Enantiomeric Ratio Research Articles

Rational design of the substrate binding pocket in epoxide hydrolase for enhanced enantioselectivity towards chiral ortho-methylstyrene oxide

Epoxide hydrolases are crucial enzymes for the stereospecific hydrolysis of racemic epoxides, serving as biocatalysts for synthesizing optically pure epoxides. The (S)-ortho-methylstyrene oxide (oMSO) is a valuable chiral building block in asymmetric organic transformations. Our lab previously elucidated the crystal structure of AuEH2, an epoxide hydrolase from Aspergillus usamii. Here, we conducted saturation mutagenesis on hotspots in the AuEH2 substrate binding pocket (SBP), creating a library of 181 single-site mutations. Screening with rac-oMSO revealed a mutational landscape of enzymatic activity and E value. Using an innovative sequence-activity relationship (innov'SAR) machine learning approach, a double mutant AuEH2A214V/S247Q was identified with obviously improvements enantiomeric ratio (E value= 28.3) for (R)-oMSO that was increased 7.9-fold over wild type. Using E. coli/Aueh2A214V/S247Q at 10 mg/mL, we dynamically resolved 20 mmol/L rac-oMSO at 25 °C, yielding (S)-oMSO with 96.7 % ees and 1,2-diol (R)-oMPED with 81.0 % eep. Molecular dynamics (MD) simulations indicated that changes in SBP residue interactions and non-covalent interactions with (R)-oMSO contributed to the increased activity and enantioselectivity of AuEH2A214V/S247Q.

Asymmetric Synthesis of the Indolo[2,3‐α]quinolizine Alkaloid (+)‐Cuscutamine via a Diastereoselective Intramolecular Pictet‐Spengler Reaction

AbstractA concise total synthesis of Cuscutamine was achieved involving a diastereoselective intramolecular acyl iminium ion mediated Pictet‐Spengler reaction. This synthetic strategy provided a direct route to (11S, 13R)‐Cuscutamine (2) in only 3 steps with an overall yield of 56 % and an enantiomeric ratio of 88 %. The diastereoselectivity of the reaction is consistent with previous studies that proceed via more highly puckered transition states and the present results thus reveal the importance of the steric contributions to the observed diastereoselectivity.

Unconventional Ligand‐to‐Catalyst Ratio for the Distal C−C Bond Formation via C−H Activation and Chain‐Walking

AbstractA mechanism‐navigated enantioselective alkylation of an indole C2 C−H bond with an internal alkene was developed. Under the unconventional ligand‐to‐metal catalyst ratio, a moderate enantiomeric ratio (er), ranging from 70:30 to 88:12, was achieved with our original ligand. We propose that chain‐walking and bond formation are mechanistically independent and suggest that hydridoiridium generated from the C−H source does not play a role in the chain‐walking process.

Biogeochemical conditions controlling the intensity of paralytic shellfish poisoning (PSP) outbreak caused by Alexandrium blooms: Results from 6-year field observations in Jinhae-Masan Bay, Korea

Previous field observations from 2018 to 2019 revealed that paralytic shellfish poisoning (PSP) caused by the blooms of toxic dinoflagellate Alexandrium species occurred under low concentrations of dissolved inorganic nitrogen (DIN) and high concentrations of dissolved organic nitrogen (DON) and humic-like fluorescent dissolved organic matter (FDOMH) in Jinhae-Masan Bay, Korea. In this study, we obtained more data for DIN, DON, FDOMH, and Alexandrium cell density from 2020 to 2023 to further validate environmental conditions for the PSP outbreak. We also measured total hydrolyzed amino acids (THAA) to determine the bioavailability of DON fueling the PSP outbreak. Over the 6-year observations, there was a consistent pattern of low DIN concentrations and high DON and FDOMH concentrations during the PSP outbreak periods. The Alexandrium cell densities, together with the PSP toxin concentrations, increased rapidly under this environmental condition. The PSP outbreak occurs when a large amount of DIN originating from the stream waters near the upstream sites is transformed into DON by biological production before entering the PSP outbreak area. The produced DON is characterized by high bioavailability based on the various AA-derived indices (enantiomeric ratio, degradation index, non-protein AA mole%, and nitrogen-normalized AA yield). In addition, the intensities of PSP outbreaks are mainly dependent on the conversion stage of DIN to DON and enhanced FDOMH. We found that the strong PSP outbreak occurred consistently under a low level of DIN (<1.0 μM) and high levels of DON (>9.0 μM) and FDOMH (>1.5 R.U.). Thus, our results suggest that the monitoring data of environmental conditions can be used to predict the PSP outbreak in the coastal oceans.

