Design and synthesis of axially chiral aryl-pyrroloindoles via the strategy of organocatalytic asymmetric (2 + 3) cyclization

Palladium-Catalyzed (3 + 2) Annulation of Azaborines with Vinyl Epoxides for Constructing Polycyclic Oxazaborolidines

A palladium-catalyzed (3 + 2) annulation of azaborines with vinyl epoxides has been established. By this strategy, various polycyclic oxazaborolidines with structural diversity were synthesized in generally high yields (up to 99%). The annulation can be scaled up and the polycyclic oxazaborolidines can be further functionalized through olefin metathesis and Heck reaction, which demonstrated good feasibility for downstream transformations. Moreover, the catalytic asymmetric version of this (3 + 2) annulation has been accomplished under the catalysis of palladium/chiral phosphoramidite ligand, producing chiral oxazaborolidines in overall good enantioselectivities (up to 98:2 er). This work not only represents the first catalytic asymmetric (3 + 2) annulation of 1,2-azaborines with vinyl epoxides but also offers an efficient strategy for constructing benzooxazaborolidine skeletons, particularly those in an enantioenriched fashion.

Asymmetric Organocatalytic 1,6-Conjugate Addition of Alkynyl 8-Methylenenaphthalen-2(8H)-one Formed In Situ: Synergistic Construction of Axial and Central Chirality

Organocatalytic enantioselective formal nucleophilic substitution reactions of α-(2-hydroxynaphthalen-8-yl)propargyl alcohols with 1-(1H-indol-3-yl)naphthalen-2-ols have been established for the first time. With the aid of a suitable chiral phosphoric acid, alkynyl 8-methylenenaphthalen-2(8H)-one was formed in situ from the corresponding α-(2-hydroxynaphthalen-8-yl)propargyl alcohol, followed by enantioselective 1,6-conjugate additions of 1-(1H-indol-3-yl)naphthalen-2-ols to afford a number of enantioenriched (Ra,Sc)-2,3-disubstituted indoles in 50-80% yields with 81-93% ee and (Sa,Sc)-2,3-disubstituted indoles in 18-40% yields with 79-96% ee. Notably, these nucleophilic substitution products were characterized by the presence of functional groups, including indole, naphthol, and alkynyl units, while exhibiting both axial and central chirality.

Recent advances in organocatalytic atroposelective reactions

Axial chirality is present in a variety of naturally occurring compounds, and is becoming increasingly relevant also in medicine. Many axially chiral compounds are important as catalysts in asymmetric catalysis or have chiroptical properties. This review overviews recent progress in the synthesis of axially chiral compounds via asymmetric organocatalysis. Atroposelective organocatalytic reactions are discussed according to the dominant catalyst activation mode. For covalent organocatalysis, the typical enamine and iminium modes are presented, followed by N-heterocyclic carbene-catalyzed reactions. The bulk of the review is devoted to non-covalent activation, where chiral Brønsted acids feature as the most prolific catalytic structure. The last part of the article discusses hydrogen-bond-donating catalysts and other catalyst motifs such as phase-transfer catalysts.

Enantioselective Synthesis of Benzodihydrofurans Bearing Axial and Central Stereogenic Elements

The enantioselective synthesis of chiral compounds containing multiple stereogenic elements via a single catalytic step is a challenging process. In the presence of α-chloronitrostyrenes and a chiral squaramide catalyst, C-C or C-N pro-axially chiral 2-naphthol substrates, featuring low barriers to enantiomerization, underwent a remote diastereo- and enantioselective domino Michael/O-alkylation. It provided the desired benzodihydrofurans bearing two stereogenic carbon atoms and a configurationally stable C-C or a C-N bond, thanks to a high increase of the barrier to rotation upon dihydrofurannulation.

Enantioselective synthesis of molecules with multiple stereogenic elements

This review explores the fascinating world of molecules featuring multiple stereogenic elements, unraveling the different strategies designed over the years for their enantioselective synthesis. Specifically, (dynamic) kinetic resolutions, desymmetrisations and simultaneous installation of stereogenic elements exploiting either metal- or organo-catalysis are the principal approaches to efficiently create and control the three-dimensional shapes of these attractive molecules. Although most molecules presented in this review possess a stereogenic carbon atom in combination with a stereogenic axis, other combinations with helices or planes of chirality have started to emerge, as well as molecules displaying more than two different stereogenic elements.

