Determination Of Absolute Configuration Research Articles

Enantioselective synthesis and absolute configuration determination of hydroxywilfordic acid in sesquiterpene pyridine alkaloids.

An enantioselective synthetic route to hydroxywilfordic acid, a key subunit of sesquiterpene pyridine alkaloids such as wilfortrine, was developed. Asymmetric cyanation using Jacobsen's (R,R)-amino-thiourea and hydrolysis were performed to afford chiral α-hydroxy-α-methyl acid as the (S)-isomer. Naturally derived hydroxywilfordate prepared by methanolysis of wilfortrine was found to be the (R)-isomer upon comparison with the synthetic compound.

Definition of Right-handed Quartz and Determination of Absolute Configuration

Definition of Right-handed Quartz and Determination of Absolute Configuration

Pyrene-based bisboronic sensors for multichannel enantioselective recognition of tartaric acid

New chiral bisboronic receptors based on pyrene-excimer were synthesized and applied for the determination of absolute configuration and enantiomeric composition of tartaric acid. The distinction was visualized by portable UV lamp in certain concentration range. There were three ways for the sensors to enantioselectively recognize tartaric acid, including the fluorescence spectrum, UV–vis spectrum, and CD spectrum. The enantiomeric excess of tartaric acid was measured quantitatively by using fluorescence analytical technique. Compared with the previous chiral boronic acid sensors, sensor 1 showed weaker background fluorescence and excellent chiral recognition ability of D/l-tartaric acid throughout the whole pH range.

Highly Sensitive, Simple, and Cost- and Time-Effective Method to Determine the Absolute Configuration of a Secondary Alcohol Using Competing Enantioselective Acylation Coupled with LC/MS.

The absolute-configuration determination of natural products and synthetic compounds with stereogenic centers is very important because stereoisomers dramatically and differentially affect many crucial properties, such as physical behaviors and biological functions. Despite several established methods for determining the absolute configuration, significant unmet needs for new methods still exist owing to the specific limitations of established methodologies. Here, we present a simple, optimized, new chemical-derivative method that utilizes competing enantioselective acylation followed by LC/MS analysis, and we demonstrate its successful application in determining the absolute configuration of a secondary alcohol in natural products with multiple reactive functional groups. This new development relies on the enantiomeric pair of homobenzotetramisole (HBTM) catalysts exhibiting adequate kinetic resolution for acylation of the secondary alcohol, and then the fast reaction was quantitatively confirmed via LC/MS as the characterization technique for the enantioselective transformations. Our new approach was successfully applied to determine the absolute configuration of one secondary alcohol in compound 1, which has other hydroxyl groups to be reacted. The identified stereocenter of 1 was verified by previously established methods including quantum chemical electronic-circular-dichroism (ECD) calculations, computational NMR-chemical-shift calculations followed by DP4+ calculations, and modified Mosher's method. In addition, our method was applied to five known naturally occurring compounds, which led to the successful verification of their absolute configurations. Our newly developed method using the HBTM catalyst provides a highly sensitive, simple, and cost- and time-effective approach and an applicable and convenient analytical method for determining the absolute configuration of one secondary alcohol in natural products.

Multi-milligram resolution and determination of absolute configuration of pentedrone and methylone enantiomers

The enantioresolution of pentedrone and methylone was carried out at a multi-milligram scale by liquid chromatography on a Chiralpak AS® stationary phase. The excellent enantioresolution using this column allowed to collect highly pure enantiomeric fractions, achieving enantiomeric ratios higher than 98%. An overall recovery of 72% was achieved for pentedrone enantiomers and 80% for methylone. Furthermore, the absolute configuration of the enantiomers of both cathinones was determined for the first time by electronic circular dichroism (ECD) spectroscopy, with the aid of theoretical calculations, as (+)‑(S) and (−)‑(R)-pentedrone, and (−)‑(S) and (+)‑(R)‑methylone.

Diastereoselective synthesis of polyfluoroalkylated α-aminophosphonic acid derivatives

The asymmetric synthesis of biorelevant aminophosphonates and aminophosphonic acids bearing trifluoromethyl, tetrafluoroethyl, perfluoropropyl or chlorodifluoromethyl group in the α-position to phosphonyl group is developed. Diastereoselective reduction of α-(poly)fluoroalkylated iminophosphonates, bearing stereodirecting α-phenylethyl group at the nitrogen atom, with sodium borohydride produces diastereomeric N-(α-phenylethyl) polyfluoroalkylphosphonates, readily separated by column chromatography. The highly stereoselective reaction of (R)-N-tert-butylsulfinylimine of tetrafluoropropanal with generated in situ diethyltrimethysilylphosphite leads to (R,R)-N-(tert-butylsulfinyl)tetrafluopropylphosphonate. Sequential N- and O-deprotection affords enantiomerically pure polyfluoroalkylphosphonate and aminophosphonic acids. The presence of α-phenylethyl group at the nitrogen atom allows easy determination of absolute configuration of the newly formed stereogenic center by the 31P,19F NMR method.

