High Enantiomeric Purity Research Articles

Enantioselectivity of Chiral Derivatives of Xanthones in Virulence Effects of Resistant Bacteria.

Antimicrobial peptides are one of the lines of defense produced by several hosts in response to bacterial infections. Inspired by them and recent discoveries of xanthones as bacterial efflux pump inhibitors, chiral amides with a xanthone scaffold were planned to be potential antimicrobial adjuvants. The chiral derivatives of xanthones were obtained by peptide coupling reactions between suitable xanthones with enantiomerically pure building blocks, yielding derivatives with high enantiomeric purity. Among 18 compounds investigated for their antimicrobial activity against reference strains of bacteria and fungi, antibacterial activity for the tested strains was not found. Selected compounds were also evaluated for their potential to inhibit bacterial efflux pumps. Compound (R,R)-8 inhibited efflux pumps in the Gram-positive model tested and three compounds, (S,S)-8, (R)-17 and (R,S)-18, displayed the same activity in the Gram-negative strain used. Studies were performed on the inhibition of biofilm formation and quorum-sensing, to which the enantiomeric pair 8 displayed activity for the latter. To gain a better understanding of how the active compounds bind to the efflux pumps, docking studies were performed. Hit compounds were proposed for each activity, and it was shown that enantioselectivity was noticeable and must be considered, as enantiomers displayed differences in activity.

Noncovalently Functionalized Commodity Polymers as Tailor‐Made Additives for Stereoselective Crystallization

AbstractStereoselective inhibition of the nucleation and crystal growth of one enantiomer aided by “tailor‐made” polymeric additives is an efficient method to obtain enantiopure compounds. However, the conventional preparation of polymeric additives from chiral monomers are laborious and limited in structures, which impedes their rapid optimization and applicability. Herein, we report a “plug‐and‐play” strategy to facilitate synthesis by using commercially available achiral polymers as the platform to attach various chiral small molecules as the recognition side‐chains through non‐covalent interactions. A library of supramolecular polymers made up of two vinyl polymers and six small molecules were applied with seeds in the selective crystallization of seven racemates in different solvents. They showed good to excellent stereoselectivity in yielding crystals with high enantiomeric purities in conglomerates and racemic compound forming systems. This convenient, low‐cost modular synthesis strategy of polymeric additives will allow for high‐efficient, economical resolution of various racemates on different scales.

Noncovalently Functionalized Commodity Polymers as Tailor-Made Additives for Stereoselective Crystallization.

Stereoselective inhibition of the nucleation and crystal growth of one enantiomer aided by "tailor-made" polymeric additives is an efficient method to obtain enantiopure compounds. However, the conventional preparation of polymeric additives from chiral monomers are laborious and limited in structures, which impedes their rapid optimization and applicability. Herein, we report a "plug-and-play" strategy to facilitate synthesis by using commercially available achiral polymers as the platform to attach various chiral small molecules as the recognition side-chains through non-covalent interactions. A library of supramolecular polymers made up of two vinyl polymers and six small molecules were applied with seeds in the selective crystallization of seven racemates in different solvents. They showed good to excellent stereoselectivity in yielding crystals with high enantiomeric purities in conglomerates and racemic compound forming systems. This convenient, low-cost modular synthesis strategy of polymeric additives will allow for high-efficient, economical resolution of various racemates on different scales.

Synthesis of Rhodium Complexes with Chiral Diene Ligands via Diastereoselective Coordination and Their Application in the Asymmetric Insertion of Diazo Compounds into E−H Bonds

AbstractA new method for the synthesis of chiral diene rhodium catalysts is introduced. The readily available racemic tetrafluorobenzobarrelene complexes [(R2‐TFB)RhCl]2 were separated into two enantiomers via selective coordination of one of them with the auxiliary S‐salicyl‐oxazoline ligand. One of the resulting chiral complexes with an exceptionally bulky diene ligand [(R,R‐iPr2‐TFB)RhCl]2 was an efficient catalyst for the asymmetric insertion of diazoesters into B−H and Si−H bonds giving the functionalized organoboranes and silanes with high yields (79–97 %) and enantiomeric purity (87–98 % ee). The stereoselectivity of separation via auxiliary ligand and that of the catalytic reaction was predicted by DFT calculations.

