Organocatalytic Asymmetric Atroposelective Construction of Axially Chiral 1,4-Distyrene 2,3-Naphthalene Diols

Organocatalytic atroposelective de novo construction of monoaxially and 1,4-diaxially chiral fused uracils with potential antitumor activity

Atropisomers bearing multiple stereogenic axes are of increasing relevance to materials science, pharmaceuticals, and catalysis. However, the catalytic enantioselective construction of these atropisomers in a single step remains synthetically challenging. We herein report the first NHC-organocatalytic enantioselective synthesis of a new class of monoaxially and 1,4-diaxially chiral fused uracil scaffolds. Preliminary studies on the antitumor activity of selected compounds demonstrated that this new class of axially chiral uracil derivatives may have potential applications in the discovery of new lead compounds in medicinal chemistry.

Biaxially chiral compounds: research and development of synthesis

With the rapid advancements in asymmetric catalysis, there is a growing need for the asymmetric selective synthesis of complex and diverse molecules with chiral axes. Axially chiral molecules are not only present in natural products and drugs but also in specialized chiral ligands and catalysts. The catalytic asymmetric synthesis of axially chiral molecules has garnered significant attention within the chemical community. To date, two asymmetric catalytic methods have been established for the synthesis of biaxially chiral molecules: transition-metal catalysis and asymmetric organocatalysis. This feature article presents a summary of the research progress made in the field of asymmetric selective synthesis of biaxially chiral molecules. This review focuses on the reaction range, limitations, and reaction mechanism of the biaxially chiral molecular synthesis method, as well as the contribution and advantages of this method in the preparation of various axially chiral compounds.

Chiral Brønsted Acid-Catalyzed Asymmetric Reaction via Vinylidene Ortho-Quinone Methides

Vinylidene ortho-quinone methides (VQMs) have been proven to be versatile and crucial intermediates in the catalytic asymmetric reaction in last decade, and thus have drawn considerable concentrations on account of the practical application in the construction of enantiomerically pure functional organic molecules. However, in comparison to the well established chiral Brønsted base-catalyzed asymmetric reaction via VQMs, chiral Brønsted acid-catalyzed reaction is rarely studied and there is no systematic summary to date. In this review, we summarize the recent advances in the chiral Brønsted acid-catalyzed asymmetric reaction via VQMs according to three types of reactions: a) intermolecular asymmetric nucleophilic addition to VQMs; b) intermolecular asymmetric cycloaddition of VQMs; c) intramolecular asymmetric cyclization of VQMs. Finally, we put forward the remained challenges and opportunities for potential breakthroughs in this area.

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.

Enantioselective Synthesis of Allylic Sulfones via Rhodium-Catalyzed Direct Hydrosulfonylation of Allenes and Alkynes

A highly regio- and enantioselective hydrosulfonylation using commercially available sodium sulfinates is reported, providing the first direct asymmetric rhodium-catalyzed hydrosulfonylation of allenes/alkynes to synthesize chiral allylic sulfones. Ligand screening studies demonstrated the indispensable role of the C1-symmetric P,N-ligand (Rax,S,S)-StackPhim for achieving both high regioselecitivity (>20:1) and enantioselectivity (up to 97% ee). Notably, the operationally simple method and mild conditions allow for the rapid preparation of chiral allylic sulfones with a wide scope of functional groups. Moreover, the use of sodium tert-butyldimethylsilyloxymethanesulfinate enables the collective synthesis of various chiral sulfone derivatives after simple transformations of the protected hydroxymethyl product.

Enantioselective Synthesis of Axially Chiral Sulfone-Containing Styrenes Based on Ion-Exchange Strategy

A novel ion exchange strategy has been developed to enable the asymmetric construction of axially chiral sulfone-containing styrenes. This approach provides a practical synthesis pathway for various axially chiral sulfone-containing styrenes with good yields, exceptional enantioselectivities, and nearly complete E/Z selectivities. Additionally, the reaction mechanism is elucidated in detail through density functional theory (DFT) calculations.