Design and Asymmetric Control of Orientational Chirality by Using the Combination of C(sp2)-C(sp) Levers and Achiral N-Protecting Group

New chiral targets of orientational chirality have been designed and asymmetrically synthesized by taking advantage of N-sulfinyl imine-directed nucleophilic addition/oxidation, Suzuki-Miyaura, and Sonogashira cross-coupling reactions. Orientation of single isomers has been selectively controlled by using aryl/alkynyl levers [C(sp2)-C(sp) axis] and tBuSO2- protecting group on nitrogen as proven by X-ray diffraction analysis. The key structural characteristic of resulting orientational products is shown by remote through-space blocking manner. Seventeen examples of multi-step synthesis were obtained with modest to good chemical yields and complete orientational selectivity.

C(sp)-C(sp) Lever-Based Targets of Orientational Chirality: Design and Asymmetric Synthesis

In this study, the design and asymmetric synthesis of a series of chiral targets of orientational chirality were conducted by taking advantage of N-sulfinylimine-assisted nucleophilic addition and modified Sonogashira catalytic coupling systems. Orientational isomers were controlled completely using alkynyl/alkynyl levers [C(sp)-C(sp) axis] with absolute configuration assignment determined by X-ray structural analysis. The key structural element of the resulting orientational chirality is uniquely characterized by remote through-space blocking. Forty examples of multi-step synthesis were performed, with modest to good yields and excellent orientational selectivity. Several chiral orientational amino targets are attached with scaffolds of natural and medicinal products, showing potential pharmaceutical and medical applications in the future.

Design and Catalytic Asymmetric Synthesis of Furan-Indole Compounds Bearing both Axial and Central Chirality

In the chemistry community, catalytic asymmetric synthesis of furan-based compounds bearing both axial and central chirality has proven to be a significant but challenging issue owing to the importance and difficulty in constructing such frameworks. In this work, we have realized the first catalytic asymmetric synthesis of five-five-membered furan-based compounds bearing both axial and central chirality via organocatalytic asymmetric (2+4) annulation of achiral furan-indoles with 2,3-indolyldimethanols with uncommon regioselectivity. By this strategy, furan-indole compounds bearing both axial and central chirality were synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities. Moreover, theoretical calculations were conducted to provide an in-depth understanding of the reaction pathway, activation mode, and the origin of the selectivity.

Nitro-Enabled Atroposelective Dynamic Kinetic Resolution of 2-Arylindoles by Phase-Transfer Catalysis

This study presents the atroposelective alkylation of 2-arylindoles catalyzed by a substituted cinchonium salt as a phase-transfer catalyst. Under the optimized reaction conditions, various substrates are employed to yield products with high enantioselectivity. The presence of an ortho-nitro group at the aromatic ring is essential for high atroposelectivity, because it facilitates favorable interactions between the catalyst and substrate. The origin of the enantioselectivity reveals favorable π-π interactions for both enantiomers and unfavorable steric strains for undesired enantiomers.

Catalytic Asymmetric Synthesis of Atropisomers Bearing Multiple Chiral Elements: An Emerging Field

With the rapid development of asymmetric catalysis, the demand for the enantioselective synthesis of complex and diverse molecules with different chiral elements is increasing. Owing to the unique features of atropisomerism, the catalytic asymmetric synthesis of atropisomers has attracted a considerable interest from the chemical science community. In particular, introducing additional chiral elements, such as carbon centered chirality, heteroatomic chirality, planar chirality, and helical chirality, into atropisomers provides an opportunity to incorporate new properties into axially chiral compounds, thus expanding the potential applications of atropisomers. Thus, it is important to perform catalytic asymmetric transformations to synthesize atropisomers bearing multiple chiral elements. In spite of challenges in such transformations, in recent years, chemists have devised powerful strategies under asymmetric organocatalysis or metal catalysis, synthesizing a wide range of enantioenriched atropisomers bearing multiple chiral elements. Therefore, the catalytic asymmetric synthesis of atropisomers bearing multiple chiral elements has become an emerging field. This review summarizes the rapid progress in this field and indicates challenges, thereby promoting this field to a new horizon.

