Counterion-Mediated Enantioconvergent Synthesis of Axially Chiral Medium Rings

Inherently Chiral Seven- and Eight-Membered Rings: Enantioselective Synthesis and Applications

Compared to smaller or larger rings, seven- and eight-membered carbo- and heterocycles are typically nonplanar and exhibit greater conformational rigidity. This property alone can impart chirality to certain 7- or 8-membered ring systems. Herein, we summarize recent achievements in the enantioselective synthesis of this class of inherently chiral medium rings, including both ring construction and ring modification, as well as the applications of chiral ligands and catalysts derived from these rigid cyclic scaffolds in asymmetric catalysis.

Enhanced Pyridine-Oxazoline Ligand-Enabled Pd(II)-Catalyzed Aminoacetoxylation of Alkenes for the Asymmetric Synthesis of Biaryl-Bridged 7-Membered N-Heterocycles and Atropisomers

A new class of binaphthyl unit-enhanced pyridine-oxazoline ligands was developed to promote the Pd-catalyzed enantioselective intramolecular 7-exo aminoacetoxylation of unactivated biaryl alkenes. Biaryl-bridged 7-membered N-heterocycles bearing a chiral center were obtained in good yields with excellent enantioselectivities (up to 99:1 er). Computational investigations on a series of biaryl-bridged 7-membered rings provided insights into the rotational barrier of the potentially chiral biaryl unit by the substituent effect including the heteroatom, the protecting group, and the chiral center. The kinetic resolution of racemic axially chiral biaryls via intramolecular enantioselective aminoacetoxylation of alkenes has also been achieved, affording previously inaccessible biaryl-bridged 7-membered N-heterocycles bearing both a chiral center and a chiral axis, as well as axially chiral biaryl amino alcohols.

Synthesis of All Ring Sizes of Medium-Sized Heterocycles Bridged Biaryls via VQM-Enabled Diversity-Oriented Synthetic Strategy

Herein, for the first time, the controllable, accurate, and diverse synthesis of all ring sizes of medium-sized (8- to 11-membered) indole-derived bridged biaryls has been realized by using ingeniously designed o-alkynylnaphthols that feature cyclic amines with adjustable ring sizes. The transformation may proceed through a DBN-mediated in-situ generation of vinylidene ortho-quinone methides/indole-ring formation/ring expansion cascade sequence, which is characterized by acceptable to excellent yields and good functional group tolerance.

Access to Chiral Bridged Biaryls via Brønsted Acid-Catalyzed Asymmetric Addition of Alcohols to Fluoroalkylated Biaryl Oxazepines

We disclose herein a chiral phosphoric-acid-catalyzed enantioselective addition reaction of alcohols to fluoroalkylated biaryl 1,3-oxoazepines, which furnished a wide range of bridged biaryls bearing a fluoroalkylated quaternary carbon stereocenter on the seven-membered ring in high yields (up to 99%) with excellent enantioselectivities (up to 98% ee). Our method can be used for the modification of several natural products and bioactive molecules. Preliminary studies revealed that the products obtained in this reaction exhibit good in vitro bioactivities against two plant pathogens.

Enantiocontrolled Cyclization to Form Chiral 7- and 8-Membered Rings Unified by the Same Catalyst Operating with Different Mechanisms

Chiral medium-sized rings, albeit displaying attractive properties for drug development, suffer from numerous synthetic challenges due to difficult cyclization steps that must take place to form these unusually strained, atropisomeric rings from sterically crowded precursors. In fact, catalytic enantioselective cyclization methods for the formation of chiral seven-membered rings are unknown, and the corresponding eight-membered variants are also sparse. In this work, we present a substrate preorganization-based, enantioselective, organocatalytic strategy to construct seven- and eight-membered rings featuring chirality that is intrinsic to the ring in the absence of singular stereogenic atoms or single bond axes of chirality. The reactions proceed under mild conditions and with high levels of stereocontrol. Notably, the same bifunctional iminophosphorane chiral catalyst orchestrates the cyclization of substrates of two different ring sizes, under two different mechanistic paradigms. We envision that the mechanistic and ring size versatility of this method could guide further applications of asymmetric catalysis to other challenging cyclization reactions.

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.

Synthesis of Alkene Atropisomers with Multiple Stereogenic Elements via Catalytic Asymmetric Rearrangement of 3-Indolylmethanols

Catalytic enantioselective preparation of alkene atropisomers with multiple stereogenic elements and discovery of their applications have become significant but challenging issues in the scientific community due to the unique structures of this class of atropisomers. We herein report the first catalytic atroposelective preparation of cyclopentenyl[b]indoles, a new kind of alkene atropisomers, with stereogenic point and axial chirality via an unusual rearrangement reaction of 3-indolylmethanols under asymmetric organocatalysis. Notably, this novel type of alkene atropisomers have promising applications in developing chiral ligands or organocatalysts, discovering antitumor drug candidates and fluorescence imaging materials. Moreover, the theoretical calculations have elucidated the possible reaction mechanism and the non-covalent interactions to control the enantioselectivity. This approach offers a new synthetic strategy for alkene atropisomers with multiple stereogenic elements, and represents the first catalytic enantioselective rearrangement reaction of 3-indolylmethanols, which will advance the chemistry of atropisomers and chiral indole chemistry.

