Chiral Auxiliary Research Articles

Determining the Absolute Configuration of Small Molecules by Diffusion NMR Experiments

AbstractEnantiomers are ubiquitous in many areas of science, such as pharmaceuticals, agriculture, and food. Nuclear magnetic resonance (NMR) alone is not able to differentiate enantiomers as their spectra are identical. However, these can be distinguished using chiral auxiliaries (such as chiral complexing agents) that form diastereomeric complexes, but absolute identification is still troublesome, usually requiring a chemical reaction with a chiral derivatizing agent. Here, we propose a new method that uses a hybrid mixture of solvating agents in a simple comparison of diffusion NMR experiments, which can discriminate enantiomers in both frequency and diffusion domains, dubbed CHIMERA (CHIral Micelle Enantiomer Resolving Agent). The new method was assessed for twenty‐three small chiral molecules using a combination of BINOL and (−)‐DMEB, a chiral surfactant, and initial results indicate that absolute configuration can be obtained from a simple experiment.

Visible‐Light‐Induced Stereoselective C(sp3)−C(sp3)−H Glycosylation for the Synthesis of C‐Glycoamino Acid Derivatives

AbstractChiral C‐Glycosyl amino acids are prevalent structural motifs found in pharmaceuticals and natural products, making them a significant dimension for exploration in modern pharmaceutical and medicinal chemistry. However, the current synthesis methods for these compounds face numerous environmental and practical challenges, highlighting the critical need to establish more sustainable approaches. Herein, we report a simple and powerful strategy for the stereoselective synthesis of C‐glycoamino acids derivatives. This method features the design and use of (−)/(+)‐camphorsultam as an ideal chiral auxiliary, shielding one prochiral face of the glycine α‐carbon efficiently and imposing a high kinetic barrier for the epimerization of thus‐generated chiral α‐carbon that two configuration products of R and S could be achieved independently. It proceeds under mild conditions, shows broad scope, and allows for the direct stereoselective glycosylation without metal and chiral ligand.

“Boron Ylide” Enables Stereoselective Construction of gem‐Diborylcyclopropanes

AbstractThe stereoselective cyclopropanation of olefins with “boron ylide” is disclosed for the first time, providing a modular strategy for the synthesis of stereospecific diboryl‐functionalized cyclopropanes. The chiral gem‐diborylcyclopropanes are synthesized with excellent enantioselectivity with the aid of a chiral auxiliary. Based on the powerful transformable ability of boryl group, those challenging multi‐quaternary carbon centers in cyclopropane units have been facilely constructed with excellent stereoselectivity. Control experiments indicate that the boryl groups are necessary for both chemoselectivity and stereoselectivity control.

Ligand-Enabled Cu-Catalyzed Stereoselective Synthesis of P-Stereogenic ProTides.

Nucleoside analogues have seen significant advancements in treating viral infections and cancer through ProTide technology, leading to a series of FDA-approved drugs such as sofosbuvir, tenofovir alafenamide, and remdesivir. The stereochemical configuration at the phosphorus center of ProTides significantly influences their pharmacological properties, necessitating efficient stereoselective synthesis. Traditional methods using chiral auxiliaries or nonracemic phosphorylating agents are labor-intensive and inefficient, while recent organocatalytic approaches, despite their promise, still face limitations. Herein, we present a novel approach employing chiral metal complexes for the stereoselective assembly of P-stereogenic ProTides via asymmetric P-O bond formation. This approach leverages a chiral metal catalyst to activate the electrophilic phosphorylating reagent, facilitating a base-promoted nucleophilic replacement pathway. Our protocol, featuring mild reaction conditions and broad applicability, enables the highly stereoselective synthesis of previously inaccessible (S,RP) and (R,SP)-ProTide derivatives. The practical utility of this method is demonstrated through the preparation of pharmaceutically relevant ProTide targets and mechanistic studies were conducted to elucidate the reaction pathway, offering significant advancements for drug development and pharmaceutical research.

"Boron Ylide" Enables Stereoselective Construction of gem-Diborylcyclopropanes.

The stereoselective cyclopropanation of olefins with "boron ylide" is disclosed for the first time, providing a modular strategy for the synthesis of stereospecific diboryl-functionalized cyclopropanes. The chiral gem-diborylcyclopropanes are synthesized with excellent enantioselectivity with the aid of a chiral auxiliary. Based on the powerful transformable ability of boryl group, those challenging multi-quaternary carbon centers in cyclopropane units have been facilely constructed with excellent stereoselectivity. Control experiments indicate that the boryl groups are necessary for both chemoselectivity and stereoselectivity control.