Organocatalytic diastereo- and atroposelective construction of N-N axially chiral pyrroles and indoles

The construction of N-N axially chiral motifs is an important research topic, owing to their wide occurrence in natural products, pharmaceuticals and chiral ligands. One efficient method is the atroposelective dihydropyrimidin-4-one formation. We present herein a direct catalytic synthesis of N-N atropisomers with simultaneous creation of contiguous axial and central chirality by oxidative NHC (N-heterocyclic carbenes) catalyzed (3 + 3) cycloaddition. Using our method, we are able to synthesize structurally diverse N-N axially chiral pyrroles and indoles with vicinal central chirality or bearing a 2,3-dihydropyrimidin-4-one moiety in moderate to good yields and excellent enantioselectivities. Further synthetic transformations of the obtained axially chiral pyrroles and indoles derivative products are demonstrated. The reaction mechanism and the origin of enantioselectivity are understood through DFT calculations.

The rapid construction and biological evaluation of densely substituted pyrrolo[1,2-a]indoles via a BF3·OEt2-assisted cascade approach

Lewis-acid cascade reactions promoted by BF3·OEt2 are reported for the synthesis of highly substituted pyrrolo[1,2-a]indoles and congeners of benzofuro[2,3-b]indoles. These reactions are highly regio- and diastereoselective towards generating up to five contiguous stereogenic centers, including two vicinal quaternary centers. Furthermore, an established cascade approach and the mechanism proposed herein are well supported by quantum chemistry calculations. In addition, a self-dimerization intermediate was trapped and isolated to establish a strategy for potential access to both pyrrolo and benzo indole derivatives, leaving sufficient freedom for broadening. Furthermore, in-silico molecular docking and all atomistic molecular dynamic (MD) simulation analysis suggests that the synthesized pyrrolo[1,2-a]indole derivatives stably bind at the active site of the mycobacterial secreted tyrosine phosphatase B (MptpB) enzyme, an emerging anti-mycobacterial drug target. Deep learning-based affinity predictions and MMPBGBSA-based energy calculations of the docked poses are presented herein.

Yb(OTf)3-Catalyzed Formal (4 + 3) Cycloaddition Reactions of 3-Benzylideneindoline-2-thiones with Donor-Acceptor Cyclopropanes

A Yb(OTf)3-catalyzed formal (4 + 3) cycloaddition reaction of donor-acceptor cyclopropanes with 3-benzylideneindoline-2-thiones as sulfur-containing 4π components has been successfully achieved. A series of functionalized 5,10-dihydro-2H-thiepino[2,3-b]indole derivatives were synthesized with good yields and moderate to good diastereoselectivity. The reaction described herein represented the inaugural (4 + 3) cycloaddition of 3-benzylideneindoline-2-thiones.

Enantioselective Alkynylation of 2-Aryl-3H-indol-3-ones via Cooperative Catalysis of Copper/Chiral Phosphoric Acid

A facile enantioselective alkynylation of cyclic ketimines attached to a neutral functional group utilizing the dual Cu(I)-CPA catalysis is described. The strategy of the alkynylation of 2-aryl-3H-indol-3-one directly to chiral propargylic amines containing indolin-3-one moiety in good yields and enantioselectivities. Moreover, gram-scale synthesis of chiral propargylamines based C2-quaternary indolin-3-ones was performed. The synthetic applications were confirmed by transformations of the products with no decrease in the yield and enantioselectivity.

