Three Groups Good, Four Groups Bad? Atropisomerism inortho-Substituted Diaryl Ethers

Catalytic Enantioselective Synthesis of Axially Chiral Diaryl Ethers Via Asymmetric Povarov Reaction Enabled Desymmetrization

Axially chiral diaryl ethers represent a distinct class of atropisomers, characterized by a unique dual C─O axes system, which have been found in a variety of natural products, pharmaceuticals, and ligands. However, the catalytic enantioselective synthesis of these atropoisomers poses significant challenges, due to the difficulty in controlling both chiral C─O axes, and their more flexible conformations. Herein, an efficient protocol for catalytic enantioselective synthesis of axially chiral diaryl ethers is presented using organocatalyzed asymmetric Povarov reaction-enabled desymmetrization, followed by aromatizations. This method yields a wide range of novel quinoline-based diaryl ether atropoisomers in good yields and high enantioselectivities. Notably, various aromatization protocols are developed, resulting in a diverse set of polysubstituted quinoline-containing diaryl ether atropisomers. Thermal racemization studies suggested excellent configurational stabilities for these novel diaryl ether atropisomers (with racemization barriers up to 38.1 kcal mol-1). Moreover, this research demonstrates for the first time that diaryl ether atropisomers lacking the bulky t-Bu group can still maintain a stable configuration, challenging the prior knowledge in the field. The fruitful derivatizations of the functional group-rich chiral products further underscore the value of this method.

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Enantioselective Synthesis of Heteroatom-Linked Non-Biaryl Atropisomers

Atropisomers hold significant fascination, not only for their prevalence in natural compounds but also for their biological importance and wide-ranging applications as chiral materials, ligands, and organocatalysts. While biaryl and heterobiaryl atropisomers are commonly studied, the enantioselective synthesis of less abundant heteroatom-linked non-biaryl atropisomers presents a formidable challenge in modern organic synthesis. Unlike classical atropisomers, these molecules allow rotation around two bonds, resulting in low barriers to enantiomerization through concerted bond rotations. In recent years the discovery of new configurationally stable rare non-biaryl scaffolds such as aryl amines, aryl ethers and aryl sulfones as well as innovative methodologies to control their configuration have been disclosed in the literature and constitute the topic of this minireview.

Evolution in the asymmetric synthesis of biaryl ethers and related atropisomers

Axially chiral biaryl ethers and related compounds hold valuable potential in natural products, medicinal chemistry, and catalysis; however, their asymmetric syntheses have always been overlooked compared to other biaryl/hetero-biaryl atropisomers. Unlike the later class molecules bearing a single chiral axis, the former category possesses a unique type of atropisomerism bearing two potential axes. Due to their great importance in diverse research domains, catalytic atropselective biaryl ether synthesis has recently witnessed an upsurge. This highlight article provides an elaborated view on the developments of catalytic synthetic methods that have been explored to achieve dual axial chirality in biaryl ethers and related scaffolds.

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.

Atroposelective Synthesis of Axially Chiral Diaryl Ethers by N-Heterocyclic-Carbene-Catalyzed Sequentially Desymmetric/Kinetic Resolution Process

We describe herein an N-heterocyclic-carbene-catalyzed atroposelective synthesis of axially chiral diaryl ethers. Through a sequentially enantioselective desymmetric process and a kinetic resolution process, the products could be constructed in good yields with excellent enantiopurities. Both alcohols and phenols were compatible with this catalytic system. The axially chiral carboxylic acids derived from the esters were proven to be potential chiral ligands for asymmetric synthesis, for example, Rh(III)-catalyzed enantioselective C-H functionalization.

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.

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.

Catalytic Asymmetric Synthesis of Axially Chiral Diaryl Ethers through Enantioselective Desymmetrization

Axially chiral diaryl ethers are a type of unique atropisomers bearing two potential axes, which have potential applications in a variety of research fields. However, the catalytic enantioselective synthesis of these diaryl ether atropisomers is largely underexplored when compared to the catalytic asymmetric synthesis of biaryl or other types of atropisomers. Herein, we report a highly efficient catalytic asymmetric synthesis of diaryl ether atropisomers through an organocatalyzed enantioselective desymmetrization protocol. The chiral phosphoric acid-catalyzed asymmetric electrophilic aromatic aminations of the symmetrical 1,3-benzenediamine type substrates afforded a series of diaryl ether atropisomers in excellent yields and enantioselectivities. The facile construction of heterocycles by the utilizations of the 1,2-benzenediamine moiety in the products provided access to a variety of structurally diverse and novel azaarene-containing diaryl ether atropisomers.

