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.

C(sp)-C(sp) Lever-Based Targets of Orientational Chirality: Design and Asymmetric Synthesis

In this study, the design and asymmetric synthesis of a series of chiral targets of orientational chirality were conducted by taking advantage of N-sulfinylimine-assisted nucleophilic addition and modified Sonogashira catalytic coupling systems. Orientational isomers were controlled completely using alkynyl/alkynyl levers [C(sp)-C(sp) axis] with absolute configuration assignment determined by X-ray structural analysis. The key structural element of the resulting orientational chirality is uniquely characterized by remote through-space blocking. Forty examples of multi-step synthesis were performed, with modest to good yields and excellent orientational selectivity. Several chiral orientational amino targets are attached with scaffolds of natural and medicinal products, showing potential pharmaceutical and medical applications in the future.

Investigating the diastereoselective synthesis of a macrocycle under Curtin-Hammett control

This work sheds new light on the stereoselective synthesis of chiral macrocycles containing twisted aromatic units, valuable π-conjugated materials for recognition, sensing, and optoelectronics. For the first time, we use the Curtin-Hammett principle to investigate a chiral macrocyclisation reaction, revealing the potential for supramolecular π-π interactions to direct the outcome of a dynamic kinetic resolution, favouring the opposite macrocyclic product to that expected under reversible, thermodynamically controlled conditions. Specifically, a dynamic, racemic perylene diimide dye (1 : 1 P : M) is strapped with an enantiopure (S)-1,1'-bi-2-naphthol group (P-BINOL) to form two diastereomeric macrocyclic products, the homochiral macrocycle (PP) and the heterochiral species (PM). We find there is notable selectivity for the PM macrocycle (dr = 4 : 1), which is rationalised by kinetic templation from intramolecular aromatic non-covalent interactions between the P-BINOL π-donor and the M-PDI π-acceptor during the macrocyclisation reaction.

Bioactive atropisomers: Unraveling design strategies and synthetic routes for drug discovery

Atropisomerism, an expression of axial chirality caused by limited bond rotation, is a prominent aspect within the field of medicinal chemistry. It has been shown that atropisomers of a wide range of compounds, including established FDA-approved drugs and experimental molecules, display markedly different biological activities. The time-dependent reversal of chirality in atropisomers poses complexity and obstacles in the process of drug discovery and development. Nonetheless, recent progress in understanding atropisomerism and enhanced characterization methods have greatly assisted medicinal chemists in the effective development of atropisomeric drug molecules. This article provides a comprehensive review of their special design thoughts, synthetic routes, and biological activities, serving as a reference for the synthesis and biological evaluation of bioactive atropisomers in the future.

Organocatalytic Synthesis of Furan-Embedded Styrene Atropisomers

Herein, we report the first synthesis of furan-embedded styrene atropisomers via the reaction between 1-(aryl-ethynyl)-naphthalen-2-ol and γ-hydroxyenone. The reaction proceeds through in situ furan formation from γ-hydroxyenone. The styrene derivatives were obtained in moderate to good yields with high diastereoselectivities with a catalytic amount of PTSA. Few applications such as triazole formation and cross-coupling reactions have been demonstrated. A preliminary catalytic asymmetric version was also reported.

Dynamic Kinetic Resolution of 2-(Quinolin-8-yl)Benzaldehydes: Atroposelective Iridium-Catalyzed Transfer Hydrogenative Allylation

An atroposelective Ir-catalyzed dynamic kinetic resolution (DKR) of 2-(quinolin-8-yl)benzaldehydes/1-naphthaldehydes by transfer hydrogenative coupling of allyl acetate is disclosed. The allylation reaction takes place with simultaneous installation of central and axial chirality, reaching high diastereoselectivities and excellent enantiomeric excesses when ortho-cyclometalated iridium-DM-BINAP is used as the catalyst. The racemization of the substrates occurs through a designed transient Lewis acid-base interaction between the quinoline nitrogen atom and the aldehyde carbonyl group.

Construction of C-B axial chirality via dynamic kinetic asymmetric cross-coupling mediated by tetracoordinate boron

Catalytic dynamic kinetic asymmetric transformation (DyKAT) provides a powerful tool to access chiral stereoisomers from racemic substrates. Such transformation has been widely employed on the construction of central chirality, however, the application in axial chirality remains underexplored because its equilibrium of substrate enantiomers is limited to five-membered metalacyclic intermediate. Here we report a tetracoordinate boron-directed dynamic kinetic asymmetric cross-coupling of racemic, configurationally stable 3-bromo-2,1-azaborines with boronic acid derivatives. A series of challenging C-B axially chiral compounds were prepared with generally good to excellent enantioselectivities. Moreover, this transformation can also be extended to prepare atropisomers bearing adjacent C-B and C-C diaxes with excellent diastereo- and enantio-control. The key to the success relies on the rational design of a reversible tetracoordinate boron intermediate, which is supported by theoretical calculations that dramatically reduces the rotational barrier of the original C-B axis and achieves the goal of DyKAT.