Comprehensive analysis of chemical and enantiomeric stability of terpenes in Cannabis sativa L. flowers.

Cannabis sativa L. is renowned for its medicinal and recreational uses. With the increasing global legalization of C.sativa L.-based products for medicinal purposes, there is a growing need for well-characterized products. While the stability of cannabinoids such as tetrahydrocannabinol and cannabidiol is well understood, information on the chemical and enantiomeric stability of terpenes remains scarce. This is despite the fact that terpenes are also thought to have pharmacological activity and may contribute to the overall effect of C.sativa L. To address these challenges, four analytical methods based on chiral, polar, and apolar gas chromatographic separation combined with either MS or FID detection were developed and validated. These methods successfully separated and quantified a total of 29 terpenes, including 13 enantiomers and 5 diastereomers specific to C.sativa L. Furthermore, terpenes and authentic C.sativa L. flowers and extracts were subjected to UV and heat treatments to observe potential degradation reactions over time. Each terpene generates a unique pattern of degradation products resulting in a diverse array of oxidation and cyclization products. P-cymene was identified as a major product of terpene aging. Notably, no enantiomeric conversion was detected, suggesting that the formation of (-)-α-pinene in cannabis extracts, for example, originates from other terpenes. Terpenes have different degradation rates, even though they are structurally similar. In addition, cultivar- and growth-condition-specific enantiomeric ratios were observed in C.sativa L., confirming that enantiomer production is species-specific and has to be considered for therapeutical applications.

Urinary D-amino acid profiles in cats with chronic kidney disease.

Chronic kidney disease (CKD) is highly prevalent in domestic cats. This study aimed to compare urinary D-amino acid levels between control and CKD-afflicted cats as a novel noninvasive method for assessing CKD. Cats were divided into control and CKD stage II groups in accordance with the International Renal Interest Society guidelines. The urinary DL-amino acid levels of the cats were analyzed using chiral tandem liquid chromatography-tandem mass spectrometry, and their medical records were investigated. The CKD group had considerably lower urinary D-amino acid concentrations and enantiomeric ratios than the control group. The total urinary D-amino acid contents significantly correlated with blood parameters (creatinine and urea nitrogen). These findings may contribute towards the detection of CKD stage II in domestic cats.

Optical Relay Sensing of Cryptochiral Alcohols Displaying α-, β-, γ- and δ-Stereocenters or Chirality by Virtue of Isotopic Substitution.

A reaction-based optical relay sensing strategy that enables accurate determination of the concentration and enantiomeric ratio (er) of challenging chiral alcohols exhibiting stereocenters at the α-, β-, γ- or even δ-position or hard-to-detect cryptochirality arising from H/D substitution is described. This unmatched application scope is achieved with a conceptually new sensing approach by which the alcohol moiety is replaced with an optimized achiral sulfonamide chromophore to minimize the distance between the covalently attached chiroptical reporter unit and the stereogenic center in the substrate. The result is a remarkably strong, red-shifted CD induction that increases linearly with the sample er. The CD sensing part of the tandem assay is seamlessly coupled to a redox reaction with a quinone molecule to generate a characteristic UV response that is independent of the enantiopurity of the alcohol and thus allows determination of the total analyte concentration. The robustness and utility of the CD/UV relay are further verified by chromatography-free asymmetric reaction analysis with small aliquots of crude product mixtures, paving the way toward high-throughput chiral compound screening workflows which is a highly sought-after goal in the pharmaceutical industry.