Asymmetric (4+n) Cycloadditions of Indolyldimethanols for the Synthesis of Enantioenriched Indole-Fused Rings

Catalytic asymmetric construction of chiral indole-fused rings has become an important issue in the chemical community because of the significance of such scaffolds. In this work, we have accomplished the first catalytic asymmetric (4+2) and (4+3) cycloadditions of 2,3-indolyldimethanols by using indoles and 2-naphthols as suitable reaction partners under the catalysis of chiral phosphoric acids, constructing enantioenriched indole-fused six-membered and seven-membered rings in high yields with excellent enantioselectivities. In addition, this approach is used to realize the first enantioselective construction of challenging tetrahydroindolocarbazole scaffolds, which are found to show promising anticancer activity. More importantly, theoretical calculations of the reaction pathways and activation mode offer an in-depth understanding of this class of indolylmethanols. This work not only settles the challenges in realizing catalytic asymmetric cycloadditions of indolyldimethanols but also provides a powerful strategy for the construction of enantioenriched indole-fused rings.

Pseudo-Diastereodivergent Synthesis of Chiral Fluorenes Bearing Bis-1,3-Nonadjacent Stereogenic Centers via Organocatalytic Desymmetrization of meso-Epoxides

We report an enantio- and diastereodivergent synthesis of enantioenriched fluorenes bearing bis-1,3-nonadjacent stereocenters with broad substrate scope and high enantioselectivity (up to 99% ee) under low catalyst loading (0.1 mol %). The key to the success of this method is the pseudo-diastereodivergent desymmetrization of stereoisomers of meso-epoxides enabled by the same organocatalyst. Furthermore, some of the chiral fluorenes obtained exhibit high fluorescence quantum yields (up to 76.6%), as evidenced by photophysical properties studies.

Organocatalytic intramolecular (4 + 2) annulation of enals with ynamides: atroposelective synthesis of axially chiral 7-aryl indolines

Catalytic enantioselective transformation of alkynes has become a powerful tool for the synthesis of axially chiral molecules. Most of these atroposelective reactions of alkynes rely on transition-metal catalysis, and the organocatalytic approaches are largely limited to special alkynes which act as the precursors of Michael acceptors. Herein, we disclose an organocatalytic atroposelective intramolecular (4 + 2) annulation of enals with ynamides. This method allows the efficient and highly atom-economical preparation of various axially chiral 7-aryl indolines in generally moderate to good yields with good to excellent enantioselectivities. Computational studies were carried out to elucidate the origins of regioselectivity and enantioselectivity. Furthermore, a chiral phosphine ligand derived from the synthesized axially chiral 7-aryl indoline was proven to be potentially applicable to asymmetric catalysis.

Asymmetric Synthesis of Axially Chiral Molecules via Organocatalytic Cycloaddition and Cyclization Reactions

Atropisomeric molecules are present in many natural products, biologically active compounds, chiral ligands and catalysts. Many elegant methodologies have been developed to access axially chiral molecules. Among them, organocatalytic cycloaddition and cyclization have attracted much attention because they have been widely used in the asymmetric synthesis of biaryl/heterobiaryls atropisomers via construction of carbo- and hetero-cycles. This strategy has undoubtedly become and will continue to be a hot topic in the field of asymmetric synthesis and catalysis. This review aims to highlight the recent advancements in this field of atropisomer synthesis by using different organocatalysts in cycloaddition and cyclization strategies. The construction of each atropisomer, its possible mechanism, the role of catalysts, and its potential applications are illustrated.

(Z)-Selective Synthesis of Bromofluoroalkenes via the TMSCF2Br-Mediated Tandem Reaction with para-Quinone Methides

We report herein a tandem reaction of para-quinone methides with TMSCF₂Br to construct bromofluoroalkenes in a Z-selective manner. While TMSCF₂Br has been documented as the precursor of difluoro carbene, it exhibits another possibility in this transformation, a formal bromofluoro carbene surrogate. The alkenyl bromide unit of the products could directly engage in a variety of transformations.

Catalytic asymmetric de novo construction of 4-pyrrolin-2-ones via intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones

The chiral phosphoric acid-catalyzed asymmetric intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones has been established. This convergent protocol leads to a facile and enantioselective de novo construction of a wide range of fully substituted 4-pyrrolin-2-ones bearing a fully substituted carbon atom in good yields and with excellent enantioselectivities (26 examples, 72-95% yields and 87-99% ee).

