiocyanato oxindoles was realized. This transformation allowed for the synthesis of a series of novel tetrahydro‐β‐carboline‐1‐thione spirooxindoles in moderate to excellent yields (up to 99%) with generally good diastereoselectivities (up to >20:1). The structure of one product was determined by an X‐ray crystal structural analysis.     

Organocatalytic Asymmetric Annulation of ortho‐Alkynylanilines: Synthesis of Axially Chiral Naphthyl‐C2‐indoles

A chiral Brønsted base catalyzed asymmetric annulation of ortho‐alkynylanilines has been developed to access axially chiral naphthyl‐C2‐indoles via vinylidene ortho‐quinone methide (VQM) intermediates. This strategy provides a unique organocatalytic atroposelective route to axially chiral aryl‐C2‐indole skeletons with excellent enantioselectivity and functional‐group tolerance. This transformation was applicable to decagram‐scale preparation (50.0 g) with perfect enantioselectivity through simple recrystallization. Moreover, the utility of this reaction was demonstrated by a variety of transformations towards chiral naphthyl‐C2‐indoles for a series of carbon–heteroatom bond formations. Furthermore, the prepared axially chiral naphthyl‐C2‐indoles were applied as a chiral skeleton for organocatalytic aza‐Baylis–Hillman reaction and asymmetric formal [4+2] tandem cyclization to give the corresponding adducts in high yields with improved enantioselectivity and diastereoselectivity.     

Catalytic Asymmetric Formal [3+3] Cycloaddition of an Azomethine Ylide with 3‐Indolylmethanol: Enantioselective Construction of a Six‐Membered Piperidine Framework

A catalytic asymmetric formal [3+3] cycloaddition of 3‐indolylmethanol and an in situ‐generated azomethine ylide has been established to construct a chiral six‐membered piperidine framework with two stereogenic centers. This approach not only represents the first enantioselective cycloaddition of isatin‐derived 3‐indolylmethanol, but also has realized an unusual enantioselective formal [3+3] cycloaddition of azomethine ylide rather than its common [3+2] cycloadditions. Besides, this protocol combines the merits of a multicomponent reaction and organocatalysis, which efficiently assembles a variety of isatin‐derived 3‐indolylmethanols, aldehydes, and amino esters into structurally diverse spiro[indoline‐3,4′‐pyridoindoles] with one all‐carbon quaternary stereogenic center in high yields and excellent enantioselectivities (up to 93 % yield, >99 % enantiomeric excess (ee)). Although the diastereoselectivity of the reaction is generally moderate, most of the diastereomers can be separated by using column chromatography followed by preparative TLC.     

Catalytic Asymmetric Intramolecular [4+2] Cycloaddition of In Situ Generated ortho‐Quinone Methides

Herein, we describe the first catalytic asymmetric intramolecular [4+2] cycloaddition of in situ generated ortho‐quinone methides. In the presence of a confined chiral imidodiphosphoric acid catalyst, various salicylaldehydes react with dienyl alcohols to give transient ortho ‐quinone methide intermediates, which undergo an intramolecular [4+2] cycloaddition to provide highly functionalized furanochromanes and pyranochromanes in excellent diastereoselectivity and enantioselectivity.     

Palladium-Catalyzed Asymmetric [4+3] Cyclization of Trimethylenemethane: Regio-, Diastereo-, and Enantioselective Construction of Benzofuro[3,2-b]azepine Skeletons

The palladium-catalyzed asymmetric [4+3] cyclization of trimethylenemethane donors with benzofuran-derived azadienes furnishes chiral benzofuro[3,2-b]azepine frameworks in high yields (up to 98 %) with exclusive regioselectivities and excellent stereoselectivities (up to >20:1 d.r., >99 % ee). This catalytic asymmetric [4+3] cyclization of Pd-trimethylenemethane can enrich the arsenal of Pd-TMM reactions in organic synthesis. In addition, this strategy provides an alternative approach to chiral azepines by a transition-metal-catalyzed asymmetric [4+3] cyclization.     