Rhodium(I)-Catalyzed Asymmetric Hydroarylative Cyclization of 1,6-Diynes to Access Atropisomerically Labile Chiral Dienes

Axially chiral open-chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)-catalyzed hydroarylative cyclization of 1,6-diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3-dienes that are conformationally labile (ΔG≠ (rac)=26.6-28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N-oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6-diyne to give a rhodacyclic intermediate, followed by σ-bond metathesis between the arene C-H bond and the Rh-C(vinyl) bond, with subsequent C-C reductive elimination being enantio-determining and turnover-limiting. The DFT-established mechanism is consistent with the experimental studies. The coupled products of quinoline N-oxides undergo facile visible light-induced intramolecular oxygen-atom transfer, affording chiral epoxides with complete axial-to-central chirality transfer.

Rhodium-catalyzed atroposelective access to trisubstituted olefins via C-H bond olefination of diverse arenes

The atroposelective synthesis of axially chiral acyclic olefins remains a daunting challenge due to their relatively lower racemization barriers, especially for trisubstituted ones. In this work, atroposelective C-H olefination has been realized for synthesis of open-chain trisubstituted olefins via C-H activation of two classes of (hetero)arenes in the coupling with sterically hindered alkynes. The employment of phenyl N-methoxycarbamates as arene reagents afforded phenol-tethered olefins, with the carbamate being a traceless directing group. The olefination of N-methoxy-2-indolylcarboxamides afforded the corresponding chiral olefin by circumventing the redox-neutral [4 + 2] annulation. The reactions proceeded with excellent Z/E selectivity, chemoselectivity, regioselectivity, and enantioselectivity in both hydroarylation systems.

Atroposelective Synthesis of Axially Chiral Styrenes Connecting an Axially Chiral Naphthyl-indole Moiety Using Chiral Phosphoric Acid Catalysis

The organocatalytic asymmetric reactions of C2-unsubstituted racemic naphthyl-indoles with orthoalkynylnaphthols were employed to synthesize axially chiral styrenes connected to an axially chiral naphthyl-indole unit. Utilizing chiral phosphoric acid as the catalyst, these axially chiral styrenes were prepared in good yields (up to 96%) and excellent stereoselectivity (up to >99.9% ee, >20:1 dr, and >99:1 E/Z) in mild conditions. Moreover, further synthetic transformations were achieved with high yields and excellent stereocontrol.

Stepwise Asymmetric Allylic Substitution-Isomerization Enabled Mimetic Synthesis of Axially Chiral B,N-Heterocycles

Axially chiral molecules bearing multiple stereogenic axes are of great importance in the field of organic chemistry. However, the efficient construction of atropisomers featuring two different types of stereogenic axes has rarely been explored. Herein, we report the novel atroposelective synthesis of configurationally stable axially chiral B,N-heterocycles. By using stepwise asymmetric allylic substitution-isomerization (AASI) strategy, diaxially chiral B,N-heterocycles bearing B-C and C-N axes that are related to the moieties of axially chiral enamines and arylborons were also obtained. In this case, all four stereoisomers of diaxially chiral B,N-heterocycles were stereodivergently afforded in high enantioselectivities. Density functional theory (DFT) studies demonstrated that the NH⋅⋅⋅π interactions played a unique role in the promotion of stereospecific isomerization, thereby leading to the highly efficient central-to-axial chirality transfer.

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.

An Isolable Vinylidene ortho-Quinone Methide: Synthesis, Structure and Reactivity

Commonly, an elusive intermediate is generated from a precursor and then trapped and consumed in a reaction. Vinylidene ortho-quinone methides (VQMs) have been demonstrated as transient axially chiral intermediates in asymmetric catalysis due to their orthogonal π-bonds forming an allene motif. The current understanding of VQMs is primarily based on time-resolved absorption, trapping experiments and computational studies. Herein, we report the first isolation and comprehensive characterization of a VQM, including crystallographic analysis. The disturbed aromaticity of the VQM led to its high reactivity as an electrophile or a 4π-component capable of asymmetric dearomatization of an electron-deficient phenyl group. Notably, the VQM could be isolated in enantiomerically enriched form, and the subsequent transformation was stereospecific, indicating that the generation of the VQM was involved in the enantiodetermining step. This study paves the way for the direct application of VQMs as starting materials.

