Racemic Mandelic Acid Research Articles

High-Enantioselectivity Adsorption Separation of Racemic Mandelic Acid and Methyl Mandelate by Robust Chiral UiO-68-Type Zr-MOFs.

Mandelic acid and its analogues are highly valuable medical intermediates and play an important role in the pharmaceutical industry, biochemistry, and life sciences. Therefore, effective enantioselective recognition and separation of mandelic acid are of great significance. In this study, two of our recently reported chiral amine-alcohol-functionalized UiO-68-type Zr-HMOFs 1 and 3 with high chemical stability, abundant binding sites, and large chiral pores were selected as chiral selectors for the enantioselective separation of mandelic acid (MA), methyl mandelate (MM), and other chiral molecules containing only one phenyl. Materials 1 and 3 exhibited excellent enantioselective separation performance for MA and MM. Especially for the separation of racemate MA, the enantiomeric excess values reached 97.3 and 98.9%, which are the highest reported values so far. Experimental and density functional theory (DFT) computational results demonstrated that the introduction of additional phenyls on the chiral amine alcohol pendants in 3 had somewhat impact on the enantioselective adsorption and separation of MA or MM compared with 1, but it was not significant. Further research on the enantioselective separation of those chiral adsorbates containing only one phenyl by material 1 indicated the crucial role of the groups directly bonded to the chiral carbons of the adsorbates in the selective separation of enantiomers, especially showing higher enantioselectivity for the adsorbates with two hydrogen-bonding groups directly bonded to its chiral carbon.

Highly active and recyclable immobilized multiple enzymes for one-pot enantioselective cascade reactions: Synthesis of (R)- and (S)-α-amino acids from racemic α-hydroxy acids

Cascade biotransformation of racemic alcohols to enantiopure amines is highly wanted in pharmaceutical manufacturing but requires green and efficient solution. Here we developed the first immobilized enzyme-based approach for this transformation, being cleaner and more productive than whole-cells biocatalysis, and more economic and higher-yielding than using isolated enzymes. As demonstration, the conversion of racemic mandelic acids to high-value (R)- or (S)-phenylglycines was achieved by immobilized enantioselective alcohol oxidase, racemase, catalase, and (S)- or (R)-transaminase. Co-immobilization of the four enzymes on Ni-NTA functionalised carriers gave high enzyme loadings (192–195 mg enzyme/g carrier), loading efficiencies (97–98%), and free enzyme activities (107–113%). Immobilized enzymes transformed racemic mandelic acids to the corresponding four (S)-phenylglycines in 91–99% ee and 82–95% yields or five (R)-phenylglycines in 93–99% ee and 83–94% yields. These catalysts showed good recyclability, retaining 86–98% productivities in the fifth reaction cycles.

Separation of chiral drugs through dual chiral ionic liquid functionalized composite membrane and study on chiral recognition mechanism

With the increase of demand for single enantiomer drugs, chiral separation methods have been rapidly developed. Benefiting from the characteristics of simple preparation, low equipment requirement, continuous operation and easy scale-up, membrane separation method has great potential in realizing large-scale separation of chiral drugs. In this study, by means of ion exchange, three different amino acids were combined with mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrin ([β-CD-BIM]) to synthesize three dual chiral ionic liquids: [β-CD-BIM][L-Cys], [β-CD-BIM][L-Asp] and [β-CD-BIM][L-Lys]. Then, they were introduced into cellulose acetate membrane by interfacial polymerization to fabricate chiral composite membrane for enantioseparation. In order to improve the separation effect of the composite membrane for chiral molecule, the filtration conditions were investigated and optimized. Under the optimal conditions, for the chiral composite membrane applying [β-CD-BIM][TsO], [β-CD-BIM][L-Cys], [β-CD-BIM][L-Asp] and [β-CD-BIM][L-Lys], the e.e.% of racemic mandelic acid was 33.9%, 47.9%, 31.3% and 60.4% respectively, and the e.e.% obtained for racemic naproxen was 5.5%, 10.3%, 5.7% and 8.7%, individually. Finally, the separation mechanism of different dissociation form for chiral molecule and the recognition mechanism of different chiral ionic liquids were studied by molecular simulation. These findings reveal the excellent enantioseparation performance of prepared chiral membrane, providing a new perspective on the design of chiral membranes.

