Combining Organocatalysis with Central-to-Axial Chirality Conversion: Atroposelective Hantzsch-Type Synthesis of 4-Arylpyridines

Understanding the axial chirality control of quinidine-derived ammonium cation-directed O-alkylation: a computational study

As a privileged chiral scaffold, cinchona alkaloid and its derivatives have reached remarkable success in catalytic asymmetric organic synthesis. In addition to the wide applications of point chirality control, Smith and co-workers recently discovered a quinidine-derived ammonium cation-catalyzed O-alkylation of tetralones, providing an effective approach for the synthesis of axially chiral biaryls. Using density functional theory (DFT) calculations, we studied the reaction mechanism and origins of enantioselectivity of this novel transformation. A stepwise strategy is adopted to ensure efficient and thorough exploration of the massive conformational space of transition state. Our computations suggested that enolate oxygen forms two hydrogen bonds with the chiral ammonium catalyst, and the non-covalent interactions between the cationic benzylic fragment and the methoxy group of enolate plays a critical role in determining the enantioselectivity.

Traceless point-to-axial chirality exchange in the atropselective synthesis of biaryls/heterobiaryls

A strategy for establishing a chiral axis from stereogenic centers via elimination or oxidative aromatization with a high level of chirality transfer was developed.

Atroposelective Catalytic Asymmetric Allylic Alkylation Reaction for Axially Chiral Anilides with Achiral Morita-Baylis-Hillman Carbonates

A highly efficient method to access axially chiral anilides through asymmetric allylic alkylation reaction with achiral Morita-Baylis-Hillman carbonates by using a biscinchona alkaloid catalyst was reported. Through the atroposelective approach, a broad range of axially chiral anilide products with different acyl groups, such as substituted phenyl, naphthyl, alkyl, enyl, styryl, and benzyl, were generated with very good yields, moderate to excellent cis: trans ratios, and good to excellent enantioselectivities. The reaction can be scaled up, and the synthetic utility of axially chiral anilides was proved by transformations. Moreover, the linear free energy relationship analysis was introduced to investigate the reaction.

Construction of Axially Chiral Compounds via Asymmetric Organocatalysis

Axially chiral compounds have received much attention from chemists because of their widespread appearance in natural products, biologically active compounds, and useful chiral ligands in asymmetric catalysis. Because of the importance of this structural motif, the catalytic enantioselective construction of axially chiral scaffolds has been intensively investigated, and great progress has been accomplished. However, the majority of methodologies in this field focus on the use of metal catalysis, whereas approaches involving organocatalysis have started to emerge only recently. This Account describes certain advances in the organocatalytic asymmetric synthesis of axially chiral compounds involving the following strategies: kinetic resolution, desymmetrization, cyclization/addition, direct arylation, and so on. We began our investigation by developing a highly efficient strategy for the kinetic resolution of axially chiral BINAM derivatives involving a chiral Brønsted acid-catalyzed imine formation and transfer hydrogenation cascade process, thereby providing a convenient route to generate chiral BINAM derivatives in high yields with excellent enantioselectivities. The desymmetrization of 1-aryltriazodiones (ATADs) through an organocatalyzed tyrosine clicklike reaction wherein a nucleophile was added to the ATAD afforded an interesting type of axially chiral N-arylurazole in an excellent remote enantiocontrolled manner. We then focused on a direct construction strategy involving cyclization and the addition strategy given the inherent limitations of the kinetic resolution in terms of the chemical yield and the desymmetrization in terms of the substrate scope. By utilizing the catalytic enantioselective Paal-Knorr reaction, we disclosed a general and efficient cyclization method to access enantiomerically pure arylpyrroles. The direct heterocycle formation and the stepwise method, which was executed in a one-pot fashion containing enantioselective cyclization and subsequent aromatization, were successfully applied for the construction of diverse axially chiral arylquinazolinones catalyzed by chiral Brønsted acids. We discovered the asymmetric organocatalytic approach to construct axially chiral styrenes through the 1,4-addition of arylalkynals in good chemical yields and enantioselectivities. Such structural motifs are important precursors for further transformations into biologically active compounds and useful synthetic intermediates and may have potential applications in asymmetric syntheses as olefin ligands or organocatalysts. To further tackle this challenge, we accomplished the phosphoric acid-catalyzed enantioselective direct arylative reactions of 2-naphthol and 2-naphthamine with quinone derivatives to deliver efficient access to a class of axially chiral BINOL and NOBIN derivatives in good yields with excellent enantioselectivities under mild reaction conditions. Most importantly, we discovered that the azo group can effectively perform as a directing and activating group for organocatalytic formal aryl C-H functionalization via formal nucleophilic aromatic substitution of azobenzene derivatives. Thus, a wide range of axially chiral arylindoles were synthesized in good yields with excellent enantioselectivities. We anticipate that this strategy will foster the development of many other transformations and motivate a new enthusiasm for organocatalytic enantioselective aryl functionalization. Moreover, SPINOLs are fundamental synthetic precursors in the construction of other chiral organocatalysts and ligands. We have successfully developed a phosphoric acid-catalyzed enantioselective approach for SPINOLs. This approach is highly convergent and functional-group-tolerant for the efficient generation of SPINOLs with good results, thus delivering practical access to this privileged structure.

trans-1,2-Diaminocyclohexane-based sulfonamides as effective hydrogen-bonding organocatalysts for asymmetric Michael–hemiacetalization reaction

Simple sulfonamide organocatalysts deliver unparalleled efficiency in the asymmetric Michael–hemiacetalization cascade.

