NHC-catalyzed atropoenantioselective synthesis of axially chiral biaryl amino alcohols via a cooperative strategy

Atroposelective Construction of C─B Axial Chirality via N-Heterocyclic Carbene-Catalyzed Dynamic Kinetic Resolution

C─B axially chiral architectures are valuable in materials science and medicinal chemistry, but their enantioselective synthesis remains a challenge. Herein, we report an efficient method for the enantioselective synthesis of C─B axially chiral 1,2-azaborines through N-heterocyclic carbene-catalyzed dynamic kinetic resolution. The treatment of racemic 1,2-azaborine-based arylaldehyde with a chiral N-heterocyclic carbene catalyst under oxidative conditions in the presence of an alcohol leads to atroposelective esterification with up to 97% yield and 98% ee. The practicality of this method has been demonstrated by the late-stage functionalization, gram-scale synthesis, and further synthetic transformations. Mechanistic studies indicate that the chiral N-heterocyclic carbene catalyst differentiates between rapidly equilibrating atropoisomeric 1,2-azaborine-based arylaldehyde. DFT studies suggest that the formation of the Breslow intermediate via [Cs]HCO3-assisted [1,2]-proton transfer is the enantioselectivity-determining step.

Chiral Brønsted acid-catalyzed intermolecular hydrophenoxylation of ynamides with biaryl phenols via sequential desymmetrization/kinetic resolution

Herein we disclose the first catalytic asymmetric intermolecular hydrophenoxylation of ynamides with biaryl phenols under chiral Brønsted acid catalysis, providing axially chiral phenols. This enantioselective reaction proceeds through a sequential desymmetrizaiton/kinetic resolution process, which features mild reaction conditions and represents a rare paradigm of an organocatalytic asymmetric intermolecular reaction of ynamides.

Catalytic Atroposelective Electrophilic Amination to Access Axially Chiral Diaryl Phenols

An enantioselective synthesis of axially chiral diaryl phenols containing sulfonamide groups has been achieved involving an electrophilic amination of 1,1'-biaryl-2,6-diols with N-sulfonyl quinone diimines, catalyzed by a chiral phosphoric acid. This atroposelective reaction offers a modular approach to enantiopure diaryl phenols, with good-to-excellent yields.

Enantiopure Turbo Chirality Targets in Tri-Propeller Blades: Design, Asymmetric Synthesis, and Computational Analysis

Enantiopure turbo chirality in small organic molecules, without other chiral elements, is a fascinating topic that has garnered significant interest within the chemical and materials science community. However, further research into and application of this concept have been severely limited by the lack of effective asymmetric tools. To date, only a few enantiomers of turbo chiral targets have been isolated, and these were obtained through physical separation using chiral HPLC, typically on milligram scales. In this work, we report the first asymmetric approach to enantiopure turbo chirality in the absence of other chiral elements such as central and axial chirality. This is demonstrated by assembling aromatic phosphine oxides, where three propeller-like groups are anchored to a P(O) center via three axes. Asymmetric induction was successfully carried out using a chiral sulfonimine auxiliary, with absolute configurations and conformations unambiguously determined by X-ray diffraction analysis. The resulting turbo frameworks exhibit three propellers arranged in either a clockwise (P,P,P) or counterclockwise (M,M,M) configuration. In these arrangements, the bulkier sides of the aromatic rings are oriented toward the oxygen atom of the P=O bond rather than in the opposite direction. Additionally, the orientational configuration is controlled by the sulfonimine auxiliary as well, showing that one of the Naph rings is pushed away from the auxiliary group (-CH2-NHSO2-tBu) of the phenyl ring. Computational studies were conducted on relative energies for the rotational barriers of a turbo target along the P=O axis and the transition pathway between two enantiomers, meeting our expectations. This work is expected to have a significant impact on the fields of chemistry, biomedicine, and materials science in the future.