A Dynamic Kinetic Resolution Approach to Axially Chiral Diaryl Ethers by Catalytic Atroposelective Transfer Hydrogenation

Diaryl ethers are widespread in biologically active compounds, ligands and catalysts. It is known that the diaryl ether skeleton may exhibit atropisomerism when both aryl rings are unsymmetrically substituted with bulky groups. Despite recent advances, only very few catalytic asymmetric methods have been developed to construct such axially chiral compounds. We describe herein a dynamic kinetic resolution approach to axially chiral diaryl ethers via a Brønsted acid catalyzed atroposelective transfer hydrogenation (ATH) reaction of dicarbaldehydes with anilines. The desired diaryl ethers could be obtained in moderate to good chemical yields (up to 79 %) and high enantioselectivities (up to 95 % ee) under standard reaction conditions. Such structural motifs are interesting precursors for further transformations and may have potential applications in the synthesis of chiral ligands or catalysts.

Atropisomerism in the Pharmaceutically Relevant Realm

Atropisomerism is a conformational chirality that occurs when there is hindered rotation about a σ-bond. While atropisomerism is exemplified by biaryls, it is observed in many other pharmaceutically relevant scaffolds including heterobiaryls, benzamides, diarylamines, and anilides. As bond rotation leads to racemization, atropisomers span the gamut of stereochemical stability. LaPlante has classified atropisomers based on their half-life of racemization at 37 °C: class 1 (t_1/2 < 60 s), class 2 (60 s < t_1/2 < 4.5 years), and class 3 (t_1/2 > 4.5 years). In general, class-3 atropisomers are considered to be suitable for drug development. There are currently four FDA-approved drugs that exist as stable atropisomers, and many others are in clinical trials or have recently appeared in the drug discovery literature. Class-1 atropisomers are more prevalent, with ∼30% of recent FDA-approved small molecules possessing at least one class-1 axis. While class-1 atropisomers do not possess the requisite stereochemical stability to meet the classical definition of atropisomerism, they often bind a given target in a specific set of chiral conformations.Over the past decade, our laboratory has embarked on a research program aimed at leveraging atropisomerism as a design feature to improve the target selectivity of promiscuous lead compounds. Our studies initially focused on introducing class-3 atropisomerism into promiscuous kinase inhibitors, resulting in a proof of principle in which the different atropisomers of a compound can have different selectivity profiles with potentially improved target selectivity. This inspired a careful analysis of the binding conformations of diverse ligands bound to different target proteins, resulting in the realization that the sampled dihedral conformations about a prospective atropisomeric axis played a key role in target binding and that preorganizing the prospective atropisomeric axis into a desired target's preferred conformational range can lead to large gains in target selectivity.As atropisomerism is becoming more prevalent in modern drug discovery, there is an increasing need for strategies for atropisomerically pure samples of pharmaceutical compounds. This has led us and other groups to develop catalytic atroposelective methodologies toward pharmaceutically privileged scaffolds. Our laboratory has contributed examples of atroposelective methodologies toward heterobiaryl systems while also exploring the chirality of less-studied atropisomers such as diarylamines and related scaffolds.This Account will detail recent encounters with atropisomerism in medicinal chemistry and how atropisomerism has transitioned from a "lurking menace" into a leverageable design strategy in order to modulate various properties of biologically active small molecules. This Account will also discuss recent advances in atroposelective synthesis, with a focus on methodologies toward pharmaceutically privileged scaffolds. We predict that a better understanding of the effects of conformational restriction about a prospective atropisomeric axis on target binding will empower chemists to rapidly "program" the selectivity of a lead molecule toward a desired target.

Approaches toward Atropisomerically Stable and Conformationally Pure Diarylamines

Diarylamines possess two potentially atropisomeric C-N axes; however, there are few examples of atropisomerically stable diarylamines in the literature, as the contiguous axes can allow for low energy racemization pathways via concerted bond rotations. Herein, we describe highly atropisomerically stable diarylamines that possess barriers to racemization of 30-36 kcal/mol, corresponding to half-lives to racemization on the decade to century time scale at room temperature. Investigation of the factors that led to the high stereochemical stability suggests that increased conjugation of the aniline lone pair of electrons into a more electron-deficient aryl ring, coupled with intramolecular hydrogen-bonding, locked the corresponding axis into a defined planar conformation, disfavoring the lower energy racemization pathways.