Organocatalytic (Z/E)-Selective Synthesis of 3-Vinylnaphthofurans via a Formal (3 + 2) Cycloaddition

A formal (3 + 2) cycloaddition of 1,4-enediones with 2-naphthols was established under the catalysis of trifluoromethanesulfonic acid as an organocatalyst, leading to the efficient synthesis of structurally diverse 3-vinylnaphthofurans with high yields and excellent (Z/E)-selectivities (up to 96% yield, all >20:1 Z/E). This formal (3 + 2) cycloaddition involved a cascade reaction process, and the intramolecular hydrogen bond in the structure of 3-vinylnaphthofurans should play an important role in controlling the (Z/E)-selectivity of the newly formed vinyl group. Moreover, this class of 3-vinylnaphthofurans was discovered to have an axial chirality. This work provides an organocatalytic approach for constructing multi-substituted vinylnaphthofurans via a cascade reaction with excellent control of the (Z/E)-selectivity, which will serve as a useful strategy for synthesizing vinylnaphthofurans via in situ construction of the furan core and formation of the vinyl group.

Switchable synthesis of natural-product-like lawsones and indenopyrazoles through regioselective ring-expansion of indantrione

Lawsones and indenopyrazoles are the prevalent structural motifs and building blocks in pharmaceuticals and bioactive molecules, but their synthesis has always remained challenging as no comprehensive protocol has been outlined to date. Herein, a metal-free, ring-expansion reaction of indantrione with diazomethanes, generated in situ from the N-tosylhydrazones, has been developed for the synthesis of lawsone and indenopyrazole derivatives in acetonitrile and alcohol solvents, respectively. It provides these valuable lawsone and pyrazole skeletons in good yields and high levels of diastereoselectivity from simple and readily available starting materials. DFT calculations were used to explore the mechanism in different solutions. The synthetic application example also showed the prospects of this method for the preparation of valuable compounds.

Organocatalytic Atroposelective Synthesis of Indole Derivatives Bearing Axial Chirality: Strategies and Applications

Catalytic atroposelective syntheses of axially chiral compounds have stimulated extensive interest in multiple communities, such as synthetic chemistry, biochemistry, and materials science, because of the intriguing characteristics of atropisomerism. In particular, atropisomeric indole derivatives, which contain a kind of five-membered heterocyclic framework, are widely distributed in a number of natural alkaloids, biologically relevant compounds, chiral ligands, and chiral organocatalysts. Hence, the catalytic atroposelective synthesis of indole derivatives bearing axial chirality is of considerable importance and has become an emerging focus of research. However, there are substantial challenges associated with the atroposelective synthesis of indole derivatives, including remote ortho-substituents around the chiral axis, a lower barrier for rotation, and a weaker configurational stability than that of atropisomeric six-membered biaryls. Therefore, the development of effective strategies toward the catalytic atroposelective synthesis of indole derivatives has become an urgent task.In order to tackle these challenges and to accomplish the task, our group devised a unique strategy of designing indole-derived platform molecules and developing organocatalytic enantioselective transformations of such platform molecules to synthesize atropisomeric indole derivatives; asymmetric organocatalysis has tremendous advantages and was the research area recognized by the Nobel Prize in Chemistry in 2021. This Account summarizes our endeavors in the organocatalytic atroposelective synthesis of indole derivatives bearing axial chirality. In brief, we devised and developed a series of indole-derived platform molecules, such as indolylmethanols, (hetero)aryl indoles, oxindole-based styrenes, N-aminoindoles, and indole-based homophthalic anhydrides, by introducing different functional groups onto the indole ring to achieve new reactivity and modulate the reactive site of the indole ring. As a result, these indole-derived platform molecules possess versatile and unique reactivity and are capable of undergoing a variety of organocatalytic enantioselective transformations for preparing structurally diversified indole derivatives with axial chirality.We used these strategies to accomplish the atroposelective synthesis of plenty of indole derivatives with axial chirality, including (hetero)aryl indoles, alkene-indoles, oxindole-based styrenes, N-pyrrolylindoles, and isochromenone-indoles. In addition, we gave a thorough and detailed understanding of the designed reaction by investigating the reaction pathway and activation mode. More importantly, we studied the biological activity of some products and performed catalyst design on the basis of atropisomeric indole moieties, which are helpful for disclosing more applications of indole derivatives bearing axial chirality.In the future, the organocatalytic atroposelective synthesis of indole derivatives bearing axial chirality will indubitably remain a frontier topic in the research area of asymmetric catalysis and chiral indole chemistry despite challenging issues, for instance, the atroposelective synthesis of novel indole derivatives bearing an unconventional chiral axis, the development of atropisomeric indole derivatives into powerful catalysts or ligands, and the discovery of atroposelective indole derivatives as potent drug candidates. We hope our efforts summarized in this Account will encourage chemists worldwide to devise innovative strategies toward solving the challenging issues that remain in this field, thus promoting its development to a higher level.