Optical Relay Sensing of Cryptochiral Alcohols Displaying α‐, β‐, γ‐ and δ‐Stereocenters or Chirality by Virtue of Isotopic Substitution

AbstractA reaction‐based optical relay sensing strategy that enables accurate determination of the concentration and enantiomeric ratio (er) of challenging chiral alcohols exhibiting stereocenters at the α‐, β‐, γ‐ or even δ‐position or hard‐to‐detect cryptochirality arising from H/D substitution is described. This unmatched application scope is achieved with a conceptually new sensing approach by which the alcohol moiety is replaced with an optimized achiral sulfonamide chromophore to minimize the distance between the covalently attached chiroptical reporter unit and the stereogenic center in the substrate. The result is a remarkably strong, red‐shifted CD induction that increases linearly with the sample er. The CD sensing part of the tandem assay is seamlessly coupled to a redox reaction with a quinone molecule to generate a characteristic UV response that is independent of the enantiopurity of the alcohol and thus allows determination of the total analyte concentration. The robustness and utility of the CD/UV relay are further verified by chromatography‐free asymmetric reaction analysis with small aliquots of crude product mixtures, paving the way toward high‐throughput chiral compound screening workflows which is a highly sought‐after goal in the pharmaceutical industry.

An electrochemical 3D platform using poly (alizarin red S) and magnetite nanoparticles for rapid recognition and determination of tryptophan enantiomers in whole blood samples

The construction of a simple and convenient electrochemical platform for the recognition of enantiomers is crucial for the life sciences and medical fields. Herein, a portable electrochemical 3D platform based on a carbon paste electrode modified (CPE) with poly (alizarin red S) (PARS) and magnetite nanoparticles (Fe3O4NPs) for the recognition and determination of tryptophan enantiomers from whole blood samples was developed. Several methods, including scanning electron microscopy (SEM), differential pulse voltammetry (DPV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), were used to evaluate the morphologies and electrochemical behaviors of the modified electrode. The synthesized magnetite nanoparticles are studied using SEM with energy-dispersive X-ray analysis (EDAX). Under the optimum experimental conditions, the current intensity ratio of d-Trp to l-Trp (ID/IL) was found to be 1.32 at PARS/Fe3O4NPs/CPE, according to the results of differential pulse voltammetry. Furthermore, the developed platform showed a good linear correlation between response current intensity and tryptophan enantiomer concentration, ranging from 0.001 to 0.2 mM. The detection limits for both d-Trp and l-Trp were found to be 0.3 µM. This study demonstrates that the designed platform is capable of rapidly detecting the enantiomeric ratio in a non-racemic tryptophan mixed solution. Finally, the proposed platform was used to identify the Trp enantiomers in human whole blood, confirming the effectiveness of the modified electrode in analyzing real samples. The electrochemical 3D platform is thus used not only to recognize tryptophan enantiomers but also exhibits a significant promise for practical uses.

Rh-Catalyzed Enantioselective Single-Carbon Insertion of Alkenes.

The interest in the discovery and development of skeletal editing processes that selectively insert, exchange, or delete an atom in organic molecules has significantly increased over the last few years. However, processes of this class that proceed through the creation of a chiral center with high asymmetric induction have been largely unexplored. Herein, we report an enantioselective single-carbon insertion in aryl- and alkyl-substituted alkenes mediated by a catalytically generated chiral Rh-carbynoid and phosphate nucleophiles that produce enantioenriched allylic phosphates (enantiomeric ratio (e.r.) = 89.5:10.5-99.5:0.5). The key to the process was a diastereo- and enantioselective cyclopropanation of the alkene with a chiral Rh-carbynoid and the formation of a transient cyclopropyl-I(III) intermediate. The addition of the phosphate nucleophile provided a cyclopropyl-I(III)-phosphate intermediate that undergoes disrotatory ring opening following the Woodward-Hoffmann-DePuy rules. This process led to a chiral intimate allyl cation-phosphate pair that evolved with excellent enantioretention. The evidence of an SN1-like SNi mechanism is provided by linear free-energy relationship studies, kinetic isotope effects, X-ray crystallography, and control experiments. We demonstrated the utility of the enantioenriched allylic phosphates in late-stage N-H allylations of natural products and drug molecules and in cross-coupling reactions that occurred with excellent enantiospecificity.