Development of axially chiral urazole scaffolds for antiplant virus applications against potato virus Y

BACKGROUND

Potato virus Y (PVY) was first discovered by Smith in 1931 and is currently ranked as the fifth most significant plant virus. It can cause severe damage to plants from the family Solanaceae, which results in billions of dollars of economic loss worldwide every year. To discover new antiviral drugs, a class of multifunctional urazole derivatives bearing a stereogenic CN axis were synthesized with excellent optical purities for antiviral evaluations against PVY.

RESULTS

The absolute configurations of the axially chiral compounds exhibited obvious distinctions in antiviral bioactivities, with several of these enantio-enriched axially chiral molecules showing excellent anti-PVY activities. In particular, compound (R)-9f exhibited remarkable curative activities against PVY with a 50% maximal effective concentration (EC₅₀ ) of 224.9 μg mL^-1 , which was better than that of ningnanmycin (NNM), which had an EC₅₀ of 234.0 μg mL^-1 . And the EC₅₀ value of the protective activities of compound (R)-9f was 462.2 μg mL^-1 , which was comparable to that of NNM (442.0 μg mL^-1 ). The mechanisms of two enantiomer of the axially chiral compounds 9f were studied by both molecule docking and defensive enzyme activity tests.

CONCLUSION

Mechanistic studies demonstrated that the axially chiral configurations of the compounds played significant roles in the molecule PVY-CP (PVY Coat Protein) interactions and could enhance the activities of the defense enzymes. The (S)-9f showed only one carbon-hydrogen bond and one π-cation interaction between the chiral molecule and the PVY-CP amino acid sites. In contrast, the (R)-enantiomer of 9f exhibited three hydrogen bonding interactions between the carbonyl groups and the PVY-CP active sites of ARG157 and GLN158. The current study provides significant information on the roles that axial chiralities play in plant protection against viruses, which will facilitate the development of novel green pesticides bearing axial chiralities with excellent optical purities. © 2023 Society of Chemical Industry.

Diaryl-Pyrano-Chromenes Atropisomers: Stereodynamics and Conformational Studies

The dynamic scenario of di-aryls-pyrano-chromenes was investigated using DFT calculations. The symmetry of the chromene scaffold and the presence of two ortho-substituted aryls substituents can generate two syn/anti diastereoisomers and conformational enantiomers with different rotational barriers. The relative conformations and configurations were derived using NOESY-1D experiments. Depending on the energies related to the conformational exchange, the experimental energy barriers were determined through Dynamic NMR, Dynamic HPLC or kinetic studies. The atropisomeric pairs were resolved in the latter scenario, and their absolute configuration was assigned using the ECD/TD-DFT method.

Chiral Spectroscopy of Nanostructures

ConspectusChirality is ubiquitous in the universe and in living creatures over detectable length scales from the subatomic to the galactic, as exemplified in the two extremes by subatomic particles (neutrinos) and spiral galaxies. Between them are living creatures that display multiple levels of chirality emerging from hierarchically assembled asymmetric building blocks. Not too far from the bottom of this pyramid are the foundational building blocks with chiral atomic centers on sp³ carbon atoms exemplified by l-amino acids and d-sugars that are self-assembled into higher-order structures with increasing dimensions forming highly complex, amazingly functional, and energy-efficient living systems. The organization and materials employed in their construction inspired scientists to replicate complex living systems via the self-assembly of chiral components. Multiple studies pointed to unexpected and unique electromagnetic properties of chiral structures with nanoscale and microscale dimensions, including giant circular dichroism and collective circularly polarized scattering that their constituent units did not possess.To address the wide variety of chiral geometries observed in continuous materials, singular particles, and their complex systems, multiple analytic techniques are needed. Simultaneously, their spectroscopic properties create a pathway to multiple applications. For example, mirror-asymmetric vibrations at chiral centers formed by sp³ carbon atoms lead to optical activity for the infrared (IR) wavelength regions. At the same time, understanding the optical activity in, for example, the IR region enables biomedical applications because multiple modalities of biomedical imaging and vibrational optical activity (VOA) of biomolecules are known for IR range. In turn, VOA can be realized in both absorption and emission modalities due to large magnetic transition moments, as vibrational circular dichroism (VCD) or Raman optical activity (ROA) spectroscopy. In addition to the VOA, in the range of longer wavelengths, lattice vibrational mode or phononic behavior occurs in chiral crystals and nanoassemblies, which can be readily detected by terahertz circular dichroism (TCD) spectroscopy. Meanwhile, chiral self-assembly can induce circularly polarized light emission (CPLE) regardless of the existence of chirality in coassembled fluorophores. The CPLE from self-assembled chiral materials is particularly interesting because the CPLE can originate from both circularly polarized luminescence and circularly polarized scattering (CPS). Furthermore, because self-assembled nanostructures often exhibit stronger optical activity than their building blocks owing to dimension and resonance effects, the optical activity of single assembled nanostructures can be investigated by using microscopic technology combined with chiral optics. Here, we describe the state of the art for spectroscopic methods for the comprehensive analysis of chiral nanomaterials at various photon wavelengths, addressed with special attention given to new tools emerging both for materials with self-organized hierarchical chirality and single-particle spectroscopy.