Pd/NHC sequentially catalyzed atroposelective synthesis of planar-chiral macrocycles

Planar-chiral macrocycles play a pivotal role in host-guest chemistry and drug discovery. However, compared with the synthesis of other types of chiral compounds, the asymmetric construction of planar-chiral macrocycles still remains a forbidding challenge. Herein, we report a sequential palladium and N-heterocyclic carbene catalysis to build planar-chiral macrocycles. This protocol features broad scope and good functional group tolerance, and allows a rapid assembling of planar-chiral macrocycles with excellent enantioselectivities.

Construction of Axially Chiral 4-Aminoquinolines by Cycloaddition and Central-to-Axial Chirality Conversion

A two-step strategy has been established for the enantioselective synthesis of 4-aminoquinolines possessing axial chirality. This approach involves a chiral phosphoric acid-catalyzed cycloaddition, followed by a DDQ oxidation step. The method offers efficient access to a variety of 1,1'-biaryl-2,2'-amino alcohol derivatives in excellent yields and enantioselectivities (up to 98% yield and 93% ee). Furthermore, the synthetic transformation of the products was also investigated.

Halogen-Bond-Assisted NHC-Catalyzed (Dynamic) Kinetic Resolution for the Atroposelective Synthesis of Heterobiaryls

We report a novel halogen-bond-assisted NHC-catalyzed (dynamic) kinetic resolution strategy for the synthesis of axially chiral heterobiaryls. A class of axially chiral quinolines are prepared efficiently in excellent enantioselectivities (≤98% ee) employing 3-5 mol % NHC catalyst. Mechanistic studies reveal the indispensability of 5-bromo-2-iodobenzaldehyde in this reaction, in which a pivotal halogen bonding interaction plays a crucial role in the process.

Recent Advances on Catalytic Atroposelective Synthesis of Planar-Chiral Macrocycles

Planar-chiral skeletons widely exist in natural products, bioactive compounds, and other functional molecules. Although significant progress has been made in the field of asymmetric synthesis of centrally or axially chiral molecules over the past years, enantioselective constructing of planar chirality is still a big obstacle and numerous efforts have been made in this field. Previous works have mainly focused on the assembly of planar-chiral [2,2]-paracyclophanes and metallocenes. This Minireview describes recent advancements in asymmetric catalytic synthesis of planar-chiral macrocycles, including ansa chain construction, plane formation and asymmetric transformation strategies. It is anticipated that this Minireview will sever as a source of inspiration for developing new unconventional procedures for access to planar-chiral skeletons.

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

Carbene-Catalyzed and Pnictogen Bond-Assisted Access to PIII-Stereogenic Compounds

Intermolecular pnictogen bonding (PnB) catalysis has received increased interest in non-covalent organocatalysis. It has been demonstrated that organic electron-deficient pnictogen atoms can act as prospective Lewis acids. Here, we present a catalytic approach for the asymmetric synthesis of chiral PIII compounds by combining intramolecular PnB interactions and carbene catalysis. Our design features a pre-chiral phosphorus molecule bearing two electron-withdrawing benzoyl groups, resulting in the formation of a σ-hole at the P atom. X-ray and non-covalent interaction (NCI) analysis indicate that the model substrates exhibit intrinsic PnB interaction between the oxygen atom of the formyl group and the phosphorus atom. This induces a conformational locking effect, leading to the crystallization of the phosphorus substrate in a preferred conformation (P212121 chiral group). Under the catalysis of N-heterocyclic carbene, the aldehyde moiety activated by the pnictogen bond selectively reacts with an alcohol to yield the corresponding chiral monoester/phosphorus product with excellent enantioselectivity. This Lewis acidic phosphorus center, aroused by the non-polarized intramolecular pnictogen bond interaction, assists in conformational and selective regulations, providing unique opportunities for catalysis and beyond.