Stereospecific formation of vanadium mandelato complexes with [Fe(2,2′-bipyridine)3]2+ as a counter ion

Two vanadium complexes of mandelic acid having [Fe(bpy)3]2+ as counterion, [Fe(bpy)3][V2O4(rac-mand)2]·4.9H2O·0.1CH3CN (1, FeV2L2) and (H3O)[Fe(bpy)3]4[V3O7(S-mand)2]3·28H2O (2, FeV3L2) (bpy = 2,2’-bipyridine, mand2– = mandelato ligand, C8H6O32–) have been synthesized and characterized by single crystal X-ray diffraction and spectral methods. The FeSO4—bpy—KVO3—H2mand—H2O—CH3CN system exhibits a stereospecific behaviour: while from the system including racemic mandelic acid only the complex of the V2L2 type (1) could be obtained in crystalline form, the system with S-mandelic acid afforded the V3L2 (2) complex as the single crystalline product. All vanadium atoms exhibit tetragonal pyramidal coordination geometry with oxygen donor atoms of the oxido ligands and carboxylate anion. The stereospecific behaviour was investigated using the 51 V NMR spectroscopy, which revealed different composition of systems with racemic mandelic acid and S-mandelic acid after some preliminary period (≈ 15 days). The compound 2 is chiral non-racemic compound (space group P21212), the structure of which contains Δ-[Fe(bpy)3]2+ cations and [V3O7(S-mand)2]3– anions.Graphical abstract

Engineering of Hydroxymandelate Oxidase and Cascade Reactions for High-Yielding Conversion of Racemic Mandelic Acids to Phenylglyoxylic Acids and (R)- and (S)-Phenylglycines

An alcohol oxidase (AOx) for the (S)-enantioselective oxidation of 4-hydroxymandelic acid (4-HMA) to 4-hydroxyphenylglyoxylic acid (4-HPGA) with enhanced activity was developed by directed evolution of hydroxymandelate oxidase (HMO) through three rounds of iterative saturation mutagenesis. The engineered HMO mutant A80G-T159S-T162Q (HMOTM) catalyzed the oxidation of (S)-4-HMA to 4-HPGA with a 23-fold enhancement in catalytic efficiency (kcat/KM). Substrate docking simulation on HMOTM suggested that A80G reoriented FMN, while T159S and T162Q formed hydrogen bonds with the carboxylic group of the substrate, thus facilitating substrate binding and catalysis. (S)-Enantioselective HMOTM was used together with mandelic acid racemase (MR) and catalase (KatE) to achieve high-yielding oxidation of rac-4-HMA to 4-HPGA with either purified enzymes (up to 93% yield and 426 mM) or Escherichia coli (HMC) cells expressing the three enzymes (up to 93% yield and 140 mM). The HMOTM-MR-KatE cascades were applied for the oxidation of seven other substituted MAs, producing the corresponding phenylglyoxylic acids with 90–99% conversions using purified enzymes or whole cells. Efficient conversion of racemic α-hydroxy acids to (S)- or (R)-α-amino acids was achieved by combining HMOTM-MR-KatE with (S)-enantioselective transaminase (EcαTA) or (R)-enantioselective transaminase (DpgAT), together with glutamate dehydrogenase (GluDH). Coupling of E. coli (HMC) with E. coli (E-G) expressing EcαTA and GluDH for one-pot biotransformation of five rac-MAs produced the corresponding (S)-phenylglycines (PGs) in 91–99% ee with 73–98% conversions. Using E. coli (HMC) and E. coli (D-G) expressing DpgAT and GluDH enabled the production of five (R)-PGs with 93–99% ee and 73–98% conversions from the corresponding rac-MAs. The engineered AOx, AOx-MR-KatE cascades, and AOx-MR-Kat-GluDH-(S)- or (R)-TA cascades provide useful tools for highly active and enantioselective oxidation of racemic hydroxyacids and high-yielding conversion of racemic hydroxyacids to ketoacids and enantiopure (S)- or (R)-aminoacids, respectively, which are challenging and useful chemical reactions.