Atropostatin: Design and Total Synthesis of an Atropisomeric Lactone–Atorvastatin Prodrug

Atorvastatins play an important role in the inhibition of HMG-CoA reductase, an enzyme present in the liver that takes part in the biosynthesis of cholesterol. In this article, we report the total synthesis of a lactone–atorvastatin prodrug with additional atropisomeric features. Conformational and experimental studies of model compounds were designed to test the stability of the chiral axis. Docking calculations were performed to evaluate the constant inhibition of a library of atorvastatins. Full synthesis of the best candidate was achieved and thermally stable atropisomeric lactone–atorvastatin was obtained. The absolute configuration of the chiral axis of the atropisomers was assigned by means of chiroptical ECD spectroscopy coupled with TD-DFT calculations.

Sulfoxide-Directed or 3d-Metal Catalyzed C-H Activation and Hypervalent Iodines as Tools for Atroposelective Synthesis

ConspectusThe expanding applications of atropisomeric compounds combined with the growing diversity of such chiral molecules translate into an urgent need for innovative synthetic strategies allowing their rapid, efficient, and sustainable synthesis. Recently, the C-H activation approach has provided new opportunities for synthesizing axially chiral compounds. The two complementary approaches allowing implementation of the C-H activation methodology toward the synthesis of the chiral molecules imply either ortho-functionalization of the preexisting prochiral or atropo-unstable biaryl substrates or direct C-H arylation of sterically encumbered aromatics. The first approach required the preinstallation of a directing group on a biaryl precursor, which drastically limits the diversity of thus generated products. To tackle this important synthetic limitation, we have envisioned using a chiral sulfoxide as both directing group and chiral auxiliary. Indeed, in addition to efficiently coordinating the Pd-catalyst thus allowing chiral induction, the sulfoxide moiety can be easily removed, via the sulfoxide/lithium exchange, after the C-H activation step, thus guaranteeing an almost unlimited postdiversification of the atropisomeric products. The efficiency and generality of this concept could be illustrated by developing atropo-diastereoselective oxidative Heck reaction, direct acetoxylation, and iodination, as well as direct arylation. Besides, the synthetic utility of this methodology was demonstrated by designing an expedient synthesis of a direct steganone precursor. This unique transformation also allowed us to build up unprecedented triaryl scaffolds with two perfectly controlled chiral axes, original chiral skeletons for new ligand design. While considering the atroposelective direct arylations, the clear antagonism between the harsh reaction conditions frequently required for the coupling of two sterically hindered compounds and the atropo-stability of the new product, resulted in the scarcity of such transformations. To solve this fundamental challenge, we have focused on the application of a low-valent cobalt catalyst, prompted to catalyze C-H activation of indoles at the C2 position under extremely mild reaction conditions (room temperature). Accordingly, atroposelective C2-arylation of indoles could be achieved using an original carbene ligand and delivering the uncommon atropoisomerically pure indoles in excellent yields and enantioselectivities. Detailed combined experimental and theoretical mechanistic studies shed light on the mechanism of this transformation, providing strong evidence regarding the origin of the enantioselectivity. Finally, the antagonism between steric hindrance required to guarantee the atropo-stability of a molecule and harsh reaction conditions required to couple two partners is a strong limitation not only for the development of atroposelective C-H arylation reaction but also for the development of direct synthesis of the C-N axially chiral compounds. Despite the long history and incredible advances achieved in Ullmann-Goldberg and Buchwald-Hartwig couplings, atroposelective versions of such transformations have remained unprecedented until recently. Our idea to tackle this challenging issue consisted in using hypervalent iodines as highly reactive coupling partners, thus allowing the desired N-arylations to occur at room temperature. This hypothesis could be validated by reporting first atropo-diastereoselective Cu-catalyzed N-arylation, using sulfoxide λ³-iodanes as the coupling partners. Subsequently, the enantioselective version of this atroposelective N-arylation was successfully established by using a chiral Cu-complex bearing a BOX ligand. In conclusion, we report herein designing tailored-made solutions to provide new synthetic strategies to construct the atropisomeric molecules, including biaryls and C-N axially chiral molecules.