Atroposelective catalysis

Atropisomeric compounds-stereoisomers that arise from the restricted rotation about a single bond-have attracted widespread attention in recent years due to their immense potential for applications in a variety of fields, including medicinal chemistry, catalysis and molecular nanoscience. This increased interest led to the invention of new molecular motors, the incorporation of atropisomers into drug discovery programmes and a wide range of novel atroposelective reactions, including those that simultaneously control multiple stereogenic axes. A diverse set of synthetic methodologies, which can be grouped into desymmetrizations, (dynamic) kinetic resolutions, cross-coupling reactions and de novo ring formations, is available for the catalyst-controlled stereoselective synthesis of various atropisomer classes. In this Review, we generalize the concepts for the catalyst-controlled stereoselective synthesis of atropisomers within these categories with an emphasis on recent advancements and underdeveloped atropisomeric scaffolds beyond stereogenic C(sp2)-C(sp2) axes. We also discuss more complex systems with multiple stereogenic axes or higher-order stereogenicity.

Carbene-Catalyzed Atroposelective Construction of Chiral Diaryl Ethers

Atropoisomeric chemotypes of diaryl ethers-related scaffolds are prevalent in naturally active compounds. Nevertheless, there remains considerable research to be carried out on the catalytic asymmetric synthesis of these axially chiral molecules. In this instance, we disclose an N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. NHC desymmetrization produces axially chiral diaryl ether atropisomers with high yields and enantioselectivities in moderate circumstances. Chiral diaryl ether compounds may be precursors for highly functionalized diaryl ethers with bioactivity and chiral ligands for asymmetric catalysis.

Synthesis of axially chiral diaryl ethers via NHC-catalyzed atroposelective esterification

Axially chiral diaryl ethers bearing two potential axes find unique applications in bioactive molecules and catalysis. However, only very few catalytic methods have been developed to construct structurally diverse diaryl ethers. We herein describe an NHC-catalyzed atroposelective esterification of prochiral dialdehydes, leading to the construction of enantioenriched axially chiral diaryl ethers. Mechanistic studies indicate that the matched kinetic resolutions play an essential role in the challenging chiral induction of flexible dual-axial chirality by removing minor enantiomers via over-functionalization. This protocol features mild conditions, excellent enantioselectivity, broad substrate scope, and applicability to late-stage functionalization, and provides a modular platform for the synthesis of axially chiral diaryl ethers and their derivatives.

Mechanism and Origin of Stereoselectivity for the NHC-Catalyzed Desymmetrization Reaction for the Synthesis of Axially Chiral Biaryl Aldehydes

Organocatalytic desymmetrization reaction is a powerful tool for constructing axial chirality, but the theoretical study on the origin of stereoselectivity still lags behind even now. In this work, the N-heterocyclic carbene (NHC)-catalyzed desymmetrization reaction of biaryl frameworks for the synthesis of axially chiral aldehydes has been selected and theoretically investigated by using density functional theory (DFT). The fundamental pathway involves several steps, i.e., desymmetrization, formation of Breslow oxidation, esterification, and NHC regeneration. The desymmetrization and formation of Breslow processes have been identified as stereoselectivity-determining and rate-determining steps. Further weak interaction analyses proved that the C-H···O hydrogen bond and C-H···π interactions are responsible for the stability of the key stereoselective desymmetrization transition states. This research contributes to understanding the nature of NHC-catalyzed desymmetrization reactions for the synthesis of axially chiral compounds.

Atroposelective Synthesis of Aldehydes via Alcohol Dehydrogenase-Catalyzed Stereodivergent Desymmetrization

Axially chiral aldehydes have emerged recently as a unique class of motifs for drug design. However, few biocatalytic strategies have been reported to construct structurally diverse atropisomeric aldehydes. Herein, we describe the characterization of alcohol dehydrogenases to catalyze atroposelective desymmetrization of the biaryl dialdehydes. Investigations into the interactions between the substrate and key residues of the enzymes revealed the distinct origin of atroposelectivity. A panel of 13 atropisomeric monoaldehydes was synthesized with moderate to high enantioselectivity (up to >99% ee) and yields (up to 99%). Further derivatization allows enhancement of the diversity and application potential of the atropisomeric compounds. This study effectively expands the scope of enzymatic synthesis of atropisomeric aldehydes and provides insights into the binding modes and recognition mechanisms of such molecules.