Chemoenzymatic One‐Pot Cascade for the Construction of Asymmetric C−C and C−P Bonds via Formal C−H Activation

AbstractThe integration of organocatalysis and enzyme catalysis in one‐pot cascade processes allows for the efficient construction of complex molecular architectures with high levels of stereocontrol. However, challenges related to reaction compatibility between both processes are often a limitation for the development of efficient synthetic routes. In this study, we describe the combination of an enzymatic aerobic oxidation followed by the squaramide‐mediated asymmetric formation of C−P and C−C bonds to access important building blocks such as chiral α‐hydroxyphosphonates and β‐nitro alcohols in good yields and enantiomeric ratios. This sequential process is conducted in a one‐pot fashion within a biphasic system and represents a pioneering example of a chemoenzymatic cascade involving aerobic biooxidation and an organocatalytic step operating under hydrogen‐bond activation mode and under mild reaction conditions.

Enantioselective Synthesis of Dihydrobenzofurans, Dihydrobenzosulfones, and Dihydroindoles by Merging One-pot Intramolecular Heck-Matsuda Reactions from Anilines with Redox-Relay Process.

A one-pot tandem process was developed aiming at the concise and expeditious enantioselective synthesis of dihydrobenzofuran, dihydrobenzosulfone, and dihydroindole scaffolds under mild, and open-flask conditions. This process combines the in situ generation of aryldiazonium salt directly from the anilines in methanol telescoped to an intramolecular Heck-Matsuda reaction linked to a redox relay process to provide the final products as the dimethyl acetals. These Heck products were smoothly converted into the corresponding primary alcohols or esters. The robustness and the efficiency of the protocol are demonstrated by the synthesis of 24 enantioenriched dihydrobenzofurans, dihydrobenzosulfones, and dihydroindoles in overall yields up to 78 % in enantiomeric ratios up to 99 : 1 by a sequential 5-step protocol.

Atroposelective Synthesis of Axial Biaryls by Dynamic Kinetic Resolution Using Engineered Imine Reductases.

Axially chiral biaryl compounds are ubiquitous scaffolds in natural products, bioactive molecules, chiral ligands and catalysts, but biocatalytic methods for their asymmetric synthesis are limited. Herein, we report a highly efficient biocatalytic route for the atroposelective synthesis of biaryls by dynamic kinetic resolution (DKR). This DKR approach features a transient six-membered aza-acetal-bridge-promoted racemization followed by an imine reductase (IRED)-catalyzed stereoselective reduction to construct the axial chirality under ambient conditions. Directed evolution of an IRED from Streptomyces sp. GF3546 provided a variant (S-IRED-Ss-M11) capable of catalyzing the DKR process to access a variety of biaryl aminoalcohols in high yields and excellent enantioselectivities (up to 98 % yield and >99 : 1 enantiomeric ratio). Molecular dynamics simulation studies on the S-IRED-Ss-M11 variant revealed the origin of its improved activity and atroposelectivity. By exploiting the substrate promiscuity of IREDs and the power of directed evolution, our work further extends the biocatalysts' toolbox to construct challenging axially chiral molecules.

Rh(II)-Catalyzed Enantioselective S-Alkylation of Sulfenamides with Acceptor-Acceptor Diazo Compounds Enables the Synthesis of Sulfoximines Displaying Diverse Functionality.