Organocatalytic Enantioselective Construction of Conformationally Stable C(sp2)-C(sp3) Atropisomers

Nonbiaryl atropisomers are molecules defined by a stereogenic axis featuring at least one nonarene moiety. Among these, scaffolds bearing a conformationally stable C(sp²)-C(sp³) stereogenic axis have been observed in natural compounds; however, their enantioselective synthesis remains almost completely unexplored. Herein we disclose a new class of chiral C(sp²)-C(sp³) atropisomers obtained with high levels of stereoselectivity (up to 99% ee) by means of an organocatalytic asymmetric methodology. Multiple molecular motifs could be embedded in this class of C(sp²)-C(sp³) atropisomers, showing a broad and general protocol. Experimental data provide strong evidence of the conformational stability of the C(sp²)-C(sp³) stereogenic axis (up to t_1/2^25 °C >1000 y) in the obtained compounds and show kinetic control over this rare stereogenic element. This, coupled with density functional theory calculations, suggests that the observed stereoselectivity arises from a Curtin-Hammett scenario establishing an equilibrium of intermediates. Furthermore, the experimental investigation led to evidence of the operating principle of central-to-axial chirality conversions.

Atropisomerism in Diarylamines: Structural Requirements and Mechanisms of Conformational Interconversion

In common with other hindered structures containing two aromatic rings linked by a short tether, diarylamines may exhibit atropisomerism (chirality due to restricted rotation). Previous examples have principally been tertiary amines, especially those with cyclic scaffolds. Little is known of the structural requirement for atropisomerism in structurally simpler secondary and acyclic diarylamines. In this paper we describe a systematic study of a series of acyclic secondary diarylamines, and we quantify the degree of steric hindrance in the ortho positions that is required for atropisomerism to result. Through a detailed experimental and computational analysis, the role of each ortho-substituent on the mechanism and rate of conformational interconversion is rationalised. We also present a simple predictive model for the design of configurationally stable secondary diarylamines.

Synthesis of an Ellagitannin Component, the Macaranoyl Group with a Tetra-ortho-Substituted Diaryl Ether Structure

Herein, a practical synthesis of the macaranoyl group contained in ellagitannins, i.e., a C-O digallate structure with a tetra-ortho-substituted diaryl ether bond, is described. The methodology involved an oxa-Michael addition/elimination reaction between a brominated ortho-quinone monoketal and a phenol with a hexahydroxydiphenoyl moiety in the presence of 18-crown-6 under dark conditions, followed by reductive aromatization. The existence of rotamers originating from the constructed ether moiety is discussed as well.

Synthesis of six-membered silacycles by borane-catalyzed double sila-Friedel-Crafts reaction

We have developed a catalytic synthetic method to prepare phenoxasilins. A borane-catalyzed double sila-Friedel–Crafts reaction between amino group-containing diaryl ethers and dihydrosilanes can be used to prepare a variety of phenoxasilin derivatives in good to excellent yields. The optimized reaction conditions were also applicable for diaryl thioethers to afford their corresponding six-membered silacyclic products. The gram-scale synthesis of a representative bis(dimethylamino)phenoxasilin and the transformation of its amino groups have also been demonstrated.

Non-biaryl atropisomerism at the C–B bond in sterically hindered aminoarylboranes

Sterically hindered aminoarylboranes with atropisomerism about the C–B bond were prepared and resolved by chiral stationary phase HPLC.

Effect of Triclosan and Chloroxylenol on Bacterial Membranes

Triclosan and chloroxylenol are broad-spectrum biocides used extensively in healthcare and consumer products. They have been suggested to perturb the structure of bacterial membranes, but studies so far have not considered that most bacterial membranes contain large amounts of branched-chain lipids. Here, molecular dynamics simulation is used to examine the effect of the two biocides on membranes consisting of lipids with methyl-branched chains, cyclopropanated chains, and nonbranched chains. It is shown that triclosan and chloroxylenol induced a phase transition in membranes from a liquid-crystalline to a liquid-ordered phase irrespective of the presence and nature of branching groups. At high concentration, chloroxylenol promoted chain interdigitation. Our results suggest that triclosan and chloroxylenol decrease the degree of fluidity of membranes and that this effect is more pronounced in bacterial membranes. As a result, their biocidal activity could be associated with a change in the function of membrane proteins.