Separation of mandelic acid enantiomers using solid-liquid biphasic systems with chiral ionic liquids

This work aims to take full advantage of ionic liquids’ (ILs) “designer solvent” nature in order to expand the applicability of solid-liquid biphasic systems (SLBS) as alternative chiral resolution techniques. To this purpose, twelve chiral ILs, bearing chirality on the cation or the anion, were used as chiral selectors in SLBS to selectively precipitate mandelic acid enantiomers. The precipitation studies were performed using aqueous solutions of the chiral ILs or their chiral precursors, where the impact of the chemical structure and chirality was investigated. The most efficient chiral ILs were employed to evaluate the influence of different operational conditions on the formed precipitate, namely resolution time, resolution speed, temperature, initial concentration of chiral IL, initial concentration of racemic mandelic acid and water content. When compared to their precursors, chiral ILs showed an enhanced ability to induce enantioselective precipitation. The composition of the precipitate formed, and the molecular-level mechanisms leading to enantioselective precipitation, were also uncovered. Due to cooperative interactions between the chiral IL and the target mandelic acid enantiomer, the [N4444][D-Phe]-based SLBS proposed allowed to obtain maximum enantiomeric excess of 51% in a single-step, without any further additives.

Direct enantioseparation of mandelic acid by high-performance liquid chromatography using a phenyl column precoated with a small amount of cyclodextrin additive in a mobile phase.

Direct enantioseparation of mandelic acid by high-performance liquid chromatography (HPLC) with a reversed phase column and a mobile phase containing a small amount of hydroxylpropyl-β-cyclodextrin (HP-β-CD) was studied as an efficient method for saving consumption of the CD additive. As a result, it was proposed that racemic mandelic acid can be analyzed with a phenyl column by using a mobile phase composed of 10 mM ammonium acetate buffer (pH 4.2) and 0.02% (w/v) HP-β-CD at a flow rate of 1.0 mL/min at 40°C after the passage of 10 mM ammonium acetate buffer (pH 4.2) containing 0.1% (w/v) HP-β-CD as a precoating mobile phase for 60 min. It is suggested that HP-β-CD is bound with a phenyl group on the surface of the stationary phase to allow a phenyl column to act as a transient chiral column, and injected mandelic acid can form the ternary complex with the adsorbed HP-β-CD. The longer retention time of D-mandelic acid than the L-isomer for HPLC can be explained from the higher stability of the HP-β-CD complex with D-mandelic acid, which was confirmed by CE experiment with HP-β-CD as a selector. The efficiency of a phenyl column compared with other stationary phases was also discussed.

High level and enantioselective production of L-phenylglycine from racemic mandelic acid by engineered Escherichia coli using response surface methodology

L-Phenylglycine (L-PHG) is a member of unnatural amino acids, and becoming more and more important as intermediate for pharmaceuticals, food additives and agrochemicals. However, the existing synthetic methods for L-PHG mainly rely on toxic cyanide chemistry and multistep processes. To provide green, safe and high enantioselective alternatives, we envisaged cascade biocatalysis for the one-pot synthesis of L-PHG from racemic mandelic acid. A engineered E. coli strain was established to co-express mandelate racemase, D-mandelate dehydrogenase and L-leucine dehydrogenase and catalyze a 3-step reaction in one pot, enantioselectively transforming racemic mandelic acid to give L-PHG (e.e. >99 %). After the conditions for biosynthesis of L-PHG optimized by response surface methodology, the yield and space-time yield of L-PHG can reach 87.89 % and 79.70 g·L−1·d−1, which was obviously improved. The high-yielding and enantioselective synthetic methods use cheap and green reagents, and E. coli whole-cell catalysts, thus providing green and useful alternative methods for manufacturing L-PHG.

One-Pot Synthesis of Phenylglyoxylic Acid from Racemic Mandelic Acids via Cascade Biocatalysis.