DBU Promoted Polysubstituted Arene Formation via a Michael Addition/Cyclization/Elimination Cascade Reaction

The straightforward construction of polysubstituted arenes is essential in both synthetic chemistry and medicinal chemistry. Herein, we reported a DBU promoted Michael addition/cyclization/elimination cascade reaction between vinylogous malononitrile derivatives and chlorinated nitrostyrenes for the synthesis of polysubstituted arenes. The method features mild reaction conditions, wide substrate scope and high yield. Interestingly, preliminary study of the enantioselective version of this cascade was conducted to give chiral biaryl atropisomers with up to 40% ee through center-to-axial chirality transfer strategy.

Addressing Atropisomerism in the Development of Sotorasib, a Covalent Inhibitor of KRAS G12C: Structural, Analytical, and Synthetic Considerations

Nearly a century after its first description, configurationally stable axial chirality remains a rare feature in marketed drugs. In the development of the KRAS^G12C inhibitor sotorasib (LUMAKRAS/LUMYKRAS), an axially chiral biaryl moiety proved a critical structural element in engaging a "cryptic" protein binding pocket and enhancing inhibitor potency. Restricted rotation about this axis of chirality gave rise to configurationally stable atropisomers that demonstrated a 10-fold difference in potency. The decision to develop sotorasib as a single-atropisomer drug gave rise to a range of analytical and synthetic challenges, whose resolution we review here.Assessing the configurational stability of differentially substituted biaryl units in early inhibitor candidates represented the first challenge to be overcome, as differing atropisomer stability profiles called for differing development strategies (e.g., as rapidly equilibrating rotamers vs as single atropisomers). We relied on a range of NMR, HPLC, and computational methods to assess atropisomer stability. Here, we describe the various variable-temperature NMR, time-course NMR, and chiral HPLC approaches used to assess the configurational stability of axially chiral bonds displaying a range of rotational barriers.As optimal engagement of the "cryptic" pocket of KRAS^G12C was ultimately achieved with a configurationally stable atropisomeric linkage, the second challenge to be overcome entailed preparing the preferred (M)-atropisomer of sotorasib on industrial scale. This synthetic challenge centered on the large-scale synthesis of an atropisomerically pure building block comprising the central azaquinazolinone and pyridine rings of sotorasib. We examined a range of strategies to prepare this compound as a single atropisomer: asymmetric catalysis, chiral chromatographic purification, and classical resolution. Although chiral liquid and simulated moving bed chromatography provided expedient access to initial multikilo supplies of this key intermediate, a classical resolution process was ultimately developed that proved significantly more efficient on metric-ton scale. To avoid discarding half of the material from this resolution, this process was subsequently refined to enable thermal recycling of the undesired atropisomer, providing an even more efficient commercial process that proved both robust and green.While the preparation of sotorasib as a single atropisomer significantly increased both the analytical and synthetic complexity of its development, the axially chiral biaryl linkage that gave rise to the atropisomerism of sotorasib proved a key design element in optimizing sotorasib's binding to KRAS^G12C. It is hoped that this review will help in outlining the range of analytical techniques and synthetic strategies that can be brought to bear in addressing the challenges posed by such axially chiral compounds and that this account may provide helpful guidelines for future efforts aimed at the development of such single atropisomer, axially chiral pharmaceutical agents.