Bridged Biaryl Atropisomers by Organocatalyzed Kinetic Asymmetric Alcoholysis

We disclose herein an asymmetric synthesis of axially chiral oxazepine-containing bridged biaryls via CPA-catalyzed kinetic asymmetric alcoholysis. Control experiments showed that this CPA-catalyzed alcoholysis was reversible, and lowering the reaction temperature could almost suppress the reversible reaction, thus providing a series of axially chiral oxazepine-containing bridged biaryl compounds in good to excellent enantioselectivities. The gram-scale reactions and facile derivatizations of the enantioenriched products demonstrate the practical utility of this reaction.

Organocatalytic diastereo- and atropo-selective construction of eight-membered bridged (hetero)biaryls via asymmetric intramolecular [3 + 2] cycloaddition

An unprecedented and straightforward route for the asymmetric construction of privileged atroposelective bridged (hetero)biaryl eight-membered scaffolds has been accomplished through chiral phosphoric acid catalyzed asymmetric intramolecular [3 + 2] cycloaddition of innovative (hetero)biaryl aldehydes with 3-aminooxindole hydrochlorides. A class of eight-membered bridged (hetero)biaryl lactones fused to spiro[pyrrolidine-oxindole] derivatives, possessing both chiral C-C/C-N axes and multiple contiguous stereocenters, were obtained in good yields with excellent enantioselectivities and diastereoselectivities in one step through this direct strategy. In addition, the good scalability and derivatization of the title compounds demonstrated their synthetic utility.

Atroposelective synthesis of biaxial bridged eight-membered terphenyls via a Co/SPDO-catalyzed aerobic oxidative coupling/desymmetrization of phenols

Bridged chiral biaryls are axially chiral compounds with a medium-sized ring connecting the two arenes. Compared with plentiful methods for the enantioselective synthesis of biaryl compounds, synthetic approaches for this subclass of bridged atropisomers are limited. Here we show an atroposelective synthesis of 1,3-diaxial bridged eight-membered terphenyl atropisomers through an Co/SPDO (spirocyclic pyrrolidine oxazoline)-catalyzed aerobic oxidative coupling/desymmetrization reaction of prochiral phenols. This catalytic desymmetric process is enabled by combination of an earth-abundant Co(OAc)2 and a unique SPDO ligand in the presence of DABCO (1,4-diaza[2.2.2]bicyclooctane). An array of diaxial bridged terphenyls embedded in an azocane can be accessed in high yields (up to 99%) with excellent enantio- (>99% ee) and diastereoselectivities (>20:1 dr).

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.

Ruthenium-Catalyzed Carbonylation of α-Aminoaryl-Tethered Alkylidenecyclopropanes: Synthesis of Eight-Membered Benzolactams

The synthesis of medium-sized lactams is a great challenge because of the unfavorable transannular interactions and entropic barriers in the transition state. We have developed a ruthenium-catalyzed carbonylation of α-aminoaryl-tethered alkylidenecyclopropanes (ACPs) that allows for the efficient preparation of valuable eight-membered benzolactams under ligand-free conditions. The amino group served a dual role of both directing group and nucleophile to facilitate the metallacycle formation and the carbonylation.

Diastereo- and Enantioselective Synthesis of Eight-Membered N-Heterocycles from Benzofuran-Derived Azadienes and Ynones

Enantioselective synthesis of eight-membered N-heterocycles represents a long-standing challenge in organic synthesis. Here, by combining the squaramide and DBU catalysis, a sequential asymmetric conjugate addition/cyclization reaction between benzofuran-derived azadienes and ynones has been well-developed, providing straightforward access to chiral eight-membered N-heterocycles in high yields with stereoselectivities. This protocol features the use of a bifunctional squaramide catalyst for controlling the enantioselectivity of products, while the DBU is utilized to achieve intramolecular cyclization and improve the diastereoselectivity of products.

Rh(III)-Catalyzed Enaminone-Directed C-H Coupling with Diazodicarbonyls for Skeleton-Divergent Synthesis of Isocoumarins and Naphthalenes

Diversity-oriented synthesis is tremendously useful for expanding the explorable chemical space but restricted by the limited available toolbox of skeleton-diversification chemistry. We report herein Rh(III)-catalyzed coupling of enaminones and diazodicarbonyls for skeleton-divergent synthesis of isocoumarins and naphthalenes. The diazodicarbonyl ring size and pH dependence of the skeleton-forming process demonstrates the achievement of both substrate- and reagent-controlled skeletal diversity generation in a single type of system. An intriguing C-C bond cleavage reactivity is critical for enabling facile synthetic access to isocoumarins.

Catalytic asymmetric synthesis of medium-sized bridged biaryls

Despite the persistent presence of medium-sized (seven- to nine-membered) scaffolds in natural products and biologically active molecules, their asymmetric syntheses have always been considered a formidable task; therefore, they have remained underdeveloped when compared to the enantioselective synthesis of five- and six-membered ring scaffolds. One important class of such medium-sized ring frameworks includes seven- to nine-membered biaryl bridged carbo- and heterocycles. These medium-ring-sized biaryl frameworks possess more configurational stability than the related smaller ring structures and are common features of valuable natural products, bioactive compounds, chiral catalysts, and molecular motors. Due to these exciting properties and broad applications, over the last few years, the catalytic enantioselective synthesis of medium-sized bridged biaryls has seen an upsurge. This highlight article describes the development of organocatalysed and transition-metal catalysed transformations for procuring seven-, eight-, and nine-membered bridged biaryls bearing a chiral axis/one or more asymmetric carbon centres.