Lepadins I-K, 3- O-(3'-Methylthio)acryloyloxy-decahydroquinoline Esters from a Bahamian Ascidian Didemnum sp. Assignment of Absolute Stereostructures.

Three decahydroisoquinoline alkaloids, lepadins I-K, were isolated from a specimen of Didemnum sp. collected in the Bahamas. The structures of the new compounds were assigned by an integrated analysis of MS, IR, and 1H, 13C, and 2D NMR spectra. Like previously reported lepadins, the structures of the new compounds contain a decahydroquinoline heterocyclic core in lepadin I, and a new variation, an octahydroquinoline in lepadin J, but differ from earlier reported compounds by acylation of the 3-hydroxyl group by a rare 3'-methylthioacrylate. The absolute configuration of lepadin I was solved by interpretation of NOE measurements, and exciton coupled circular dichroism (ECCD) of the corresponding N- p-bromobenzoyl derivative. The latter constitutes a general method for determination of absolute configuration of the entire lepadin family. The configuration of the remote side-chain secondary carbinol was solved by the modified Mosher's esters method. Lepadin I inhibited butyrylcholineesterase (BuChE, IC50 3.1 μM), but only weakly inhibited acetylcholineesterase (AChE) (10% at 100 μM).

Diagnostic absolute configuration determination of clavulanate potassium: A comprehensive investigation of chiroptical spectroscopies and theoretical calculations

Clavulanate potassium (CP) is a classic β-lactamase inhibitor, which is extensively used as a combination drug in antibiotics therapy. As a chiral drug, CP is the only effective stereoisomer revealing prominent activities. To determine the chirality of CP efficiently, we provide an accurate and convenient method based on the comprehensive investigation of circular dichroism spectra and theoretical calculations. The results demonstrate two distinguishable cotton effects in electronic circular dichroism (ECD) and a sequence of cotton effect signs at specific wavenumbers in vibrational circular dichroism (VCD) can be regarded as diagnostic characteristics for the determination of absolute configuration. Additionally, the corresponding spectroscopic appearance of CP was elucidated with the help of theoretical analysis of molecular orbitals, natural bond orbitals and molecular electrostatic potential. The explicit absolute configuration determination of CP facilitates the thorough understanding in the stereochemistry of clavulanate potassium and its derivatives.

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time.

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

Determination of the Absolute Configuration of (S)‐N‐(1‐Aryl‐allyl)‐3,5‐dinitrobenzamides and Their Elution Order on Brush‐Type Chiral Stationary Phases

A series of ten enantiomerically pure (S)‐N‐(1‐aryl‐allyl)‐3,5‐dinitrobenzamides (S‐DNBs) was prepared using enzymatic resolution and chiral chromatography. Enzymatic resolution of corresponding 1‐aryl‐allylamines using Candida antarctica lipase B (CaLB) was efficient for amines with no steric hindrance near the stereogenic center and S‐DNB amides were prepared by acylation of the obtained S‐amine. When steric effects interrupted enzymatic resolution, racemic DNB amides were resolved using a brush‐type chiral column (CSP‐A) developed in our laboratory. Previously reported behavior of CaLB in kinetic resolution of amines was considered a starting point for the determination of absolute configuration (AC). The AC of prepared S‐DNB amides was anticipated using the elution order of prepared DNB amides on CSP‐A and commercial Whelk‐O1 columns and comparison with DNB amides obtained after acylation of (S)‐amines. The comparison between experimental electronic circular dichroism (ECD) spectra with those obtained by conformational analysis and ECD calculations of representative compounds allowed us to verify the AC of prepared DNB amides.

Configurational Study of an Aporphine Alkaloid from Annona purpurea

Purpureine (1), norpurpureine (2), and 3-hydroxyglaucine (4) were isolated from the leaves of Annona purpurea. A vibrational circular dichroism study for the absolute configuration determination of 1 provides evidence for the mutually dependent atropisomerism, local chirality of the sole stereogenic center, and the geometry of the tetrahedral nitrogen atom in aporphine alkaloids. The observed change in the optical rotation sign between 2 and its hydrochloride 3 might explain why this compound has been reported as dextrorotatory and levorotatory from the same botanical source. Furthermore, 1H and 13C NMR spectra of 1, 2 and 4 were fully assigned for the first time.