Catalyst-Controlled Divergent Reactions of 2,3-Disubstituted Indoles with Propargylic Alcohols: Synthesis of 3H-Benzo[b]azepines and Axially Chiral Tetrasubstituted Allenes

Catalyst-controlled divergent reactions of 2,3-disubstituted indoles with propargylic alcohols were developed for the first time. In the presence of TsOH or B(C₆F₅)₃ as catalyst, 2,3-disubstituted indoles reacted smoothly with 3-alkynyl-3-hydroxyisoindolinones to afford 3H-benzo[b]azepines by selective C2(sp²)-C3(sp²) ring expansion of indoles. In contrast, decreasing the catalyst strength (e.g., with chiral phosphoric acid) interrupted the cascade reactions, affording axially chiral tetrasubstituted allenes bearing an adjacent chiral quaternary carbon stereocenter. Control experiments provided insights into the reaction mechanism.

Iminosugar-Phosphines as Organocatalysts in the [3 + 2] Cycloaddition of Allenoates and N-Tosylimines

Iminosugar derivatives containing a pyrrolidine-phosphine moiety were prepared from carbohydrates and used as catalysts in the [3 + 2] cycloaddition reaction between alkyl allenoates and electron-deficient imines. The corresponding 1,2,3,5-tetrasubstituted pyrrolines were obtained in good yields and diastereoselectivities but with moderate enantiocontrol. The stereochemical outcome of the reaction depends on the substituent at the nitrogen atom and hydroxyl groups, the configuration of the stereogenic centers and the distance between the diphenylphosphine group and the pyrrolidine skeleton of the catalyst. The preparation of both enantiomers of the catalyst allowed the corresponding enantiomeric pyrrolines to be obtained with similar yields, diastereo- and enantioselectivities.

Chiral Brønsted Acid Catalyzed Enantioselective Synthesis of Spiro-Isoindolinone-Indolines via Formal [3 + 2] Cycloaddition

A novel organocatalytic asymmetric formal [3 + 2] cycloaddition of 3-substituted 1H-indoles with in situ generated 3-hydroxy-isoindolinone-derived β,γ-alkynyl-α-ketimines has been developed. A variety of biologically relevant chiral spiro-isoindolinone-indolines were achieved with excellent yields (up to 99%) and enantioselectivity (up to 99% ee) under mild conditions. The gram-scale reaction of this methodology and several interesting transformations of the products have been demonstrated.

Cu(I)-Catalyzed Asymmetric Arylation of Pyrroles with Diaryliodonium Salts toward the Synthesis of N-N Atropisomers

We report herein that copper(I) catalysis using a bis(phosphine) dioxide ligand can catalyze the desymmetric C-H arylation of prochiral bipyrroles. More than 50 nitrogen-nitrogen atropisomers were achieved in good to excellent yields with excellent enantioselectivities (≤97% yield, ≤98% ee). The reaction proceeds under mild conditions with good functional group compatibility on arenes and diaryliodonium salts. Moreover, this principle enables iterative arylation of the bipyrroles to enantioselectively arylate different positions during the catalysis of copper.