Phenylglyoxylic acid (PGA) are key building blocks and widely used to synthesize pharmaceutical intermediates or food additives. However, the existing synthetic methods for PGA generally involve toxic cyanide and complex processes. To explore an alternative method for PGA biosynthesis, we envisaged cascade biocatalysis for the one-pot synthesis of PGA from racemic mandelic acid. A novel mandelate racemase named ArMR showing higher expression level (216.9 U·mL-1 fermentation liquor) was cloned from Agrobacterium radiobacter and identified, and six recombinant Escherichia coli strains were engineered to coexpress three enzymes of mandelate racemase, d-mandelate dehydrogenase and l-lactate dehydrogenase, and transform racemic mandelic acid to PGA. Among them, the recombinant E. coli TCD 04, engineered to coexpress three enzymes of ArMR, LhDMDH, and LhLDH, can transform racemic mandelic acid (100 mM) to PGA with 98% conversion. Taken together, we provide a green approach for one-pot biosynthesis of PGA from racemic mandelic acid.

Biosynthesis of Phenylglyoxylic Acid by LhDMDH, a Novel d-Mandelate Dehydrogenase with High Catalytic Activity.

d-Mandelate dehydrogenase (DMDH) has the potential to convert d-mandelic acid to phenylglyoxylic acid (PGA), which is a key building block in the field of chemical synthesis and is widely used to synthesize pharmaceutical intermediates or food additives. A novel NAD+-dependent d-mandelate dehydrogenase was cloned from Lactobacillus harbinensi (LhDMDH) by genome mining and expressed in Escherichia coli BL21. After being purified to homogeneity, the oxidation activity of LhDMDH toward d-mandelic acid was approximately 1200 U·mg-1, which was close to four times the activity of the probe. Meanwhile, the kcat/ Km value of LhDMDH was 28.80 S-1·mM-1, which was distinctly higher than the probe. By coculturing two E. coli strains expressing LhDMDH and LcLDH, we developed a system for the efficient synthesis of PGA, achieving a 60% theoretical yield and 99% purity without adding coenzyme or cosubstrate. Our data supports the implementation of a promising strategy for the chiral resolution of racemic mandelic acid and the biosynthesis of PGA.

Front Cover Picture: One‐Pot Enantioselective Synthesis of d‐Phenylglycines from Racemic Mandelic Acids, Styrenes, or Biobased l‐Phenylalanine via Cascade Biocatalysis (Adv. Synth. Catal. 24/2017)

Front Cover Picture: One‐Pot Enantioselective Synthesis of <scp>d</scp>‐Phenylglycines from Racemic Mandelic Acids, Styrenes, or Biobased <scp>l</scp>‐Phenylalanine <i>via</i> Cascade Biocatalysis (Adv. Synth. Catal. 24/2017)

One‐Pot Enantioselective Synthesis of d‐Phenylglycines from Racemic Mandelic Acids, Styrenes, or Biobased l‐Phenylalanine via Cascade Biocatalysis

AbstractEnantiopure d‐phenylglycine and its derivatives are an important group of chiral amino acids with broad applications in thepharmaceutical industry. However, the existing synthetic methods for d‐phenylglycine mainly rely on toxic cyanide chemistry and multistep processes. To provide green and safe alternatives, we envisaged cascade biocatalysis for the one‐pot synthesis of d‐phenylglycine from racemic mandelic acid, styrene, and biobased l‐phenylalanine, respectively. Recombinant Escherichia coli (LZ110) was engineered to coexpress four enzymes to catalyze a 3‐step reaction in one pot, transforming mandelic acid (210 mM) to give enantiopure d‐phenylglycine in 29.5 g L−1 (195 mM) with 93% conversion. Using the same whole‐cell catalyst, twelve other d‐phenylglycine derivatives were also produced from the corresponding mandelic acid derivatives in high conversion (58–94%) and very high ee (93–99%). E. coli (LZ116) expressing seven enzymes was constructed for the transformation of styrene to enantiopure d‐phenylglycine in 80% conversion via a one‐pot 6‐step cascade biotransformation. Twelve substituted d‐phenylglycines were also produced from the corresponding styrene derivatives in high conversion (45–90%) and very high ee (92–99%) via the same cascade reactions. A nine‐enzymeexpressing E. coli (LZ143) was engineered to transform biobased l‐phenylalanine to enantiopure d‐phenylglycine in 83% conversion via a one‐pot 8‐step transformation. Preparative biotransformations were also demonstrated. The high‐yielding synthetic methods use cheap and green reagents (ammonia, glucose, and/or oxygen), and E. coli whole‐cell catalysts, thus providing green and useful alternative methods for manufacturing d‐phenylglycine.magnified image