Assigning the absolute configuration of single aliphatic molecules by visual inspection

Deciphering absolute configuration of a single molecule by direct visual inspection is the next step in compound identification, with far-reaching implications for medicinal chemistry, pharmacology, and natural product synthesis. We demonstrate the feasibility of this approach utilizing low temperature atomic force microscopy (AFM) with a CO-functionalized tip to determine the absolute configuration and orientation of a single, adsorbed [123]tetramantane molecule, the smallest chiral diamondoid. We differentiate between single enantiomers on Cu(111) by direct visual inspection, and furthermore identify molecular dimers and molecular clusters. The experimental results are confirmed by a computational study that allowed quantification of the corresponding intermolecular interactions. The unique toolset of absolute configuration determination combined with AFM tip manipulation opens a route for studying molecular nucleation, including chirality-driven assembly or reaction mechanisms.

Enantioselective multiple heartcut two-dimensional ultra-high-performance liquid chromatography method with a Coreshell chiral stationary phase in the second dimension for analysis of all proteinogenic amino acids in a single run

A multiple heartcut (MHC) 2D-UHPLC method with UV detection has been developed for the enantioselective analysis of complex amino acid mixtures in a single run. The MHC method is based on an achiral gradient RPLC separation with 1.8 μm C18 phase (100 × 2.1 mm ID column) in the first dimension (1D) and enantioselective isocratic separation on a tert-butylcarbamoylquinine-based 2.7 μm Coreshell particle column (50 × 3 mm ID) in the second dimension (2D). Pre-column derivatization has been performed with Sanger’s reagent (2,4-dinitrofluorobenzene) yielding chromogenic 2,4-dinitrophenylated amino acids (DNP-AAs). Heartcuts of 40 μL fractions of the 1D peaks were sampled into the 2D system via a two-position four-port dual valve connected to two loop decks each equipped with six 40 μL parking loops. Using this setup, 25 amino acids (20 proteinogenic plus allo-Thr, allo-Ile, homoserine (Hse), Orn, β-Ala) have been analyzed enantioselectively in a fully automated manner with a single chiral column within 130 min total run time (1D and 2D). All 2D separations together took 101.5 min (29 cuts with 3.5 min run time each) and thus the total analysis time was quite efficiently utilized. Faster separations were restricted by some software constraints which did not allow to adjust run times in 2D individually. The practical utility of this enantioselective MHC method is documented by application for the absolute configuration determination of the amino acids in gramicidin and bacitracin. Further optimizations should lead to a generic enantioselective amino acid analyzer for the quality control of synthetic peptides and the structural characterization of non-ribosomal peptides.

Chiral resolution, absolute configuration determination, and stereo-activity relationship study of IDO1 inhibitor NLG919

NLG919 (1) with two chiral carbon atoms on its chemical structure is a potent indoleamine 2,3-dioxygenase 1 (IDO1) inhibitor. We developed an effective way to prepare all stereoisomers of 1, the key step being the chiral resolution of racemic intermediate 2. The optimal resolution solvent system was identified as dichloromethane and n-pentane or petroleum ether. Using (−)-di-p-toluoyl-d-tartaric acid as resolution reagent, optical pure (R)-2 (e.e. > 99%, yield = 70%) was obtained. The mechanism of chiral resolution was clarified through single-crystal X-ray diffraction of the diastereomeric salt. The absolute configurations of four stereoisomers of 1 were established through electronic circular dichroism spectra, quantum chemical calculation and transition metal method. Their IDO1 inhibitory activity was assessed by pharmacological experiments in vitro and in mouse, demonstrating that S configuration of C5 played an important role on the inhibition of IDO1, while the stereochemistry on C2′ exerted little effect on the IDO1 inhibitory activity in mouse.