Kinetic Resolution of Acyclic Tertiary Propargylic Alcohols by NHC-Catalyzed Enantioselective Acylation

We report herein an efficient NHC-catalyzed kinetic resolution of acyclic tertiary propargylic alcohols that provides them in high to excellent enantioselectivity. This is the first example of kinetic resolution realized by enantioselective acylation. The recovered enantioenriched alcohols can be facilely converted into other valuable compounds such as densely functionalized tertiary alcohols and carbmates in high yields and excellent stereopurity. Density functional theory calculations were performed to determine the reaction mechanism and to understand the origin of enantiodiscrimination.

Enantioselective Synthesis of Axially Chiral Diaryl Ethers via NHC Catalyzed Desymmetrization and Following Resolution

Axially chiral diaryl ethers are present in numerous natural products and bioactive molecules. However, only few catalytic enantioselective approaches have been established to access diaryl ether atropisomers. Herein, we report the N-heterocyclic carbene-catalyzed enantioselective synthesis of axially chiral diaryl ethers via desymmetrization of prochiral 2-aryloxyisophthalaldehydes with aliphatic alcohols, phenol derivatives, and heteroaromatic amines. This reaction features mild reaction conditions, good functional group tolerance, broad substrate scope and excellent enantioselectivity. The utility of this methodology is illustrated by late-stage functionalization, gram-scale synthesis, and diverse enantioretentive transformations. Control experiments and DFT calculations support the association of NHC-catalyzed desymmetrization with following kinetic resolution to enhance the enantioselectivity.

Access to Axially Chiral Aryl Aldehydes via Carbene-Catalyzed Nitrile Formation and Desymmetrization Reaction

An approach utilizing N-heterocyclic carbene for nitrile formation and desymmetrization reaction is developed. The process involves kinetic resolution, with the axially chiral aryl monoaldehydes obtained in moderate yields with excellent optical purities. These axially chiral aryl monoaldehydes can be conveniently transformed into functionalized molecules, showing great potential as catalysts in organic chemistry.

Atroposelective Synthesis of Planar-Chiral Indoles via Carbene Catalyzed Macrocyclization

Indole-based planar-chiral macrocycles are widely found in natural products and bioactive molecules. However, in sharp contrast to the preparation of indole-based axially chiral structures, the enantioselective catalysis of indole-based planar-chiral macrocycles is still a formidable task so far. Here we report an N-heterocyclic carbene (NHC)-catalyzed intramolecular atroposelective macrocyclization of 3-carboxaldehyde indole/pyrroles, featuring with broad substrate scope and good functional group tolerance, and allowing for a rapid access to diverse indole/pyrrole-based planar-chiral macrocycles with various tether-lengths (10-16 members) in good yields and with excellent enantioselectivities. Importantly, the indole-based macrocyclic structures with both planar and axial chirality were directly and efficiently obtained through this protocol with excellent enantioselectivities and diastereoselectivities. In addition, these synthesized planar-chiral macrocycles offer many possibilities for chemists to develop new catalysts or ligands, as well as new reactions.

Enantioselective Synthesis of C-O Axially Chiral Diaryl Ethers by NHC-Catalyzed Atroposelective Desymmetrization

Axially chiral diaryl ethers, a distinguished class of atropisomers possessing unique dual C-O axis, hold immense potential for diverse research domains. In contrast to the catalytic enantioselective synthesis of conventional single axis bearing atropisomers, the atroposelective synthesis of axially chiral ethers containing flexible C-O axis remains a significant challenge. Herein, we demonstrate the first N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. Mechanistically, the reaction proceeds via NHC-catalyzed desymmetrization strategy to afford the corresponding axially chiral diaryl ether atropisomers in good yields and high enantioselectivities under mild conditions. The derivatization of the synthesized product expands the utility of present strategy via access to a library of C-O axially chiral compounds. The temperature dependency and preliminary investigations on the racemization barrier of C-O bonds are also presented.