Organocatalytic atroposelective construction of axially chiral N, N- and N, S-1,2-azoles through novel ring formation approach

1,2-Azoles are privileged structures in ligand/catalyst design and widely exist in many important natural products and drugs. In this report, two types of axially chiral 1,2-azoles (naphthyl-isothiazole S-oxides with a stereogenic sulfur center and atropoisomeric naphthyl pyrazoles) are synthesized via modified vinylidene ortho-quinone methide intermediates. Diverse products are acquired in satisfying yields and good to excellent enantioselectivities. The vinylidene ortho-quinone methide intermediates bearing two hetero atoms at 5-position have been demonstrated as a platform molecule for the atroposelective synthesis of axially chiral 1,2-azoles. This finding not only enrich our knowledge of vinylidene ortho-quinone methide chemistry but also provide the easy preparation method for diverse atropisomeric heterobiaryls that were inaccessible by existing methodologies. The obtained chiral naphthyl-isothiazole S-oxides and naphthyl-pyrazoles have demonstrated their potential application in further synthetic transformations and therapeutic agents.

Organocatalytic Enantioselective Construction of Spiroketal Lactones Bearing Axial and Central Chirality via an Asymmetric Domino Reaction

The catalytic asymmetric synthesis of chiral compounds with multiple stereogenic elements via a single catalytic process is challenging. This paper proposes a domino asymmetric electrophilic halocyclization strategy for constructing heterocycloalkenyl atropisomeric spiroketal lactones. A single catalyst was utilized to realize two independent stereodetermining steps. Various spiroketal lactones containing both chiral axes and chiral centers were prepared in excellent yields with excellent enantioselectivity and diastereoselective (up to 99% ee and >20:1 dr).

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.

Access to axially chiral aryl 1,3-dienes by transient group directed asymmetric C-H alkenylations

We present a Pd-catalyzed atroposelective preparation of aryl 1,3-dienes from readily available styrenes and olefins through aldehyde derived transient chiral auxiliary, proceeding by enantioselective olefinic C-H alkenylation of styrenes via...

Synthesis of aryl-fused 1,4-oxathiepines from pyridinium 1,4-zwitterionic thiolates and vinylidene ortho-quinone methides

Synthesis of aryl-fused 1,4-oxathiepines from pyridinium 1,4-zwitterionic thiolates with vinylidene ortho-quinone methides generated in situ via a formal (3 + 4) pathway.

Construction of axially chiral styrene-type allylamines via chiral phosphoric acid-catalyzed asymmetric reductive amination

The first enantioselective synthesis of axially chiral styrene-type allylamines through chiral phosphoric acid mediated atroposelective reductive amination of 1-enal substituted 2-naphthols is achieved.

Rational Design of Axially Chiral Styrene-Based Organocatalysts and Their Application in Catalytic Asymmetric (2+4) Cyclizations

A new class of axially chiral styrene-based thiourea tertiary amine catalysts, which have unique characteristics such as an efficient synthetic route, multiple chiral elements, and multiple activating groups, has been rationally designed. These new chiral catalysts have proven to be efficient organocatalysts, enabling the chemo-, diastereo-, and enantioselective (2+4) cyclization of 2-benzothiazolimines with homophthalic anhydrides in good yields (up to 96 %) with excellent stereoselectivities (all >95:5 dr, up to 98 % ee). More importantly, theoretical calculations elucidated the important role of an axially chiral styrene moiety in controlling both the reactivity and enantioselectivity. This work not only represents the first design of styrene-based chiral thiourea tertiary amine catalysts and the first catalytic asymmetric (2+4) cyclization of 2-benzothiazolimines, but also gives an in-depth understanding of axially chiral styrene-based organocatalysts.

Rhodium-Catalyzed Atroposelective Access to Axially Chiral Olefins via C-H Bond Activation and Directing Group Migration

Axially chiral open-chain olefins represent an underexplored class of chiral platform. In this report, two classes of tetrasubstituted axially chiral acyclic olefins have been accessed in excellent enantioselectivity and regioselectivity via C-H activation of (hetero)arenes assisted by a migratable directing group en route to coupling with sterically hindered alkynes. The coupling of indoles bearing an N-aminocarbonyl directing group afforded C-N axially chiral acrylamides with the assistance of a racemic zinc carboxylate additive. DFT studies suggest a β-nitrogen elimination-reinsertion pathway for the directing group migration. Meanwhile, the employment of N-phenoxycarboxamide delivered C-C axially chiral enamides via migration of the oxidizing directing group. Experiments suggest that in both cases the (hetero)arene substrate adopts a well-defined orientation during the C-H activation, which in turn determines the disposition of the alkyne in migratory insertion. Synthetic applications of representative chiral olefins are demonstrated.