The Rh(II)-catalyzed enantioselective S-alkylation of sulfenamides with α-amide diazoacetates at 1 mol % catalyst loading to obtain sulfilimines in high yields and enantiomeric ratios of up to 99:1 is reported. The enantioenriched sulfilimine products incorporate versatile amide functionality poised for further elaboration to diverse sulfoximines with multiple stereogenic centers, including by highly diastereoselective sulfilimine and sulfoximine α-alkylation with alkylating agents and epoxides and by interconversion of the amide to N-tert-butanesulfinyl aldimines, followed by diastereoselective additions.

A Merger of Relay Catalysis with Dynamic Kinetic Resolution Enables Enantioselective β-C(sp3)-H Arylation of Alcohols.

The conceptual merger of relay catalysis with dynamic kinetic resolution strategy is reported to enable regio- and enantioselective C(sp3)-H bond arylation of aliphatic alcohols, forming enantioenriched β-aryl alcohols typically with >90 : 10 enantiomeric ratios (up to 98 : 2 er) and 36-74 % yields. The starting materials bearing neighbouring stereogenic centres can be converted to either diastereomer of the β-aryl alcohol products, with >85 : 15 diastereomeric ratios determined by the catalysts. The reactions occur under mild conditions, ensuring broad compatibility, and involve readily available aryl bromides, an inorganic base, and commercial Ru- and Pd-complexes. Mechanistic experiments support the envisioned mechanism of the transformation occurring through a network of regio- and stereoselective processes operated by a coherent Ru/Pd-dual catalytic system.

A Merger of Relay Catalysis with Dynamic Kinetic Resolution Enables Enantioselective β‐C(sp3)−H Arylation of Alcohols

AbstractThe conceptual merger of relay catalysis with dynamic kinetic resolution strategy is reported to enable regio‐ and enantioselective C(sp3)‐H bond arylation of aliphatic alcohols, forming enantioenriched β‐aryl alcohols typically with &gt;90 : 10 enantiomeric ratios (up to 98 : 2 er) and 36–74 % yields. The starting materials bearing neighbouring stereogenic centres can be converted to either diastereomer of the β‐aryl alcohol products, with &gt;85 : 15 diastereomeric ratios determined by the catalysts. The reactions occur under mild conditions, ensuring broad compatibility, and involve readily available aryl bromides, an inorganic base, and commercial Ru‐ and Pd‐complexes. Mechanistic experiments support the envisioned mechanism of the transformation occurring through a network of regio‐ and stereoselective processes operated by a coherent Ru/Pd‐dual catalytic system.

Spectroscopic Study of a Novel Binaphthyl Amine Fluorescent Probe for Chiral Recognition of D/L-Lysine.

Lysine plays a crucial role in promoting development, enhancing immune function, and improving the function of central nervous system tissues. The two configurational isomers of amino acids have significantly different effects. Currently, methods for chiral recognition of lysine have been reported; however, previous detection methods have drawbacks such as expensive equipment and complicated detection processes. Fluorescence analysis, on the other hand, boasts high sensitivity, strong selectivity, and simple operation. In this study, we synthesized four novel Binaphthyl-Amine (BINAM)-based fluorescent probes capable of specifically identifying the L-configuration of lysine among the twenty amino acids that constitute human proteins. The enantiomeric fluorescence enhancement ratio (ef or ΔIL/ΔID) reached up to 15.29, demonstrating high enantioselectivity. In addition, we assessed the probe's recognition capabilities under varying pH levels, reaction times, and metal ion conditions, along with its limit of detection (LOD) and quantum yield. Our results suggest that this probe serves as a highly stable tool for the detection of chiral lysine.