Solvent dependent hindered rotation versus epimerization in axially chiral thiohydantoin derivatives: an experimental and a computational study

5-Benzyl-3-(o-aryl)-2-thiohydantoin and 5-isobutyl-3-(o-aryl)-2-thiohydantoin derivatives (o-aryl = o-tolyl and o-bromophenyl) have been synthesized by reacting o-aryl isothiocyanates with S-phenylalanine methyl ester hydrochloride or with S-leucine methyl ester hydrochloride in the presence of triethylamine (TEA). The synthesized compounds have a chirality center at C5 of the heterocyclic ring and a chirality axis, the N3-C_(aryl) bond. The axially chiral compounds were shown to exist in unequal amounts of SM, SP, RM and RP stereoisomeric forms with a high prevalence of the P isomers over the M isomers. The isomeric assignments were done by comparing the ¹H NMR spectra with the HPLC chromatograms. The stereoisomers were resolved micropreparatively by HPLC on chiral stationary phases and the interconversion of the single isomers has been investigated. The conversion type has been determined as epimerization or rotation by the HPLC analyses. It has been found that although the stereoisomers converted to each other only by rotation in toluene, in ethanol epimerization (racemization at C5 of the heteroring) was accompanied with rotation depending on the duration, temperature of the thermal interconversion experiment and the nature of the ortho substituent. The occurrence of epimerization was also proved through H/D exchange reactions via¹H NMR experiments done in CD₃OD. The rotation and epimerization mechanisms of synthesized compounds were further elucidated by Density Functional Theory (DFT) calculations at M062X/6-311 + G** level of theory and the results were shown to be in harmony with experimental findings.

Unraveling the Pivotal Role of Atropisomerism for Cellular Internalization

The intrinsic challenge of large molecules to cross the cell membrane and reach intracellular targets is a major obstacle for the development of new medicines. We report how rotation along a single C-C bond, between atropisomers of a drug in clinical trials, improves cell uptake and therapeutic efficacy. The atropisomers of redaporfin (a fluorinated sulfonamide bacteriochlorin photosensitizer of 1135 Da) are separable and display orders of magnitude differences in photodynamic efficacy that are directly related to their differential cellular uptake. We show that redaporfin atropisomer uptake is passive and only marginally affected by ATP depletion, plasma proteins, or formulation in micelles. The α₄ atropisomer, where meso-phenyl sulfonamide substituents are on the same side of the tetrapyrrole macrocycle, exhibits the highest cellular uptake and phototoxicity. This is the most amphipathic atropisomer with a conformation that optimizes hydrogen bonding (H-bonding) with polar head groups of membrane phospholipids. Consequently, α₄ binds to the phospholipids on the surface of the membrane, flips into the membrane to adopt the orientation of a surfactant, and eventually diffuses to the interior of the cell (bind-flip mechanism). We observed increased α₄ internalization by cells of the tumor microenvironment in vivo and correlated this to the response of photodynamic therapy when tumor illumination was performed 24 h after α₄ administration. These results show that properly orientated aryl sulfonamide groups can be incorporated into drug design as efficient cell-penetrating motifs in vivo and reveal the unexpected biological consequences of atropisomerism.

A nonneglectable stereochemical factor in drug development: Atropisomerism

Chirality is one of the key factors affecting the medicinal efficacy of compounds. In addition to central chirality, sterically hindered chiral axes commonly appear in drugs and the resulting chirality is known as atropisomerism. With developments in medicinal chemistry, atropisomerism has attracted increasing attention. This review discusses the classification, biological activity, pharmacokinetics, toxicity and side effects of atropisomers, and can serve as a reference in the research and development of potential chiral drugs.

Axial chirality and affinity at the GABAA receptor of triazolobenzodiazepines

Triazolobenzodiazepines substituted with a methyl group at the C1- and C10-positions and chloro group at C2' of pendant-phenyl were prepared and their physicochemical properties were investigated. The atropisomers of 1,10-disubstituted triazolobenzodiazepines, 1d and 1f, were isolated as (a¹R, a²S) and (a¹S, a²R) isomers. Their absolute configurations were determined on the basis of CD spectra in comparison with those of stereochemically defined 9-methyl-1,4-benzodiazepin-2-ones. Examination of the affinity at the human GABA_A receptors revealed that each (a¹R, a²S) isomer of 1d and 1f possessed higher activity than its antipode (a¹S, a²R) isomer. It was also found that 1a, which behaves achirally due to the rapid conformational change, had the highest GABA_A affinity, equal to that of triazolam. Considering that each eutomer of 1d and 1f is (a¹R, a²S), the conformation of 1a at the binding site of the GABA_A receptor is expected to be (a¹R, a²S).

Synthesis, resolution, and absolute configuration determination of a vicinal amino alcohol with axial chirality. Application to the synthesis of new box and pybox ligands

New racemic vicinal amino alcohol derivatives with 4‐benzylidenecyclohexane skeleton and axial chirality have been prepared. A preparatively easy and efficient protocol for resolution of the N‐benzoylamino alcohol is described. Using a 250 × 20 mm (L × ID) Chiralpak® IA column, and the appropriate mixture of n‐hexane/ethanol/chloroform as eluent, both enantiomers of N‐benzoylamino alcohol 3 are obtained with >99% enantiomeric excess (ee) by successive injections of a solution of the racemic sample in chloroform. The obtained axially chiral vicinal amino alcohol is used to synthesize structurally novel bisoxazoline ligands in high yields.