Catalytic Asymmetric [2+3] Cyclizations of Azlactones with Azonaphthalenes

The first catalytic asymmetric [2+3] cyclization of azlactones with azonaphthalenes has been established. This strategy allowed the synthesis of a variety of chiral isatin derivatives in generally good yields and excellent enantioselectivities (up to 99 % yield, 98 % ee). The developed reaction has not only established a catalytic enantioselective [2+3] cyclization using azlactones as two‐carbon building blocks, but also enriches the chemistry of catalytic asymmetric cyclizations of azonaphthalenes. In addition, this protocol will provide a useful method for constructing enantioenriched 3,3′‐disubstituted isatin‐type frameworks.     

Organocatalytic Asymmetric Cascade Reactions of 7‐Vinylindoles: Diastereo‐ and Enantioselective Synthesis of C7‐Functionalized Indoles

The first catalytic asymmetric cascade reaction of 7‐vinylindoles has been established by the rational design of such substrates. Cascade reactions with isatin‐derived 3‐indolylmethanols in the presence of a chiral phosphoric acid derivative allow the diastereo‐ and enantioselective synthesis of C7‐functionalized indoles as well as the construction of cyclopenta[b]indole and spirooxindole frameworks (all >95:5 d.r., 94–>99 % ee). This approach not only addresses the great challenge of the catalytic asymmetric synthesis of C7‐functionalized indoles, but also provides an efficient method for constructing biologically important cyclopenta[b]indole and spirooxindole scaffolds with excellent optical purity. Investigation of the reaction pathway and activation mode has suggested that this cascade reaction proceeds through a vinylogous Michael addition/Friedel–Crafts process, in which dual H‐bonding activation of the two reactants plays a crucial role.     

Highly Diastereo‐ and Enantioselective Synthesis of Cyclohepta[b]indoles by Chiral‐Phosphoric‐Acid‐Catalyzed (4+3) Cycloaddition

A highly enantio‐ and diastereoselective formal (4+3) cycloaddition of 1,3‐diene‐1‐carbamates with 3‐indolylmethanols in the presence of a chiral phosphoric acid catalyst is reported. The approach described herein provides efficient access to 6‐aminotetrahydrocyclohepta[b]indoles in good yields with mostly complete diastereoselectivity and excellent levels of enantioselectivity (>98:2 dr and up to 98 % ee). Mild reaction conditions, facile scale‐up, and versatile derivatization highlight the practicality of this methodology. A mechanistic study suggests that cycloaddition occurs in a stepwise fashion, after the formation of an ion pair between the chiral catalytic phosphate and the intermediate carbocation.     

Asymmetric Michael Addition/Intramolecular Cyclization Catalyzed by Bifunctional Tertiary Amine–Squaramides: Construction of Chiral 2‐Amino‐4H‐chromene‐3‐Carbonitrile Derivatives

The efficient asymmetric Michael addition/intramolecular cyclization of malononitrile with dienones catalyzed by a chiral bifunctional tertiary amine–squaramide catalyst for the synthesis of chiral 2‐amino‐4H‐chromene‐3‐carbonitrile derivatives was developed. The corresponding products were obtained in good to excellent yields (up to 99 %) with excellent enantioselectivities (up to 98 % ee) for most of the bisarylidenecyclopentanones.     

Ming‐Phos/Gold(I)‐Catalyzed Stereodivergent Synthesis of Highly Substituted Furo[3,4‐d][1,2]oxazines†

A gold(I)‐catalyzed asymmetric intermolecular tandem [3+3]‐cyclization reaction of 2‐(1‐alkynyl)‐2‐ alken‐1‐ones with nitrones has been developed by using Ming‐Phos as a chiral ligand. This method enables access to the stereodivergent synthesis of highly substituted furo[3,4‐d][1,2]oxazines in excellent efficiency and stereoselectivity (up to 99% yield, 99% ee, >20 : 1 dr).     

Desilylation‐Activated Propargylic Transformation: Enantioselective Copper‐Catalyzed [3+2] Cycloaddition of Propargylic Esters with β‐Naphthol or Phenol Derivatives

A copper‐catalyzed asymmetric [3+2] cycloaddition of 3‐trimethylsilylpropargylic esters with either β‐naphthols or electron‐rich phenols has been realized and proceeds by a desilylation‐activated process. Under the catalysis of Cu(OAc)₂?H₂O in combination with a structurally optimized ketimine P,N,N‐ligand, a wide range of optically active 1,2‐dihydronaphtho[2,1‐b]furans or 2,3‐dihydrobenzofurans were obtained in good yields and with high enantioselectivities (up to 96 % ee). This represents the first desilylation‐activated catalytic asymmetric propargylic transformation.     