Chiral Phosphoric Acid Catalyzed Asymmetric Hydrolysis of Biaryl Oxazepines for the Synthesis of Axially Chiral Biaryl Amino Phenol Derivatives

The development of catalytic asymmetric reaction with water as the reactant is challenging due to the reactivity- and stereoselectivity-control issues resulted from the low nucleophilicity and the small size of water. We disclose herein a chiral phosphoric acid (CPA) catalyzed atroposelective ring-opening reaction of biaryl oxazepines with water. A series of biaryl oxazepines undergo the CPA catalyzed asymmetric hydrolysis in a highly enantioselective manner. The key for the success of this reaction is the use of a new SPINOL-derived CPA catalyst and the high reactivity of biaryl oxazepine substrates towards water under acidic conditions. Density functional theory calculations suggest that the reaction proceeds via a dynamic kinetic resolution pathway and the CPA catalyzed addition of water to the imine group is both enantio- and rate-determining.

Atroposelective desymmetrization of 2-arylresorcinols via Tsuji-Trost allylation

Palladium-catalyzed asymmetric allylic alkylation has proven to be a powerful method for the preparation of a wide variety of chiral molecules. However, the catalytic and atroposelective allylic alkylation is still rare and challenging, especially for biaryl substrates. Herein, we report the palladium-catalyzed desymmetric and atroposelective allylation, in which the palladium complex with a chiral phosphoramidite ligand enables desymmetrization of nucleophilic 2-arylresorcinols in a highly enantioselective manner. With the aid of the secondary kinetic resolution effect, a wide variety of substrates containing a hydroxymethyl group at the bottom aromatic ring are able to provide O-allylated products up to 98:2 er. Computational studies show an accessible quadrant of the allylpalladium complex and provide three plausible transition states with intra- or intermolecular hydrogen bonding. The energetically favorable transition state is in good agreement with the observed enantioselectivity and suggests that the catalytic reaction would proceed with an intramolecular hydrogen bond.

A Highly Enantioselective Homoenolate Michael Addition/Esterification Sequence of Cyclohexadienone-Tethered Enals via NHC Catalysis

Reported here is a highly enantioselective homoenolate Michael addition/esterification sequence of cyclohexadienone-tethered enals via N-heterocyclic carbene (NHC) catalysis, affording the enantiopure cis-hydrobenzofurans, cis-hydroindoles, and cis-hydroindenes. The NHC catalyst bearing a nitro group greatly enhances the stereocontrol, and a bulky N-aryl substituent of the triazolium salt in the catalyst is helpful for inhibiting the further aldol condensation after homoenolate Michael addition. The utility of this protocol is highlighted by a gram-scale experiment and versatile downstream transformations.

Atroposelective Access to 1,3-Oxazepine-Containing Bridged Biaryls via Carbene-Catalyzed Desymmetrization of Imines

We disclose herein an atroposelective synthesis of novel bridged biaryls containing medium-sized rings via N-heterocyclic carbene organocatalysis. The reaction starts with addition of the carbene catalyst to the aminophenol-derived aldimine substrate. Subsequent oxidation and intramolecular desymmetrization lead to the formation of 1,3-oxazepine-containing bridged biaryls in good yields and excellent enantioselectivities. These novel bridged biaryl products can be readily transformed into chiral phosphite ligands. Preliminary density function theory calculations suggest that the origin of enantioselectivity arises from the more favorable frontier molecular orbital interactions in the transition state leading to the major product.

Approaches to Synthesis and Isolation of Enantiomerically Pure Biologically Active Atropisomers