A Linear and Stereoselective Approach for the Synthesis of Dapoxetine from Benzaldehyde

Abstract: In addition to providing a succinct pathway for the stereoselective synthesis of dapoxetine, a potent SSRI employed in the treatment of premature ejaculation, this study highlights the strategic use of Ellman's sulfinamide as a chiral auxiliary. The key method involves the diastereoselective allylation of (S,E)-N-Benzylidenesulfinamide, resulting in the desired S-configuration critical for the pharmacological activity of dapoxetine. The utilization of readily available benzaldehyde as the starting material and 1-naphthol as a late-stage coupling partner contributes to the economic feasibility of the synthesis. Especially, the linear synthetic approach adopted in this study employs simplified and more efficient protocols for various transformations, culminating in an overall yield of 26%. This research not only presents a practical synthetic route for dapoxetine, but also underscores the importance of cost-effective and streamlined methodologies in drug development processes.

Asymmetric Synthesis and Biological Activity of Contact Pheromone of Western Flower Thrips, Frankliniella occidentalis.

Western flower thrips, Frankliniella occidentalis, is a serious worldwide pest of agriculture and horticulture, and its contact pheromone is 7-methyltricosane. Two enantiomers of 7-methyltricosane were synthesized for the first time. The centra of our strategy were chiral auxiliaries to introduce stereocenter, and Wittig coupling to connect two blocks. The overall yields of our synthesis were 29-30% with seven steps. The electroantennogram (EAG) and the contact behavioral responses revealed that (R)-, (S)- and racemic 7-methyltricosane were separately bioactive, and the racemate was the most bioactive in the male arrestant activity and the female EAG test. This result provides valuable insights, showing that the racemate could be used for the support of the control of western flower thrips, which could be more easily prepared relative to more expensive enantiopure pheromone.

Heterochiral coupling to bilateral β-turn structured azapeptides bearing two remote chiral centers

Enantioselective synthesis governed by chiral catalysts has been extensively developed, but that without any chiral auxiliaries or chiral catalysts is rare, particularly when remote stereogenic centers are involved. Here we report an enantioselectivity of heterochiral coupling in the one-pot reaction of racemic hydrazides with achiral 1,4-bis(isothiocyanine)benzene, yielding preferentially the heterochiral bilateral azapeptides over the homochiral ones. Despite bearing two hydrogen-bonded β-turn structures that allow intramolecular chiral transfer, the bilateral azapeptide products have two chiral centers separated by 14 atoms or 15 bonds, which prevent the direct intramolecular asymmetric communication between the two chiral centers. Interestingly, the heterochiral azapeptides feature intermolecular hydrogen bonding stacking between homochiral β-turns to form a superstructure of alternative M- and P-helices in the crystals. In contrast, the homochiral azapeptide counterparts adopt a β-sheet-like structure, which is less favorable compared to the helical-like superstructure from heterochiral azapeptides, accounting for the favored heterochiral coupling of the one-pot reaction. This work demonstrates enantioselective synthesis involving distant chiral centers through the formation of biomimetic superstructures, opening up new possibilities for the regulation of enantioselectivity.

LC enantioseparation of active pharmaceutical ingredients using rationally synthesized CDRs and chiral molecules with high molar absorptivity.

The synthesis of optically active compounds requires determination of ee, er, and enantiomeric purity. The aim of the present paper is to review the synthesis of several chiral derivatizing reagents (CDRs) in a rational manner, which were successful for the separation and isolation of enantiomers of a variety of active pharmaceutical ingredients and other important and useful racemates. Besides, the application of (i) certain enantiomerically pure amines, either directly or by incorporating each of them as chiral auxiliary in difluorodinitrobenzene or cyanuric chloride moieties to construct the CDR, (ii) (S)-ketoprofen and (S)-levofloxacin as chiral platforms, and (iii) a few isothiocyanates, have been suitably included. Attention is drawn to the use of water micellar mobile phase as the "green" RP-HPLC method and the use of simple achiral derivatization with ninhydrin, particularly. Synthesis of CDRs and their application for enantioseparation of racemates and detagging of certain chromophoric reagent components for obtaining native enantiomers are other interesting features included herein. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases. This comprehensive review not only highlights innovative methodologies for enantioseparation but also underscores their practical applications in controlling and ensuring the enantiomeric purity of pharmaceutical compounds.