Towards a Catalytic Atroposelective Synthesis of Diaryl Ethers via C(sp 2)-H Alkylation Using Nitroalkanes

Herein we report studies towards a small molecule catalytic approach to access atropisomeric diaryl ethers that proceeds via a C(sp ²)-H alkylation using nitroalkanes as the alkyl source. A quaternary ammonium salt derived from quinine containing a sterically hindered urea at the C-9 position was found to effect atroposelective C(sp ²)-H alkylation with moderate to good enantioselectivities across several naphthoquinone-containing diaryl ethers. Products can then be isolated in greater than 95:5 er after one round of trituration. For several substrates that were evaluated we observed a 'nitroethylated' product in similar yields and selectivities.

Atropisomerism in medicinal chemistry: challenges and opportunities

Atropisomerism is a dynamic type of axial chirality that is ubiquitous in medicinal chemistry. There are several examples of stable atropisomeric US FDA-approved drugs and experimental compounds, and in each case the atropisomers of these compounds possess drastically different biological activities. Rapidly interconverting atropisomerism is even more prevalent, and while such compounds are typically considered achiral, they bind their protein targets in an atroposelective fashion, with the nonrelevant atropisomer contributing little to the desired activities. It has been recently demonstrated that various properties of an interconverting atropisomer can be modulated through the synthesis of atropisomer stable and pure analogs. Herein we discuss examples of atropisomerism in drug discovery as well as challenges and opportunities moving forward.

Fundamental Methods in Ellagitannin Synthesis

This account describes methods for synthesizing natural ellagitannins. The ellagitannins, a class of polyphenols, has a wide variety of chemical structures and biological activities. Here, we focus on three topics, which are the synthesis of the hexahydroxydiphenoyl group, construction of the hexahydroxydiphenoyl bridge, and synthesis of C–O digallates. The hexahydroxydiphenoyl group and the C–O digallates are C–C and C–O coupled galloyl groups, respectively, both group of which are the two major components of ellagitannins. By combining methods described in this account, many ellagitannins might be synthesized.

CAr-O Rotamers in 3,3'-Disubstituted BINOL Esters

Rotamers around the C_Ar-O bond were disclosed in 3,3'-disubstituted BINOL esters by NMR spectroscopy. A bulky R¹ group increased the rotational barrier. The pivalate showed two rotamers at 2 °C, and broad signals were observed close to room temperature when R² = Ph. The highest rotational barrier was recorded for the (tetracyanocyclopentadienyl)carboxylate, and C-O rotamers were present at room temperature. DFT calculations indicated the presence of repulsion between R¹ and R² during rotation of the C_Ar-O bond.

Total synthesis of russuphelol: a case of mistaken chirality

The chlorohydroquinone tetramer, russuphelol, does not have stereocenters; however, it was reported as a chiral optically active substance with stable enantiomeric conformations. The natural product is synthesized in six steps and 14% overall yield. Synthetic material was used to experimentally investigate its chiral properties.

The nature of persistent conformational chirality, racemization mechanisms, and predictions in diarylether heptanoid cyclophane natural products

Restricted rotations of chemical bonds can lead to the presence of persistent conformational chirality in molecules lacking stereocenters.

Is nevirapine atropisomeric? Experimental and computational evidence for rapid conformational inversion

The non-nucleoside reverse transcriptase inhibitor nevirapine displays in its room temperature (1)H-NMR spectrum signals characteristic of a chiral compound. Following suggestions in the recent literature that nevirapine may display atropisomerism-and therefore be a chiral compound, due to slow interconversion between two enantiomeric conformers-we report the results of an NMR and computational study which reveal that while nevirapine does indeed possess two stable enantiomeric conformations, they interconvert with a barrier of about 76 kJ mol(-1) at room temperature. Nevirapine has a half life for enantiomerisation at room temperature of the order of seconds, is not atropisomeric, and cannot exist as separable enantiomers.

Carbamate-directed benzylic lithiation for the diastereo- and enantioselective synthesis of diaryl ether atropisomers

Diaryl ethers carrying carbamoyloxymethyl groups may be desymmetrised enantio- and diastereoselectively by the use of the sec-BuLi–(−)-sparteine complex in diethyl ether. Enantioselective deprotonation of one of the two benzylic positions leads to atropisomeric products with ca. 80:20 e.r.; an electrophilic quench typically provides functionalised atropisomeric diastereoisomers in up to 97:3 d.r.

Cu-catalyzed arylation of phenols: synthesis of sterically hindered and heteroaryl diaryl ethers

Cu-catalyzed O-arylation of phenols with aryl iodides and bromides can be performed under mild condition in DMSO/K(3)PO(4) with use of picolinic acid as the ligand for copper. This method tolerates a variety of functional groups and is effective in the synthesis of hindered diaryl ethers and heteroaryl ethers.