Defining Pathways of Anaerobic Alkane Oxidation: Synthesis of Enantiomers of 4‐Methylalkanoic Acids and (2‐Methylalkyl)malonic Acids

AbstractThe coenzyme A (CoA) esters of 4‐methylalkanoic acids are pivotal intermediates in metabolism of alkanes by anaerobic bacteria found in O2‐deprived environments. A generic method for synthesis of either (R)‐ or (S)‐acid in high enantiomeric purity from enantiomers of methyl 3‐hydroxy‐2‐methylpropionate is described for (R)‐ and (S)‐4‐methyloctanoic acid and (R)‐4‐methyldodecanoic acid. In a typical procedure silyl‐protection of methyl (S)‐3‐hydroxy‐2‐methylpropionate was followed by reduction of the ester to a primary alcohol, which was tosylated. Cu(I)‐catalysed cross‐coupling of the tosylate with propylmagnesium chloride followed by deprotection, tosylation and base‐induced reaction with di‐t‐butyl malonate, gave di‐t‐butyl (R)‐(2‐methylhexyl)malonate. Microwave heating of the diester in 2,2,2‐trifluoroethanol gave a 42 % overall yield of (R)‐4‐methyloctanoic acid, which was shown to be the enantiomer derived by metabolism of hexane by proteobacterium Aromatoleum sp. HxN1. Deprotection of the diester with trifluoroacetic acid gave (R)‐2‐(2‐methylhexyl)malonic acid, which is the biological precursor of (R)‐4‐methyloctanoic acid (via CoA esters).

Scalable Flow Electrochemical Alcohol Oxidation: Maintaining High Stereochemical Fidelity in the Synthesis of Levetiracetam.

An electrochemical flow process has been developed for an alcohol oxidation step in the synthesis of the generic epilepsy drug levetiracetam. A crucial metric in this process is the retention of high enantiomeric purity as the oxidation of the primary alcohol to the carboxylic acid proceeds via an epimerizable aldehyde intermediate. Here, three different reactor configurations are compared: undivided batch, undivided flow, and divided flow cells. The divided flow cell accesses the highest rate, throughput, and enantiomeric fidelity among the three configurations. This approach is showcased in a 200-gram scale process that retains ≥97% enantiomeric purity and highlights a unique advantage of flow electrolysis.

Lipase Catalyzed Synthesis of Enantiopure Precursors and Derivatives for β-Blockers Practolol, Pindolol and Carteolol

Sustainable methods for producing enantiopure drugs have been developed. Chlorohydrins as building blocks for several β-blockers have been synthesized in high enantiomeric purity by chemo-enzymatic methods. The yield of the chlorohydrins increased by the use of catalytic amount of base. The reason for this was found to be the reduced formation of the dimeric by-products compared to the use of higher concentration of the base. An overall reduction of reagents and reaction time was also obtained compared to our previously reported data of similar compounds. The enantiomers of the chlorohydrin building blocks were obtained by kinetic resolution of the racemate in transesterification reactions catalyzed by Candida antarctica Lipase B (CALB). Optical rotations confirmed the absolute configuration of the enantiopure drugs. The β-blocker (S)-practolol ((S)-N-(4-(2-hydroxy-3-(isopropylamino)propoxy)phenyl)acetamide) was synthesized with 96% enantiomeric excess (ee) from the chlorohydrin (R)-N-(4-(3-chloro-2 hydroxypropoxy)phenyl)acetamide, which was produced in 97% ee and with 27% yield. Racemic building block 1-((1H-indol-4-yl)oxy)-3-chloropropan-2-ol for the β-blocker pindolol was produced in 53% yield and (R)-1-((1H-indol-4-yl)oxy)-3-chloropropan-2-ol was produced in 92% ee. The chlorohydrin 7-(3-chloro-2-hydroxypropoxy)-3,4-dihydroquinolin-2(1H)-one, a building block for a derivative of carteolol was produced in 77% yield. (R)-7-(3-Chloro-2-hydroxypropoxy)-3,4-dihydroquinolin-2(1H)-one was obtained in 96% ee. The S-enantiomer of this carteolol derivative was produced in 97% ee in 87% yield. Racemic building block 5-(3-chloro-2-hydroxypropoxy)-3,4-dihydroquinolin-2(1H)-one, building block for the drug carteolol, was also produced in 53% yield, with 96% ee of the R-chlorohydrin (R)-5-(3-chloro-2-hydroxypropoxy)-3,4-dihydroquinolin-2(1H)-one. (S)-Carteolol was produced in 96% ee with low yield, which easily can be improved.

Microbial Synthesis of (S)- and (R)-Benzoin in Enantioselective Desymmetrization and Deracemization Catalyzed by Aureobasidium pullulans Included in the Blossom Protect™ Agent.

In this study, we examined the Aureobasidium pullulans strains DSM 14940 and DSM 14941 included in the Blossom Protect™ agent to be used in the bioreduction reaction of a symmetrical dicarbonyl compound. Both chiral 2-hydroxy-1,2-diphenylethanone antipodes were obtained with a high enantiomeric purity. Mild conditions (phosphate buffer [pH 7.0, 7.2], 30 °C) were successfully employed in the synthesis of (S)-benzoin using two different methodologies: benzyl desymmetrization and rac-benzoin deracemization. Bioreduction carried out with higher reagent concentrations, lower pH values and prolonged reaction time, and in the presence of additives, enabled enrichment of the reaction mixture with (R)-benzoin. The described procedure is a potentially useful tool in the synthesis of chiral building blocks with a defined configuration in a simple and economical process with a lower environmental impact, enabling one-pot biotransformation.