Chromium‐Mediated Dearomative Allylic Addition to Arenes

AbstractIn comparison with the well‐established allylic additions to C=O, C=N, and C=C double bonds, the dearomative allylic addition to arene π‐systems is heavily underdeveloped due to the severe challenge in breaking aromatic resonance stabilization. Herein, we disclose a general and efficient dearomative allylic addition to chromium‐bound arenes using allylsilanes as the pronucleophiles, thus affording convenient access to allyl‐functionalized 1,3‐cyclohexadiene building blocks. With commercially available deuterium sources, the versatility of this strategy further enables an arene dearomative 1,2‐deuteroallylation with a high level of deuterium incorporation. Finally, a preliminary try on the asymmetric dearomative hydroallylation based on a chiral auxiliary strategy is described.

Photoinduced Stereoselective Giese Addition on Coumarins

AbstractDiastereoselective Giese addition on coumarin-3-carboxylate bearing a chiral auxiliary with N-hydroxyphthalimide ester to generate the corresponding alkylated adduct has been explored. Among screened chiral auxiliaries, bulkier one with appropriate steric hindrance is better for diastereoselective addition. Mechanistic studies involving fluorescence studies and On/Off experiments shed insight onto the plausible reaction pathway.

Determining the Absolute Configuration of Small Molecules by Diffusion NMR Experiments.

Enantiomers are ubiquitous in many areas of science, such as pharmaceuticals, agriculture, and food. Nuclear magnetic resonance (NMR) alone is not able to differentiate enantiomers as their spectra are identical. However, these can be distinguished using chiral auxiliaries (such as chiral complexing agents) that form diastereomeric complexes, but absolute identification is still troublesome, usually requiring a chemical reaction with a chiral derivatizing agent. Here, we propose a new method that uses a hybrid mixture of solvating agents in a simple comparison of diffusion NMR experiments, which can discriminate enantiomers in both frequency and diffusion domains, dubbed CHIMERA (CHIral Micelle Enantiomer Resolving Agent). The new method was assessed for twenty-three small chiral molecules using a combination of BINOL and (-)-DMEB, a chiral surfactant, and initial results indicate that absolute configuration can be obtained from a simple experiment.

Unraveling the Source of Self-Induced Diastereomeric Anisochronism in Chiral Dipeptides.

Mastering of analytical methods for accurate quantitative determinations of enantiomeric excess is a crucial aspect in asymmetric catalysis, chiral synthesis, and pharmaceutical applications. In this context, the phenomenon of Self-Induced Diastereomeric Anisochronism (SIDA) can be exploited in NMR spectroscopy for accurate determinations of enantiomeric composition, without using a chiral auxiliary that could interfere with the spectroscopic investigation. This phenomenon can be particularly useful for improving the quantitative analysis of mixtures with low enantiomeric excesses, where direct integration of signals can be tricky. Here, we describe a novel analysis protocol to correctly determine the enantiomeric composition of scalemic mixtures and investigate the thermodynamic and stereochemical features at the basis of SIDA. Dipeptide derivatives were chosen as substrates for this study, given their central role in drug design. By integrating the experiments with a conformational stochastic search that includes entropic contributions, we provide valuable information on the dimerization thermodynamics, the nature of non-covalent interactions leading to self-association, and the differences in the chemical environment responsible for the anisochronism, highlighting the importance of different stereochemical arrangement and tight association for the distinction between homochiral and heterochiral adducts. An important role played by the counterion was pointed out by computational studies.

Synthesis and applications of optically pure trans-2-(3,5-dimethylphenoxy)cyclohexan-1-ol as chiral auxiliary for preparation of α-substituted carboxylic acid derivatives

Synthesis and applications of optically pure trans-2-(3,5-dimethylphenoxy)cyclohexan-1-ol as chiral auxiliary for preparation of α-substituted carboxylic acid derivatives

Direct Asymmetric Synthesis of α-Aminoimines from 1,2-Bis-N-Sulfinylimines by Using Allyl Boronic Acids.

A unique direct asymmetric synthesis of α-aminoimines is realized, through rapid and exclusive mono-allylation of chiral bis-N-sulfinylimines using allylboronic acids. The highly selective allylation was possible as electrophilic imine functional group in the product α-aminoimines remained unreactive towards allyl boronic acid nucleophiles. Notably, by varying the geometry and chiral auxiliary, all four isomers of the α-aminoimines were accessed from readily available precursors. A range of allyl nucleophiles, which are tricky to generate by other means possessing highly reactive functional groups also took part in this reaction, expanding the scope further. The applicability of the products α-aminoimines were further demonstrated by accessing a range of structurally diverse chiral cyclic and acyclic 1,2-diamines bearing adjacent stereocenters through addition of a second nucleophile or aza-Prins-type cyclization by exploiting the nucleophilicity of the tethered alkene moiety. Moreover, the leaving group aptitude of sulfinyl auxiliary attached to imine, was exploited to access valuable chiral α-aminonitriles under thermal conditions without employing any reagents. Detailed DFT calculation revealed a chair-like transition state likely operating for the allylboration reaction across imine.