Benzyl isobornyl amines: a simple and practical class of NMR discriminating agents for effective chiral recognition of acids

The design and synthesis of a few simple N-benzyl derivatives of isobornyl amine is presented. The derivatives have been assessed as chiral solvating agents for effective discrimination of the signals of some acids in NMR analysis. The single crystal X-ray analysis of the salts of (R)-mandelic acid with two of the title derivatives help to understand the supramolecular interactions and assign the induced chemical shifts in 1H NMR analysis. The title derivative is found to be suitable for quantitative determination of the enantiomeric excess of unknown enantiomeric purity as well as being efficient in resolving racemic mandelic acid.

Stereoselective biotransformation of racemic mandelic acid using immobilized laccase and (S)-mandelate dehydrogenase

Objectives(S)-Mandelate dehydrogenase (SMDH) and laccase were immobilized on chitosan. The bi-enzymatic system with immobilized SMDH and immobilized laccase was taken to catalyze the stereoselective transformation of racemic mandelic acid and (R)-mandelic acid was obtained from its racemic mixture.ResultsCharacteristics of the immobilized enzymes were valuated. The optimum pH and temperature of the immobilized SMDH were found to be pH 3.4 and 45 °C, and these of the immobilized laccase were about pH 6.0 and 55 °C, respectively. The Km value of the immobilized SMDH for racemic mandelic acid was 0.27 mM and that of the immobilized laccase for ferrocyanide was 0.99 mM. The thermal and storage stabilities of these enzymes were improved with immobilization. The enantiomeric purity of the bi-enzymatically produced (R)-mandelic acid was determined to be over 99%.ConclusionThe immobilized bi-enzymatic system for the stereoselective transformation of racemic mandelic acid showed higher productivity, faster reaction velocity, and more stable catalytic ability.Graphical abstract.

Cocrystallization out of the blue: dl-mandelic acid/ethyl-dl-mandelate cocrystal

This work focuses on a peculiar behavior of racemic mandelic acid in ethanol solution. Dissolution of racemic mandelic acid in ethanol followed by evaporation to dryness results in a dl-mandelic acid/ethyl-dl-mandelate cocrystal. This behavior indicates that racemic mandelic acid tends not only to transform into an ester in ethanol, but also to cocrystallize with untransformed acid molecules. Cocrystal formation for mandelic acid in ethanol was found to be reproducible under various conditions. dl-tropic acid and dl-phenyllactic acid that contain similar functional groups and that were tested as well, on the other hand, showed no cocrystal formation: dl-phenyllactic acid partly converted into an ester, whereas dl-tropic acid mostly recrystallized.

The pH-dependency of Diastereomeric Salt Resolutions with Amphoteric Resolving Agents

We have analysed the role of pH in the diastereomeric resolution of racemic mandelic acid with (S)-phenylalanine and (R)-3-(aminomethyl)-5-methylhexanoic acid [(R)-Pregabalin] enantiomers. The adjusted pH had an effect on the optical purity and yield of the mandelic acid enantiomeric mixtures isolated from the diastereomeric salts. We showed that the pKa of the carboxyl group of the amphoteric resolving agents should be considered during the pH optimisation of diastereomeric salt resolutions.