Atropisomerism is a stereochemical phenomenon exhibited by molecules containing a rotationally restricted σ bond. Contrary to classical point chirality, the two atropisomeric stereoisomers exist as a dynamic mixture and can be interconverted without the requirement of breaking and reforming a bond. Although this feature increases structural complexity, atropisomers have become frequent targets in medicinal chemistry projects. Their axial chirality, e.g., from axially chiral biaryl motifs, gives access to unique 3D structures. It is often desirable to have access to both enantiomers of the atropisomers via a nonselective reaction during the early discovery phase as it allows the medicinal chemistry team to probe the structure activity relationship in both directions. However, once a single atropisomer is selected, it presents several problems. First, the pure single atropisomer may interconvert to the undesired stereoisomer under certain conditions. Second, separation of atropisomers is nontrivial and often requires expensive chiral stationary phases using chromatography or additives if a salt resolution approach is chosen. Other options can be kinetic resolution using enzymes or chiral catalysts. However, apart from the high cost often associated with the two latter methods, a maximum yield of only 50% of the desired atropisomer can be obtained. The ideal approach is to install the chiral atropisomeric axis enantioselectively or employing a dynamic kinetic resolution approach. In theory, both approaches have the potential to provide a single atropisomer in quantitative yield. This Account will discuss the successes/failures and challenges we have experienced in developing methods for resolution/separation and asymmetric synthesis of atropisomeric drug candidates in one of our early phase drug development projects. Suitability for the different methods at various stages of the drug development phase is discussed. Depending on the scale and time available, a separation of a mixture of atropisomers by chromatography was sometimes preferred, whereas asymmetric- or resolution approaches were desired for long-term supply. With the use of chromatography, the impact on separation efficiency and solvent consumption, depending on the nature of the substrate, is discussed. We hope that with this Account the readers will get a better view on the challenges medicinal and process chemists meet when designing new atropisomeric drug candidates and developing processes for manufacture of a single atropisomer.

Dynamic kinetic resolution of γ,γ-disubstituted indole 2-carboxaldehydes via NHC-Lewis acid cooperative catalysis for the synthesis of tetracyclic ε-lactones

The ubiquity of ε-lactones in various biologically active compounds inspired the development of efficient and enantioselective routes to these target compounds. Described herein is the enantioselective synthesis of indole-fused ε-lactones by the N-heterocyclic carbene (NHC)-Lewis acid cooperative catalyzed dynamic kinetic resolution (DKR) of in situ generated γ,γ-disubstituted indole 2-carboxaldehydes. The Bi(OTf)3-catalyzed Friedel-Crafts reaction of indole-2-carboxaldehyde with 2-hydroxy phenyl p-quinone methides generates γ,γ-disubstituted indole 2-carboxaldehydes, which in the presence of NHC and Bi(OTf)3 afforded the desired tetracyclic ε-lactones in up to 93% yield and >99 : 1 er. Moreover, preliminary studies on the mechanism of this formal [4 + 3] annulation are also provided.

Enantioselective Synthesis of Atropisomers via Vinylidene ortho-Quinone Methides (VQMs)

Atropisomers, arising from conformational restriction, are inherently chiral due to the intersecting dissymmetric planes. Since there are numerous applications of enantiopure atropisomers in catalyst design, drug discovery, and material science, the asymmetric preparation of these highly prized molecules has become a flourishing field in synthetic chemistry. A number of catalysts, synthetic procedures, and novel concepts have been developed for the manufacture of the atropisomeric molecules. However, due to the intrinsic properties of different types of atropisomers featuring biaryl, hetero-biaryl, or non-biaryl architectures, only very few methods pass the rigorous inspection and are considered generally applicable. The development of a broadly applicable synthetic strategy for various atropisomers is a challenge. In this Account, we summarize our recent studies on the enantioselective synthesis of atropisomers using the vinylidene ortho-quinone methides (VQMs) as pluripotent intermediates.The most appealing features of VQMs are the disturbed aromaticity and axial chirality of the allene fragment. At the outset, the applications of VQMs in organic synthesis have been neglected due to their principal liabilities: ephemeral nature, extraordinary reactivity, and multireaction sites. The domestication of this transient intermediate was demonstrated by in situ catalytic asymmetric generation of VQMs, and the reactivity and selectivity were fully explored by judiciously modifying precursors and tuning catalytic systems. A variety of axially chiral heterocycles were achieved through five-, six-, seven- and nine-membered ring formation of VQM intermediates with different kinds of branched nucleophilic functional groups. The axially chiral C-N axis could be constructed from VQM intermediates via N-annulation or desymmetrization of preformed C-N scaffolds. We take advantage of the high electrophilicity of VQMs toward a series of sulfur and carbon based nucleophiles leading to atropisomeric vinyl arenes. Furthermore, chiral helical compounds were realized by cycloaddition or consecutive annulation of VQM intermediates. These achievements demonstrated that the VQMs could work as a nuclear parent for the collective synthesis of distinct and complex optically active atropisomers. Recently, we have realized the isolation and structural characterization of the elusive VQMs, which were questioned as putative intermediates for decades. The successful isolation of VQMs provided direct evidence for their existence and an unprecedented opportunity to directly investigate their reactivity. The good thermal stability and reserved reactivity of the isolated VQMs demonstrated their great potential as synthetic reagents and expanded the border of VQM chemistry.