From grape to wine: A thorough compound specific isotopic, enantiomeric and quali-quantitative investigation by means of gas chromatographic analysis

In this study, multiple analytical approaches, including simultaneous enantiomeric and isotopic analysis, were employed to thoroughly investigate the volatile fraction in Moscato giallo grape berries and wines. For the qualitative and quantitative profiling, a fast GC-QqQ/MS approach was successfully utilized. However, prior to isotopic analysis, the extracts underwent an additional concentration step, necessitating an assessment of isotopic fractionation during the concentration process. Once the absence of carbon isotopic fractionation was confirmed, this research aimed to develop a suitable gas chromatographic method for the simultaneous detection of both enantiomeric and isotopic ratios of target monoterpenoids in Moscato giallo samples. To address the limitations associated with a one-dimensional approach, multidimensional gas chromatography was employed to enhance separation before IRMS and qMS detections. Utilizing a Deans switch transfer device, the coupling of an apolar column in the first dimension and a chiral cyclodextrin-based stationary phase in the second dimension proved effective for this purpose. The data obtained from the analysis of Moscato giallo samples allowed for the assessment of natural isotopic and enantiomeric distributions in grapes and wines for the first time in the literature. Significant enantiomeric excesses were observed for the target terpenoids investigated. Regarding isotopic distribution, a consistent trend was observed for all detected target terpenols, including the linalool enantiomers. To date, this study represents the first investigation of simultaneous δ13C and chiral investigation of the main terpenoids in oenological products in the literature.

Carbon Dots in Enantioselective Sensing.

Chirality has a crucial effect on clinical, chemical and biological research since most bioactive compounds are chiral in the natural world. It is thus important to evaluate the enantiomeric ratio (or the enantiopurity) of the selected chiral analytes. To this purpose, fluorescence and electrochemical sensors, in which a chiral modifier is present, are reported in the literature. In this review, fluorescence and electrochemical sensors for enantiorecognition, in which chiral carbon dots (CDs) are used, are reported. Chiral CDs are a novel zero-dimensional carbon-based nanomaterial with a graphitic or amorphous carbon core and a chiral surface. They are nanoparticles with a high surface-to-volume ratio and good conductivity. Moreover, they have the advantages of good biocompatibility, multi-color emission, good conductivity and easy surface functionalization. Their exploitation in enantioselective sensing is the object of this review, in which several examples of fluorescent and electrochemical sensors, containing chiral CDs, are analyzed and discussed. A brief introduction to the most common synthetic procedures of chiral CDs is also reported, evidencing strengths and weaknesses. Finally, consideration concerning the potential challenges and future opportunities for the application of chiral CDs to the enantioselective sensing world are outlined.

Fe/Thiol Cooperative Hydrogen Atom Transfer Olefin Hydrogenation: Mechanistic Insights That Inform Enantioselective Catalysis.

Asymmetric hydrogenation of activated olefins using transition metal catalysis is a powerful tool for the synthesis of complex molecules, but traditional metal catalysts have difficulty with enantioselective reduction of electron-neutral, electron-rich, and minimally functionalized olefins. Hydrogenation based on radical, metal-catalyzed hydrogen atom transfer (mHAT) mechanisms offers an outstanding opportunity to overcome these difficulties, enabling the mild reduction of these challenging olefins with selectivity that is complementary to traditional hydrogenations with H2. Further, mHAT presents an opportunity for asymmetric induction through cooperative hydrogen atom transfer (cHAT) using chiral thiols. Here, we report insights from a mechanistic study of an iron-catalyzed achiral cHAT reaction and leverage these insights to deliver stereocontrol from chiral thiols. Kinetic analysis and variation of silane structure point to the transfer of hydride from silane to iron as the likely rate-limiting step. The data indicate that the selectivity-determining step is quenching of the alkyl radical by thiol, which becomes a more potent H atom donor when coordinated to iron(II). The resulting iron(III)-thiolate complex is in equilibrium with other iron species, including FeII(acac)2, which is shown to be the predominant off-cycle species. The enantiodetermining nature of the thiol trapping step enables enantioselective net hydrogenation of olefins through cHAT using a commercially available glucose-derived thiol catalyst with up to 80:20 enantiomeric ratio. To the best of our knowledge, this is the first demonstration of asymmetric hydrogenation via iron-catalyzed mHAT. These findings advance our understanding of cooperative radical catalysis and act as a proof of principle for the development of enantioselective iron-catalyzed mHAT reactions.