Determination of absolute configuration and binding efficacy of benzimidazole-based FabI inhibitors through the support of electronic circular dichroism and MM-GBSA techniques

We have previously reported benzimidazole-based compounds to be potent inhibitors of FabI for Francisella tularensis (FtFabI), making them promising antimicrobial hits. Optically active enantiomers exhibit markedly differing affinities toward FtFabI. The IC50 of benzimidazole (−)-1 is ∼100× lower than the (+)-enantiomer, with similar results for the 2 enantiomers. Determining the absolute configuration for these optical compounds and elucidating their binding modes is important for further design. Electronic circular dichroism (ECD) quantum calculations have become important in determining absolute configurations of optical compounds. We determined the absolute configuration of (−)/(+)-1 and (−)/(+)-2 by comparing experimental spectra and theoretical density functional theory (DFT) simulations of ECD spectra at the B3LYP/6-311+G(2d, p) level using Gaussian09. Comparison of experimental and calculated ECD spectra indicates that the S configuration corresponds to the (−)-rotation for both compounds 1 and 2, while the R configuration corresponds to the (+)-rotation. Further, molecular dynamics simulations and MM-GBSA binding energy calculations for these two pairs of enantiomers with FtFabI show much tighter binding MM-GBSA free energies for S-1 and S-2 than for their enantiomers, R-1 and R-2, consistent with the S configuration being the more active one, and with the ECD determination of the S configuration corresponding to (−) and the R configuration corresponding to (+). Thus, our computational studies allow us to assign (−) to (S)- and (+) to (R)- for compounds 1 and 2, and to further evaluate structural changes to improve efficacy.

Universality of Ethane-Bridged Bis-Porphyrin Structural Motiff for Chirality and Molecular Sensing

Covalently linked porphyrins are of particular interest for the scientific community as efficient artificial systems widely used in various practical applications in the numerous research and technoligical fields such as polymer and nano science, light harvesting and nonlinear optics, different sensors and catalysis, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination. Recently, we have found that a simple ethane-bridged bis-porphyrin structural motif, 1 (M = Zn, Mg, Cu, Co, 2H), can be effectively served for various application purposes owing to the specific molecular and supramolecular functionalities. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of covalent linkage between two porphyrin units resulting in the environmentally assisted syn-anti conformational switching and tweezer formation depending upon the guest structure, which can be effectively employed in different sensing areas.1-11 For example, in the case of monodentate chiral guests there is the unidirectional screw formation in the anti form of 1 (M = Zn, Mg, 2H) upon the corresponding host-guest interaction. The direction of screw governs by the size difference of ligand’s substituents at the chirogenic center resulting in chirality transfer to the bis-porphyrin host and induction of the noticeable exciton couplet CD signals in the porphyrin absorption region.1-6 This chirogenic phenomenon was efficiently utilized for determining the absolute configuration of various chiral guests, thus making it possible to apply 1 as effective and universal chirality sensors for different types of organic molecules as in solution and in solid state. To expand further the applicability of these supramolecular systems Langmuir–Schaefer (LS) thin-films on the basis of 1 have been prepared and employed as highly selective and sensitive sensors for aromatic amines.7-9 For example, the monometallated Cu complex of 1 is able to detect the presence of aniline in aqueous solution at a concentration as low as 1 nM.9 The sensory mechanism includes the corresponding host-guest interaction resulting in the syn-to-anti conformational switching, which can be easily detected by various spectroscopic methods. Another sensing application of the LS film of monometallated Co complex of 1was found in the field of detection of volatile organic compounds like alcohol vapors and oxidizing gas as NO2 by using magneto-optical surface plazmon resonance spectroscopy.10 The detection mechanism is based upon the molecular recognition correlating with the alcohol molecular structure. More simple sensing system was developed on the basis of spin-coated thin films of the monometallated Zn complex of 1.11 The obtained thin films were successfully employed to detect ammonia and acetone as the relevant analytes in aqueous solutions. This was a result of the supramolecular interaction between each of these two compounds and the bis-porphyrin active layer, inducing the corresponding tweezer conformation easily detectable by UV–vis absorption spectroscopy in the range of concentrations between 1 ppm and 20 ppm for both the analytes. Further developments and prospects toward new sensing systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed. Borovkov, Symmetry 2014, 6, 256.Borovkov, Symmetry 2010, 2, 184.Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189.A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1.V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.Giancane, V. Borovkov, Y. Inoue, L. Valli, J. Colloid Interface Sci. 2012, 385, 282.Giancane, V. Borovkov, Y. Inoue, S. Conoci, L. Valli, Soft Matter 2013, 9, 2302. Bettini, E. Maglie, R. Pagano, V. Borovkov, Y. Inoue, L. Valli, G. Giancane, Beilstein J. Nanotechnol. 2015, 6, 2154.Colombelli, M.G. Manera, V. Borovkov, G. Giancane, L. Valli, R. Rella, Sensor Actuat. B-Chem., 2017, 246, 1039.A. Buccolieri, D. Manno, A. Serra, A. Santino, M. Hasan, V. Borovkov, G. Giancane, Sensor Actuat. B-Chem., 2018, 257, 685.