Asymmetric Synthesis of Axially Chiral C-N Atropisomers

Molecules with restricted rotation around a single bond or atropisomers are found in a wide number of natural products and bioactive molecules as well as in chiral ligands for asymmetric catalysis and smart materials. Although most of these compounds are biaryls and heterobiaryls displaying a C−C stereogenic axis, there is a growing interest in less common and more challenging axially chiral C−N atropisomers. This review offers an overview of the various methodologies available for their asymmetric synthesis. A brief introduction is initially given to contextualize these axially chiral skeletons, including a historical background and examples of natural products containing axially chiral C−N axes. The preparation of different families of C−N based atropisomers is then presented from anilides to chiral five‐ and six‐membered ring heterocycles. Special emphasis has been given to modern catalytic asymmetric strategies over the past decade for the synthesis of these chiral scaffolds. Applications of these methods to the preparation of natural products and biologically active molecules will be highlighted along the text.

Recent advances in the applications of pyrazolone derivatives in enantioselective synthesis

Pyrazolones and pyrazoles, featuring nitrogen-nitrogen bonds, are two of the most important classes of heterocycles, owing to their widespread occurrence in medicinal chemistry and functional materials. The last decade has witnessed a rapid increase in the construction of chiral pyrazolone and pyrazole derivatives, with the application of pyrazolone derivatives as powerful synthons. Since our last review in 2018, a large number of new achievements has emerged in this area, requiring a timely update. Thus, this review summarizes these elegant achievements based on the multiple reactive sites of different pyrazolone synthons. In addition, important mechanisms and interesting biological investigations relating to the corresponding products are also discussed.

Aldol Reactions of Conformationally Stable Axially Chiral Thiohydantoin Derivatives

Two novel axially chiral ortho-trifluoromethylphenyl thiohydantoin derivatives have been prepared atroposelectively from the reaction of R and S alanine methyl ester HCl salts with ortho-trifluoromethylphenyl isothiocyanate in the presence of triethyl amine. It was found that after purification of the crude product by simple recrystallization, the R amino acid esters yielded thiohydantoins having solely M axial chirality whereas the S ones returned the P isomers only. This result prompted us to perform sterically controlled aldol reactions on M and P thiohydantoin atropisomers. It was found that during the aldol reaction of 3-o-trifluoromethyl-5-methylthiohydantoins, the o-trifluoromethyl group of the M isomers efficiently shielded the Si face of the intermediate and in this way, enabled the selective formation of only the R configured aldol products at C5 of the heterocyclic ring. The P thiohydantoins, on the other hand, yielded only the S C5 configured aldol products as a result of the Re face shielding of the ortho-trifluoromethyl group of intermediate enolates. A noteworthy face selectivity of the benzaldehyde molecule was not observed (anti/syn only 3/2) during the aldolization of trifluoromethylphenyl derivatives of thiohydantoins. Aldol reactions were also done using the previously synthesized axially chiral thiohydantoins with ortho-Cl, Br, and I phenyl substituents which had predominantly P conformations (P/M ratios > 95%), and the stereochemical outcomes were compared with those of the ortho-trifluoromethyl substituted ones. 80-90% face selectivity of the benzaldehyde molecule was observed for the axially chiral o-halophenyl substituted thiohydantoins. The syntheses done with axially chiral 3-ortho-trifluoromethylphenyl- and 3-ortho-iodophenyl-5-methyl thiohydantoins enabled stereoselective formation of quaternized chiral carbon centers at C5 of the thiohydantoin ring.

Solid-State and Theoretical Investigations of Some Banister-Type Macrocycles with 2,2'-Aldoxime-1,1'-Biphenyl Units

In the context of helical chirality, bridging of biphenyl units leads to banister-type compounds and the stability of the resulted atropisomers may increase dramatically if suitable changes are performed in the linker unit that coils around the biphenyl moiety. A rigorous density functional theory (DFT) study was conducted for macrocycles containing rigid oxime ether segments connected to the biphenyl backbone in order to determine how the rotation barriers are influenced by the presence of either a flexible oligoethyleneoxide or a more rigid m–xylylene component in the macrocycle. The calculated values for the racemization barrier were in good agreement with those obtained experimentally and confirm the benefit of introducing a more rigid unit in the macrocycle on the stability of atropisomers. Solid-state data were obtained and computed data were used to assess the contribution brought by supramolecular associations observed in the lattice to the stabilization of the crystal structure. Beside introducing rigidity in the linker, complexation of flexible macrocycles with alkali metal ions is also contributing to the stability of atropisomers, leading to values for the racemization barrier matching that of the rigid macrocycle. Using diethylammonium cation as guest for the macrocycle, a spectacular increase in the barrier to rotation was observed for the resulted pseudo[2]rotaxane.