Direct Asymmetric Synthesis of α‐Aminoimines from 1,2‐Bis‐N‐Sulfinylimines by Using Allyl Boronic Acids

AbstractA unique direct asymmetric synthesis of α‐aminoimines is realized, through rapid and exclusive mono‐allylation of chiral bis‐N‐sulfinylimines using allylboronic acids. The highly selective allylation was possible as electrophilic imine functional group in the product α‐aminoimines remained unreactive towards allyl boronic acid nucleophiles. Notably, by varying the geometry and chiral auxiliary, all four isomers of the α‐aminoimines were accessed from readily available precursors. A range of allyl nucleophiles, which are tricky to generate by other means possessing highly reactive functional groups also took part in this reaction, expanding the scope further. The applicability of the products α‐aminoimines were further demonstrated by accessing a range of structurally diverse chiral cyclic and acyclic 1,2‐diamines bearing adjacent stereocenters through addition of a second nucleophile or Prins‐type cyclization by exploiting the nucleophilicity of the tethered alkene moiety. Moreover, the leaving group aptitude of sulfinyl auxiliary attached to imine, was exploited to access valuable chiral α‐aminonitriles under thermal conditions without employing any reagents. Detailed DFT calculation revealed a chair‐like transition state, arising from corresponding allylboroxine species, likely operating for the allylboration reaction across imine.

A General Enantioselective α-Alkyl Amino Acid Derivatives Synthesis Enabled by Cobalt-Catalyzed Reductive Addition.

Enantioenriched unnatural amino acids represent a prevalent motif in organic chemistry, with profound applications in biochemistry, medicinal chemistry, and materials science. Herein, we report a cobalt-catalyzed aza-Barbier reaction of dehydroglycines with unactivated alkyl halides to afford unnatural α-amino esters with high enantioselectivity. This catalytic reductive alkylative addition protocol circumvents the use of moisture-, air-sensitive organometallic reagents, and stoichiometric chiral auxiliaries, enabling the conversion of a variety of primary, secondary, and even tertiary unactivated alkyl halides to α-alkyl-amino esters under mild conditions, thus leading to broad functional group tolerance. The expedient access to biologically active motifs demonstrates the practicality of this protocol by reducing the number of synthetic steps and enhancing the reaction efficiency.

Phosphine-Mediated Reductive Insertion of α-Keto Esters and Isatins into Phthalic Anhydride Derivatives.

Herein, we report an unprecedented P(NMe2)3-mediated reductive insertion of 1,2-dicarbonyl compounds including α-keto esters and isatins into phthalic anhydride-derived alkenes and phthalic anhydrides, which furnishes the corresponding isochroman-1-ones and isochroman-1,4-diones, respectively, in moderate to excellent yields with high chemo- and regioselectivity. Furthermore, the asymmetric version of the ring expansion reaction could be realized by using a chiral auxiliary strategy. Mechanistically, the nucleophilic attack of the Kukhtin-Ramirez adduct, generated from P(NMe2)3 and 1,2-dicarbonyl compound, to the anhydride derivative, followed by a cascade ring-opening and ring-closure process, affords the ring expansion product. The reaction represents a novel metal-free carbon insertion ring expansion of aliphatic rings and also the first [1 + 5] annulation involving the Kukhtin-Ramirez adducts.

Stereoselective Synthesis of α‐Azido Esters and α‐Amino Acid Derivatives via Matteson Homologation of Boronic Esters

AbstractMatteson homologation with sodium azide in DMF proved to be an excellent highly stereoselective tool for the synthesis of complex α‐azido and α‐amino acids. Both enantiomers are easily accessible via the choice of the chiral boronic ester auxiliary.

Electro‐Organic Stereoselective Dehydrogenative Homo‐Coupling of β‐Naphthylamines Derivatives

AbstractThe electrochemical stereoselective dehydrogenative homocoupling of β‐naphthylamine derivatives was investigated and successfully accomplished, the chiral diaryl amines being obtained in up to 60 % yield. The use of an enantiopure β‐naphthylamine featuring an aminoalcohol as chiral auxiliary led to the expected 2,2’‐binaphthyl diamine derivative in up to 96/4 diastereoisomeric ratio. The application of the methodology to the intramolecular cross coupling of the enantiopure bis‐N‐β‐naphthylamine 1,2‐trans‐diamino cyclohexane led to the formation of a single enantiopure diastereoisomer in 37 % yield.