Asymmetric [4+2] Annulation of C1 Ammonium Enolates with Copper‐Allenylidenes

An asymmetric catalytic decarboxylative [4+2] annulation of 4‐ethynyl dihydrobenzooxazinones and carboxylic acids has been established by cooperative copper and nucleophilic Lewis base catalysis. A C1 ammonium enolate and copper–allenylidene complex, each catalytically generated from different substrates, underwent a cascade asymmetric propargylation and lactamization process to yield optically active 3,4‐dihydroquinolin‐2‐one derivatives with excellent levels of stereoselectivity (up to 99 % ee , 95:5 d.r.).     

Catalytic Asymmetric Inverse‐Electron‐Demand Oxa‐Diels–Alder Reaction of In Situ Generated ortho‐Quinone Methides with 3‐Methyl‐2‐Vinylindoles

The first catalytic asymmetric inverse‐electron‐demand (IED) oxa‐Diels–Alder reaction of ortho‐quinone methides, generated in situ from ortho‐hydroxybenzyl alcohols, has been established. By selecting 3‐methyl‐2‐vinylindoles as a class of competent dienophiles, this approach provides an efficient strategy to construct an enantioenriched chroman framework with three adjacent stereogenic centers in high yields and excellent stereoselectivities (up to 99 % yield, >95:5 d.r., 99.5:0.5 e.r.). The utilization of ortho‐hydroxybenzyl alcohols as precursors of dienes and 3‐methyl‐2‐vinylindoles as dienophiles, as well as the hydrogen‐bonding activation mode of the substrates met the challenges of a catalytic asymmetric IED oxa‐Diels–Alder reaction.     

Chiral N,N′‐Dioxide–Scandium(III) Complex‐Catalyzed Asymmetric Friedel–Crafts Alkylation Reaction of ortho‐Hydroxybenzyl Alcohols with C3‐Substituted N‐Protected Indoles

The first Lewis acid catalyzed asymmetric Friedel–Crafts alkylation reaction of ortho‐hydroxybenzyl alcohols with C3‐substituted indoles is described. A chiral N,N′‐dioxide Sc(OTf)₃ complex served not only to promote formation of ortho‐quinone methides (o‐QMs) in situ but also induced the asymmetry of the reaction. This methodology enables a novel activation of ortho‐hydroxybenzyl alcohols, thus affording the desired chiral diarylindol‐2‐ylmethanes in up to 99 % yield and 99 % ee. A range of functional groups were also tolerated under the mild reaction conditions. Moreover, this strategy gives concise access to enantioenriched indole‐fused benzoxocines.     

One‐Pot Syntheses of 2‐(2‐Sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic Acid Derivatives via Reactions of 3‐(2‐Isothiocyanatophenyl)prop‐2‐enoic Acid Derivatives with Thiols or Sodium Sulfide

Two efficient methods for the preparation of 2‐(2‐sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid derivatives 3 under mild conditions have been developed. The first method is based on the reaction of 3‐(2‐isothiocyanatophenyl)prop‐2‐enoates 1a – 1c with thiols in the presence of Et₃N in THF at room temperature, leading to the corresponding dithiocarbamate intermediates 2 , which underwent spontaneous cyclization at the same temperature by an attack of the S‐atom at the prop‐2‐enoyl moiety in a 1,4‐addition manner (Michael addition) to give 2‐(2‐sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetates in one pot. The second method involves treatment of 3‐(2‐isothiocyanatophenyl)prop‐2‐enoic acid derivatives 1b – 1d with Na₂S leading to the formation of 2‐(2‐sodiosulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid intermediates 5 by a similar addition/cyclization sequence, which are then allowed to react with alkyl or aryl halides to afford derivatives 3 . 2‐(2‐Thioxo‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid derivatives 6 can be obtained by omitting the addition of halides.