Catalytic Atroposelective C7 Functionalisation of Indolines and Indoles

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry. In particular, axially chiral indole‐ and indoline‐based frameworks have been recognised as important heterobiaryl classes because they are the core units of bioactive natural alkaloids, chiral ligands and bioactive compounds. Among them, the synthesis of C7‐substituted indole biaryls and the analogous indoline derivatives is particularly challenging, and methods for their efficient synthesis are in high demand. Transition‐metal catalysis is considered one of the most efficient methods to construct atropisomers. Here, we report the enantioselective synthesis of C7‐indolino‐ and C7‐indolo biaryl atropisomers by means of C−H functionalisation catalysed by chiral RhJasCp complexes.

Chiral Phosphoric Acid-Catalyzed Remote Control of Axial Chirality at Boron-Carbon Bond

The previously elusive catalytic enantioselective construction of axially chiral B-aryl-1,2-azaborines with a C-B stereogenic axis has been realized through a chiral phosphoric acid-catalyzed desymmetrization strategy reported herein. The electrophilic aromatic substitution reaction of 3,5-disubsituted phenols with diazodicarboxamides could afford these axially chiral structures in good efficiency with excellent enantiocontrol. The efficient long-range stereochemical control is achieved by multiple well-defined H-bonding interactions between chiral phosphoric acid and both substrates. Meanwhile, the reaction duration could be markedly shortened with weakly acidic N-H in 1,2-azaborine acting as H-bond donor. The scalability of the reaction and facile cleavage of the N-N bond in the product further demonstrated the practicality of this method.

Synthesis of homochiral sulfanyl- and sulfoxide-substituted naphthyltriazoles and study of the conformational stability

The preparation of a series of novel homochiral atropisomeric sulfanyl- and sulfoxide-substituted naphthyltriazoles is described. The triazolization methodology used presents a new way towards novel and highly stable 1,2,3-triazole-based atropisomers, and introduces a new and complementary synthetic pathway towards 4-sulfanyl substituted 1,2,3-triazoles. Starting from sulfanyl-substituted naphthyl ketones, enantiopure amines, and 4-nitrophenyl azide, a collection of 16 sulfanyl-substituted naphthyltriazoles were obtained via the triazolization reaction in which the homochiral diastereomers are readily isolated. Subsequent monooxidation results in the preparation of several sulfoxide-substituted naphthyltriazoles. The absolute configuration of a set of diastereomeric sulfanyl- and sulfoxide-appended naphthyltriazoles was deduced via X-ray crystallography. Furthermore, the conformational stability of the atropisomers was determined experimentally, and further confirmed and analyzed with the aid of computational DFT calculations.

Construction of Axially Chiral Arylborons via Atroposelective Miyaura Borylation

Compared with the well-developed centrally chiral boron chemistry, C-B axially chiral chemistry remains elusive and challenging. Herein we report the first atroposelective Miyaura borylation of bromoarenes with unsymmetrical diboron reagents for the direct catalytic synthesis of optically active atropisomeric arylborons. This reaction features broad substrate scope and produces axially chiral arylborons with high yields and good enantioselectivities.

Analysis of Stereochemical Stability of Dynamic Chiral Molecules Using an Automated Microflow Measurement System

An automated microflow measurement system for the kinetic study of racemization of dynamic chiral molecules was developed. This system facilitated the analysis of fast racemization within several seconds at elevated temperatures owing to its rapid heating ability, high performance for controlling short residence times, and ease of connection to HPLC systems for direct measurement of the enantiomeric purity. A more precise analysis was realized by combination of microflow and common batch measurements over a broad range of temperatures.

Carbene-Catalyzed Atroposelective Annulation and Desymmetrization of Urazoles

An NHC-catalyzed atroposelective reaction between ynals and urazoles is disclosed. The reaction establishes a chiral C-N axis via an atroposelective [3 + 2] annulation/desymmetrization process. Our reaction allows efficient access to axially chiral and heteroatom-rich urazole derivatives with potential applications in bioactive molecules and catalysis.