Conformational enantiodiscrimination for asymmetric construction of atropisomers

Molecular conformations induced by the rotation about single bonds play a crucial role in chemical transformations. Revealing the relationship between the conformations of chiral catalysts and the enantiodiscrimination is a formidable challenge due to the great difficulty in isolating the conformers. Herein, we report a chiral catalytic system composed of an achiral catalytically active unit and an axially chiral 1,1'-bi-2-naphthol (BINOL) unit which are connected via a C-O single bond. The two conformers of the catalyst induced by the rotation about the C-O bond, are determined via single-crystal X-ray diffraction and found to respectively lead to the formation of highly important axially chiral 1,1'-binaphthyl-2,2'-diamine (BINAM) and 2-amino-2'-hydroxy-1,1'-binaphthyl (NOBIN) derivatives in high yields (up to 98%), with excellent enantioselectivities (up to 98:2 e.r.) and opposite absolute configurations. The results highlight the importance of conformational dynamics of chiral catalysts in asymmetric catalysis.

From Center-to-Multilayer Chirality: Asymmetric Synthesis of Multilayer Targets with Electron-Rich Bridges

Asymmetric synthesis of new atropisomerically multilayered chiral targets has been achieved by taking advantage of the strategy of center-to-multilayer chirality and double Suzuki-Miyaura couplings. Diastereomers were readily separated via flash column chromatography and well characterized. Absolute configuration assignment was determined by X-ray structural analysis. Five enantiomerically pure isomers possessing multilayer chirality were assembled utilizing anchors involving electron-rich aromatic connections. An overall yield of 0.69% of the final target with hydroxyl attachment was achieved over 11 steps from commercially available starting materials.

Highly Chiral Selective Resolution in Pillar[6]arenes Functionalized Microchannel Membranes

High flux microchannel membranes have the potential for large scale separations. However, it is prevented by poor enantioselectivity. Therefore, the development of a high-enantioselective microchannel membrane is of great importance for large scale chiral separations. In this work, chiral gold nanoparticles are incorporated into the microchannel membrane to astringe the large pores and improve the enantioselectivity. Here, the gold nanoparticles are functionalized by l-phenylalanine-derived pil-lararenes (l-Phe-P6@AuNPs) as the chiral receptor of R-phenylglycinol (R-PGC) over its enantiomer. This chiral Au NPs coated microchannel membrane (l-Phe-P6@AuNPs microchannel) shows a selectivity of 5.40 for R-PGC and a flux of 140.35 nmol·cm^-2·h^-1, where the enantioselectivity is improved, ensuring its flux. Compared with the enantioselectivity and flux of nanochannel membranes reported in literatures, the l-Phe-P6@AuNPs microchannel has the advantage for enantioselectivity and flux for chiral separation.

Synthesis of Axially Chiral Aldehydes by N-Heterocyclic-Carbene-Catalyzed Desymmetrization Followed by Kinetic Resolution

Axially chiral aldehydes have received increasing attention in enantioselective catalysis. However, only very few catalytic methods have been developed to construct structurally diverse axially chiral aldehydes. We herein describe an NHC-catalyzed atroposelective esterification of biaryl dialdehydes as a general and practical strategy for the construction of axially chiral aldehydes. Mechanistic studies indicate that coupling proceeds through a novel combination of NHC-catalyzed desymmetrization of the dialdehydes and kinetic resolution. This protocol features excellent enantioselectivity, mild conditions, good functional-group tolerance, and applicability to late-stage functionalization and provides a modular platform for the synthesis of axially chiral aldehydes and their derivatives.