Organocatalytic Atroposelective Arylation of 2-Naphthylamines as a Practical Approach to Axially Chiral Biaryl Amino Alcohols

The first phosphoric acid catalyzed direct arylation of 2-naphthylamines with iminoquinones for the atroposelective synthesis of axially chiral biaryl amino alcohols has been developed. This reaction constitutes a highly functional-group-tolerant route for the rapid construction of enantioenriched axially chiral biaryl amino alcohols, and is a rare example of 2-naphthylamines acting as nucleophiles in an organocatalytic enantioselective transformation. Furthermore, the products, which feature various halogen atoms, provide access to structurally diverse axially chiral amino alcohols through further transformations.

Organocatalytic asymmetric arylation of indoles enabled by azo groups

Arylation is a fundamental reaction that can be mostly fulfilled by electrophilic aromatic substitution and transition-metal-catalysed aryl functionalization. Although the azo group has been used as a directing group for many transformations via transition-metal-catalysed aryl carbon-hydrogen (C-H) bond activation, there remain significant unmet challenges in organocatalytic arylation. Here, we show that the azo group can effectively act as both a directing and activating group for organocatalytic asymmetric arylation of indoles via formal nucleophilic aromatic substitution of azobenzene derivatives. Thus, a wide range of axially chiral arylindoles have been achieved in good yields with excellent enantioselectivities by utilizing chiral phosphoric acid as catalyst. Furthermore, highly enantioenriched pyrroloindoles bearing two contiguous quaternary chiral centres have also been obtained via a cascade enantioselective formal nucleophilic aromatic substitution-cyclization process. This strategy should be useful in other related research fields and will open new avenues for organocatalytic asymmetric aryl functionalization.

Bifunctional organocatalysts for the asymmetric synthesis of axially chiral benzamides

Bifunctional organocatalysts bearing amino and urea functional groups in a chiral molecular skeleton were applied to the enantioselective synthesis of axially chiral benzamides via aromatic electrophilic bromination. The results demonstrate the versatility of bifunctional organocatalysts for the enantioselective construction of axially chiral compounds. Moderate to good enantioselectivities were afforded with a range of benzamide substrates. Mechanistic investigations were also carried out.

Brønsted acid-catalysed enantioselective construction of axially chiral arylquinazolinones

The axially chiral arylquinazolinone acts as a privileged structural scaffold, which is present in a large number of natural products and biologically active compounds as well as in chiral ligands. However, a direct catalytic enantioselective approach to access optically pure arylquinazolinones has been underexplored. Here we show a general and efficient approach to access enantiomerically pure arylquinazolinones in one-pot fashion catalysed by chiral phosphoric acids. A variety of axially chiral arylquinazolinones were obtained in high yields with good to excellent enantioselectivities under mild condition. Furthermore, we disclosed a method for atroposelective synthesis of alkyl-substituted arylquinazolinones involving Brønsted acid-catalysed carbon–carbon bond cleavage strategy. Finally, the asymmetric total synthesis of eupolyphagin bearing a cyclic arylquinazolinone skeleton was accomplished with an overall yield of 32% in six steps by utilizing the aforementioned methodology.

Axially chiral arylquinazolinones are structural motifs in several natural products and can also act as chiral ligands. Here, the authors show a chiral phosphoric acid-catalysed strategy to access enantiomerically pure arylquinazolinones by efficient transfer of central chirality into axial chirality.

Induction of Axial Chirality in 8-Arylquinolines through Halogenation Reactions Using Bifunctional Organocatalysts

The enantioselective syntheses of axially chiral heterobiaryls were accomplished through the aromatic electrophilic halogenation of 3-(quinolin-8-yl)phenols with bifunctional organocatalysts that control the molecular conformations during successive halogenations. Axially chiral quinoline derivatives, which have rarely been synthesized in an enantioselective catalytic manner, were afforded in moderate-to-good enantioselectivities through bromination, and an analogous protocol also enabled enantioselective iodination. In addition, this catalytic reaction, which allows enantioselective control through the use of mono-ortho-substituted substrates, allowed the asymmetric synthesis of 8-arylquinoline derivatives bearing two different halogen groups in high enantioselectivities.

Organocatalytic atroposelective synthesis of axially chiral styrenes

Axially chiral compounds are widespread in biologically active compounds and are useful chiral ligands or organocatalysts in asymmetric catalysis. It is well-known that styrenes are one of the most abundant and principal feedstocks and thus represent excellent prospective building blocks for chemical synthesis. Driven by the development of atroposelective synthesis of axially chiral styrene derivatives, we discovered herein the asymmetric organocatalytic approach via direct Michael addition reaction of substituted diones/ketone esters/malononitrile to alkynals. The axially chiral styrene compounds were produced with good chemical yields, enantioselectivities and almost complete E/Z-selectivities through a secondary amine-catalysed iminium activation strategy under mild conditions. Such structural motifs are important precursors for further transformations into biologically active compounds and synthetic useful intermediates and may have potential applications in asymmetric synthesis as olefin ligands or organocatalysts.

Many methods exist for the atroposelective synthesis of axially chiral biaryls, but axially chiral styrenes are much less explored. Here the authors report an organocatalytic procedure for the synthesis of configurationally stable, axially chiral styrenes with high enantio- and diastereoselectivities.