Catalytic Aza-Wacker Annulation: Tuning Mechanism by the Activation Mode of Amide and Enantioselective Syntheses of Melinonine-E and Strychnoxanthine

An unprecedented N-substituent of the amide was found to be crucial for the successful annulation to establish 2-azabicyclo[3.3.1]nonane and other ring skeletons in good yield. The novel catalytic aza-Wacker annulation methodology was further illustrated in the concise syntheses and the absolute configuration determinations of melinonine-E and strychnoxanthine.

Amino acids as chiral derivatizing agents for antiproliferative substituted N-benzyl isoindolinones.

The absolute configurations of the diastereomers of novel amino acid ester derivatives of 2,3-substituted isoindolinones, which are known as apoptosis activators due to their ability to inhibit the MDM2-p53 PPI, were assigned using NMR and computational methods. Procedures for diastereomer separation and determining the absolute configuration were developed to perform the study. The high significance of N-benzyl fragment for the determination of the diastereomer absolute configuration by NMR methods was established; it is determined by a number of factors inherent in this fragment and the structural features of the studied substrates. Analysis of the individual isomer activity showed that the target inhibitory effect of S- and R-isoindolinone L-valinates differs by less than 20%. It can be explained by the presence of a flexible linker between the isoindolinone core and amino acid fragment, which provides the optimal arrangement of the molecule in the hydrophobic cavity of MDM2 for both isomers.

Enantiomeric Mixtures in Natural Product Chemistry: Separation and Absolute Configuration Assignment.

Chiral natural product molecules are generally assumed to be biosynthesized in an enantiomerically pure or enriched fashion. Nevertheless, a significant amount of racemates or enantiomerically enriched mixtures has been reported from natural sources. This number is estimated to be even larger since the enantiomeric purity of secondary metabolites is rarely checked in the natural product isolation pipeline. This latter fact may have drastic effects on the evaluation of the biological activity of chiral natural products. A second bottleneck is the determination of their absolute configurations. Despite the widespread use of optical rotation and electronic circular dichroism, most of the stereochemical assignments are based on empirical correlations with similar compounds reported in the literature. As an alternative, the combination of vibrational circular dichroism and quantum chemical calculations has emerged as a powerful and reliable tool for both conformational and configurational analysis of natural products, even for those lacking UV-Vis chromophores. In this review, we aim to provide the reader with a critical overview of the occurrence of enantiomeric mixtures of secondary metabolites in nature as well the best practices for their detection, enantioselective separation using liquid chromatography, and determination of absolute configuration by means of vibrational circular dichroism and density functional theory calculations.

Self-Assembled Binuclear Cu(II)-Histidine Complex for Absolute Configuration and Enantiomeric Excess Determination of Naproxen by Tandem Mass Spectrometry.

Naproxen is one of the most consumed nonsteroidal anti-inflammatory drugs and marketed as S-naproxen since R-naproxen is hepatotoxic. In this study, chiral recognition of naproxen has been investigated by tandem mass spectrometry (MS/MS). Among all diastereomeric complexes formed between naproxen and the examined chiral selectors, including cyclodextrins (α/β/γ-CD), modified phenylalanines ( N-acetyl-phenylalanine, N-t-butoxycarbonyl-phenylalanine, N-9-fluorenylmethyloxycarbonyl-phenylalanine), amino acids (Trp, Phe, Tyr, His), glucose, tartaric acid, and vancomycin, a novel binuclear metal bound diastereomeric complexes [(M(II))2( S/ R-naproxen)(l-His)2-3H]+ (M = Cu, Ni, or Co with Cu being the best) could allow effective identification of the absolute configuration of naproxen and determination of its enantiomeric excess ( ee) through MS/MS analysis. The key candidate structure of [(Cu(II))2( S/ R-naproxen)(l-His)2-3H]+ has been revealed by means of collision-induced dissociation, ion mobility mass spectrometry and density functional theory calculations, indicating an interesting and unusual self-assembled compact geometry with the two Cu(II) ions bridged closely together (Cu-Cu distance is 3.04 Å) by the carboxylate groups of the two histidines. It was shown that the difference in dissociation efficiency between the two diastereomers was attributed to the interaction between the NH2 bond of the amino group of one histidine and the naphthyl ring of naproxen. The present report is the first to observe and characterize the complex of (Cu(II))2(His)2 with aromatic acid, which could contribute to the chiral recognition of other chiral aromatic acids, design of catalysts based on binuclear copper bound complex, as well as the better understanding of metal ion complexation by His or His-containing ligands.