Copper-Catalyzed Asymmetric Ring-Opening Reaction of Cyclic Diaryliodonium Salts with Imides

An efficient copper-catalyzed asymmetric ring-opening reaction of diaryliodonium salts with imides has been developed, affording a wide range of axially chiral 2-imidobiaryl compounds with excellent enantioselectivities and better convertibility. The potential utility of the current method has been supported by the synthesis of two known chiral ligands with better efficiency, which would be of great significance to the development of other catalytic asymmetric reactions.

Carbene-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes bearing a chiral axis between a sterically non-congested acyclic alkene and an aryl ring are difficult to prepare due to low rotational barrier of the axis. Disclosed here is an N-heterocyclic carbene (NHC) catalytic asymmetric solution to this problem. Our reaction involves ynals, sulfinic acids, and phenols as the substrates with an NHC as the catalyst. Key steps involve selective 1,4-addition of sulfinic anion to acetylenic acylazolium intermediate and sequential E-selective protonation to set up the chiral axis. Our reaction affords axially chiral styrenes bearing a chiral axis as the product with up to > 99:1 e.r., > 20:1 E/Z selectivity, and excellent yields. The sulfone and carboxylic ester moieties in our styrene products are common moieties in bioactive molecules and asymmetric catalysis.

Mechanism and origin of selectivities for NHC-catalyzed synthesis of axially chiral benzothiophene/benzofuran-fused biaryls

By performing density functional theory (DFT) calculations, we investigated and identified the fundamental pathway for N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral benzothiophene-fused biaryl using enal and 2-benzyl-benzothiophene-3-carbaldehyde, which includes...

Organocatalytic atroposelective N-alkylation: divergent synthesis of axially chiral sulfonamides and biaryl amino phenols

Axial chirality exists ubiquitously in numerous natural products and has been extensively recognized for decades in pharmaceuticals and enantioselective transformations. The development of efficient methodologies to obtain enantiopure structures bearing...

Recent advances in the asymmetric phosphoric acid-catalyzed synthesis of axially chiral compounds

In recent years, the synthesis of axially chiral compounds has received considerable attention due to their extensive application as biologically active compounds in medicinal chemistry and as chiral ligands in asymmetric catalysis. Chiral phosphoric acids are recognized as efficient organocatalysts for a variety of enantioselective transformations. In this review, we summarize the recent development of chiral phosphoric acid-catalyzed synthesis of a wide range of axially chiral biaryls, heterobiaryls, vinylarenes, N-arylamines, spiranes, and allenes with high efficiency and excellent stereoselectivity.

Organocatalytic Higher-Order [8+2] Cycloaddition for the Assembly of Atropoenantiomeric 3-Arylindolizines

We present an unprecedented atroposelective [8+2] cycloaddition reaction between pyridinium/isoquinolinium ylides and ynals. It is worth noting that this protocol represents a new example of the organocatalyzed atropoenantioselective higher-order cycloaddition reaction, providing various axial chiral 3-arylindolizines in good yields and high enantioselectivities. In addition, the obtained axially chiral 3-aryldolizines also provide many opportunities for structural transformations and potential drug discovery.

Organocatalytic atroposelective heterocycloaddition to access axially chiral 2-arylquinolines

Axially chiral heterobiaryls play a vital role in asymmetric synthesis and drug discovery. However, there are few reports on the synthesis of atropisomeric heterobiaryls compared with axially chiral biaryls. Thus, the rapid enantioselective construction of optically active heterobiaryls and their analogues remains an attractive challenge. Here, we report a concise chiral amine-catalyzed atroposelective heterocycloaddition reaction of alkynes with ortho-aminoarylaldehydes, and obtain a new class of axially chiral 2-arylquinoline skeletons with high yields and excellent enantioselectivities. In addition, the axially chiral 2-arylquinoline framework with different substituents is expected to be widely used in enantioselective synthesis.