Tandem Iridium Catalysis as a General Strategy for Atroposelective Construction of Axially Chiral Styrenes

Axially chiral styrenes are of great interest since they may serve as a class of novel chiral ligands in asymmetric synthesis. However, only recently have strategies been developed for their enantioselective preparation. Thus, the development of novel and efficient methodologies is highly desirable. Herein, we reported the first tandem iridium catalysis as a general strategy for the synthesis of axially chiral styrenes enabled by Asymmetric Allylic Substitution-Isomerization (AASI) using cinnamyl carbonate analogues as electrophiles and naphthols as nucleophiles. In this approach, axially chiral styrenes were generated through two independent iridium-catalytic cycles: iridium-catalyzed asymmetric allylic substitution and in situ isomerization via stereospecific 1,3-hydride transfer catalyzed by the same iridium catalyst. Both experimental and computational studies demonstrated that the isomerization proceeded by iridium-catalyzed benzylic C-H bond oxidative addition, followed by terminal C-H reductive elimination. Amid the central-to-axial chirality transfer, the hydroxyl of naphthol plays a crucial role in ensuring the stereospecificity by coordinating with the Ir(I) center. The process accommodated broad functional group compatibility. The products were generated in excellent yields with excellent to high enantioselectivities, which could be transformed to various axially chiral molecules.

Recent Progress on the Synthesis of Chiral Sulfones

Chiral sulfones extensively exist in drugs, agricultural chemicals, chiral organic intermediates, and functional materials. Their importance causes the rapid development of their synthetic methods in recent years. Many transition metal complex catalysts with chiral ligands and chiral organocatalysts are adopted in synthesis of chiral sulfones. Most of the methods to construct chiral sulfones are based on the reduction of unsaturated sulfones and the introduction of sulfone groups into unsaturated hydrocarbons. This review describes all classes of asymmetric reactions for synthesis of chiral sulfones.

Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Atropisomerism is a stereochemical behavior portrayed by three-dimensional molecules that bear rotationally restricted σ bond. Akin to the well-represented point-chiral molecules, atropisomerically chiral compounds are finding increasing utilities in many disciplines where molecular asymmetry is influential. This provides steady demand on atroposelective synthesis, where numerous synthetic pursuits have been rewarded with conceptually novel and streamlined methods while expanding the structural diversity of atropisomers. This review summarizes key achievements in stereoselective preparation of biaryl, heterobiaryl, and nonbiaryl atropisomers documented between 2015 and 2020. Emphasis is placed on the synthetic strategies for each structural class, while examples are cited to illustrate the potential applications of the accessed atropochiral targets.

Atropisomerism in Styrene: Synthesis, Stability, and Applications

Atropisomeric styrenes are a class of optically active compounds, the chirality of which results from restricted rotation of the C(vinyl)–C(aryl) single bond. In comparison with biaryl atropisomers, the less rigid skeleton of styrenes usually leads them to have lower rotational barriers. Although it has been overlooked for a long time, scientists have paid attention to this class of unique molecules in recent years and have developed many methods for the preparation of optically active atropisomeric styrenes. In this article, we review the development of the concept of atropisomeric styrenes, along with their isolation, asymmetric synthesis, and synthetic applications.

1 Introduction

2 The Concept of Styrene Atropisomerism

3 Early Research: Separation of Optically Active Styrenes

4 Synthesis of Optically Active Styrenes

5 Stability of the Chirality of Atropisomeric Styrenes

6 Outlook

Enantioselective Control of Both Helical and Axial Stereogenic Elements though an Organocatalytic Approach

A highly diastereo- and enantio-selective method for the asymmetric synthesis of molecules containing helicenes and stereogenic axes was developed based on organocatalysis. Various compounds bearing both helical and axial stereogenic elements were obtained in excellent enantioselectivities. The mechanism study revealed that the reaction proceeded through two stages: 1) The first cyclization produces a reaction intermediate containing a stereogenic axis. 2) The dynamic kinetic resolution of helix reaction intermediate following with cyclization generates a helix and another stereogenic axis.

Enantioselective organocatalytic activation of vinylidene-quinone methides (VQMs)

Vinylidene-quinone methides (VQMs) are highly electrophilic chiral reagents that can be generated in situ from 2-(phenylethynyl)phenols. They were characterized for the first time in 2012 but their enantioselective organocatalytic activation was addressed only very recently. Their specific reactivity has revealed innovative strategies notably for the control of axial chirality.