Stable Axially Chiral Isomers of Arylnaphthalene Lignan Glycosides with Antiviral Potential Discovered from Justicia procumbens

Arylnaphthalene lignans (ANLs) were known to have axial chirality due to the biphenyl skeleton with hindered rotation at the single bond. However, the stable ANL atropisomers have not been isolated from nature until the present study. Phytochemical separation of the methanol extract of the stems and barks of Justicia procumbens led to the isolation of 11 ANL glycosides including four pairs of new atropisomers with stable confirmations at room temperature. Their structures were deduced from elucidation of the extensive spectral data, and their absolute configurations were determined by the circular dichroism, electronic circular dichroism, and X-ray methods as well as the total synthesis of one pair of the atropisomers. The ANL compounds were evaluated for their antiviral potential, and it was found that they displayed great antiviral activity discrepancy between a pair of atropisomers due to the geometric orientation. The 1'P-oriented atropisomers showed much more significant antiviral potency than their corresponding 1'M-oriented counterparts. The biological activity discrepancy caused by the axial chirality will not only inspire synthetic design of novel ANL atropisomers to enrich the structural diversity, but also provide important hints to direct the synthetic approaches toward the antiviral drug development of ANL compounds.

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.

Slight structural modulation around a pivotal bond: high impact on enantiomeric stability

Based on an atropisomeric scaffold, structural modifications around chiral axis allowed to establish structure-rotational barrier relationships.

Enantioselective Synthesis of Five-Membered-Ring Atropisomers with a Chiral Rh(III) Complex

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry, and efficient methods for their synthesis are in high demand. This applies in particular to atropisomers derived from five-membered aromatic rings because their lower barrier for rotation among the biaryl axis limits their asymmetric synthesis. We report here an enantioselective C-H functionalization method using our chiral RhJasCp complex for the synthesis of the biaryl atropisomer types that can be accessed from three different five-membered-ring heterocycles.

Raise the anchor! Synthesis, X-ray and NMR characterization of 1,3,5-triazinanes with an axial tert-butyl group

N-t-Bu-N',N''-Disulfonamide-1,3,5-triazinanes were synthesized and characterized by X-ray single crystal structure analysis. In the course of the X-ray structure elucidation, the first solid experimental evidence of the axial position of the tert-butyl group in unconstrained hexahydro-1,3,5-triazacyclohexanes was obtained. Dynamic low-temperature NMR analysis allowed to fully investigate a rare case of crystallization-driven unanchoring of the tert-butyl group in the chair conformation of saturated six-membered cycles. DFT calculations show that the use of explicit solvent molecules is necessary to explain the equatorial position of the t-Bu group in solution. Otherwise, the axial conformer is the thermodynamically stable isomer.

4,15-Dimethyl-7,12-diazo-niatri-cyclo-[10.4.0.02,7]hexa-deca-1(12),2,4,6,13,15-hexa-ene dibromide monohydrate

The crystal structure of the viologen 4,4′-dimethyl-2,2′-dipyridyl-N,N′-tetra­methyl­ene dibromide monohydrate is presented, along with details of an improved synthesis and NMR spectroscopic analysis.

The title compound, C₁₆H₂₀N₂²⁺·2Br⁻·H₂O (1) is a member of the class of compounds called viologens. Viologens are quaternary salts of di­pyridyls and are especially useful as redox indicators as a result of their large negative one-electron reduction potentials. Compound 1 consists of a dication composed of a pair of 4-methyl­pyridine rings mutually joined at the 2-position, with a dihedral angle between the pyridine rings of 62.35 (4)°. In addition, the rings are tethered via the pyridine nitro­gen atoms by a tetra­methyl­ene bridge. Charge balance is provided by a pair of bromide anions, which are hydrogen bonded to a single water mol­ecule [D_O⋯Br = 3.3670 (15) and 3.3856 (15) Å]. The crystal structure of 1, details of an improved synthesis, and a full analysis of its NMR spectra are presented.

Asymmetric Catalysis upon Helically Chiral Loratadine Analogues Unveils Enantiomer-Dependent Antihistamine Activity

Analogues of the conformationally dynamic Claritin (loratadine) and Clarinex (desloratadine) scaffolds have been enantio- and chemoselectively N-oxidized using an aspartic acid containing peptide catalyst to afford stable, helically chiral products in up to >99:1 er. The conformational dynamics and enantiomeric stability of the N-oxide products have been investigated experimentally and computationally with the aid of crystallographic data. Furthermore, biological assays show that rigidifying the core structure of loratadine and related analogues through N-oxidation affects antihistamine activity in an enantiomer-dependent fashion. Computational docking studies illustrate the observed activity differences.