Enantioseparation of Mandelic Acid Enantiomers With Magnetic Nano‐Sorbent Modified by a Chiral Selector

In this study, R(+)-α-methylbenzylamine-modified magnetic chiral sorbent was synthesized and assessed as a new enantioselective solid phase sorbent for separation of mandelic acid enantiomers from aqueous solutions. The chemical structures and magnetic properties of the new sorbent were characterized by vibrating sample magnetometry, transmission electron microscopy, Fourier transform infrared spectroscopy, and dynamic light scattering. The effects of different variables such as the initial concentration of racemic mandelic acid, dosage of sorbent, and contact time upon sorption characteristics of mandelic acid enantiomers on magnetic chiral sorbent were investigated. The sorption of mandelic acid enantiomers followed a pseudo-second-order reaction and equilibrium experiments were well fitted to a Langmuir isotherm model. The maximum adsorption capacity of racemic mandelic acid on to the magnetic chiral sorbent was found to be 405 mg g(-1). The magnetic chiral sorbent has a greater affinity for (S)-(+)-mandelic acid compared to (R)-(-)-mandelic acid. The optimum resolution was achieved with 10 mL 30 mM of racemic mandelic acid and 110 mg of magnetic chiral sorbent. The best percent enantiomeric excess values (up to 64%) were obtained by use of a chiralpak AD-H column.

Are the Crystal Structures of Enantiopure and Racemic Mandelic Acids Determined by Kinetics or Thermodynamics?

Mandelic acids are prototypic chiral molecules where the sensitivity of crystallized forms (enantiopure/racemic compound/polymorphs) to both conditions and substituents provides a new insight into the factors that may allow chiral separation by crystallization. The determination of a significant number of single crystal structures allows the analysis of 13 enantiopure and 30 racemic crystal structures of 21 (F/Cl/Br/CH3/CH3O) substituted mandelic acid derivatives. There are some common phenyl packing motifs between some groups of racemic and enantiopure structures, although they show very different hydrogen-bonding motifs. The computed crystal energy landscape of 3-chloromandelic acid, which has at least two enantiopure and three racemic crystal polymorphs, reveals that there are many more possible structures, some of which are predicted to be thermodynamically more favorable as well as slightly denser than the known forms. Simulations of mandelic acid dimers in isolation, water, and toluene do not differentiate between racemic and enantiopure dimers and also suggest that the phenyl ring interactions play a major role in the crystallization mechanism. The observed crystallization behavior of mandelic acids does not correspond to any simple "crystal engineering rules" as there is a range of thermodynamically feasible structures with no distinction between the enantiopure and racemic forms. Nucleation and crystallization appear to be determined by the kinetics of crystal growth with a statistical bias, but the diversity of the mandelic acid crystallization behavior demonstrates that the factors that influence the kinetics of crystal nucleation and growth are not yet adequately understood.

ChemInform Abstract: Kinetic Resolution of Racemic Mandelic Acid Esters by N,N′‐Dioxide‐Scandium‐Complex‐Catalyzed Enantiomer‐Selective Acylation.

AbstractA simple and efficient acylative kinetic resolution of racemic mandelic acid esters is accomplished with a chiral N,N‐dioxide‐scandium(III) complex under mild and base‐free reaction conditions.

A novel d-mandelate dehydrogenase used in three-enzyme cascade reaction for highly efficient synthesis of non-natural chiral amino acids

A novel NAD+-dependent D-mandelate dehydrogenase was identified from Lactobacillus brevis (LbDMDH). After purified to homogeneity, the optimum pH and temperature for oxidation of D-mandelate were pH 10.0 and 40°C, and the Km and kcat were 1.1mM and 355s−1 respectively. Employing the LbDMDH together with a mandelate racemase from Pseudomonas putida and a leucine dehydrogenase (EsLeuDH) from Exiguobacterium sibiricum, we established a three-step one-pot domino reaction system for preparing chiral L-phenylglycine from racemic mandelic acid with internal cofactor recycling. Under the optimum conditions, 30.4g rac-mandelic acid (0.2M) at 1L scale had been converted into chiral L-phenylglycine, with 96.4% conversion, 86.5% isolation yield, >99% eep and 50.4gL−1d−1 space-time yield.