Single‐Point Mutant Inverts the Stereoselectivity of a Carbonyl Reductase toward β‐Ketoesters with Enhanced Activity

Enzyme stereoselectivity control is still a major challenge. To gain insight into the molecular basis of enzyme stereo-recognition and expand the source of antiPrelog carbonyl reductase toward β-ketoesters, rational enzyme design aiming at stereoselectivity inversion was performed. The designed variant Q139G switched the enzyme stereoselectivity toward β-ketoesters from Prelog to antiPrelog, providing corresponding alcohols in high enantiomeric purity (89.1-99.1 % ee). More importantly, the well-known trade-off between stereoselectivity and activity was not found. Q139G exhibited higher catalytic activity than the wildtype enzyme, the enhancement of the catalytic efficiency (kcat /Km ) varied from 1.1- to 27.1-fold. Interestingly, the mutant Q139G did not lead to reversed stereoselectivity toward aromatic ketones. Analysis of enzyme-substrate complexes showed that the structural flexibility of β-ketoesters and a newly formed cave together facilitated the formation of the antiPrelog-preferred conformation. In contrast, the relatively large and rigid structure of the aromatic ketones prevents them from forming the antiPrelog-preferred conformation.

The Asymmetric Difluoro‐Reformatsky Reaction

AbstractThe structural motif of α,α‐difluoro‐substituted carboxyl and carbonyl groups with hydroxy‐ or amino‐substituents at the stereogenic carbon in β‐position is found frequently in drugs and biologically active compounds. A straightforward method for obtaining those targets in high enantiomeric purity is given by asymmetric difluoro‐Reformatsky and imino‐difluoro‐Reformatsky reactions that lead to α,α‐difluorinated β‐hydroxy‐ and β‐amino‐carboxylates or β‐lactams. Three concepts that open a route to this type of compounds are discussed here. Firstly, chiral substrates are able to induce stereoselectivity in the formation of the newly created stereogenic center. Secondly, covalently bound and removable chiral auxiliaries are used in asymmetric syntheses and, thirdly, chiral additives and ligands at the metal direct the addition of the zinc enolate, the Reformatsky reagent, to aldehydes or imines in an enantioselective manner. The review closes with a short discussion of more recently developed difluoro‐aldol reactions that may be considered as alternatives to the Reformatsky procedure.

Mechanochemically Activated Asymmetric Organocatalytic Domino Mannich Reaction-Fluorination

Mechanochemical activation effectively mediated asymmetric organocatalytic domino Mannich addition followed by diastereoselective fluorination. The Mannich reactions of pyrazolones and to a lesser extent those of isoxazolones were effective under solvent-free ball-milling conditions. This reaction in combination with a chiral squaramide catalyst provided corresponding products in high yields and enantiomeric purities up to 99:1 e.r. and as a single diastereomer. DFT calculations revealed reasons for high diastereoselectivity.

Engineering of the 2,3-butanediol pathway of Paenibacillus polymyxa DSM 365

Paenibacillus polymyxa is a Gram-positive, non-pathogenic soil bacterium that has been extensively investigated for the production of R-,R-2,3-butanediol in exceptionally high enantiomeric purity. Rational metabolic engineering efforts to increase productivity and product titers were restricted due to limited genetic accessibility of the organism up to now. By use of CRISPR-Cas9 mediated genome editing, six metabolic mutant variants were generated and compared in batch fermentations for the first time. Downstream processing was facilitated by completely eliminating exopolysaccharide formation through the combined knockout of the sacB gene and the clu1 region, encoding for the underlying enzymatic machinery of levan and paenan synthesis. Spore formation was inhibited by deletion of spoIIE, thereby disrupting the sporulation cascade of P. polymyxa. Optimization of the carbon flux towards 2,3-butanediol was achieved by deletion of the lactate dehydrogenase ldh1 and decoupling of the butanediol dehydrogenase from its natural regulation via constitutive episomal expression. The improved strain showed 45 % increased productivity, reaching a final concentration of 43.8 g L−1 butanediol. A yield of 0.43 g g−1 glucose was achieved, accounting for 86 % of the theoretical maximum.

Biocatalytic Epoxidation of Cyclooctene to 1,2-Epoxycyclooctane by a Newly Immobilized Aspergillus niger Lipase

A newly immobilized Aspergillus niger lipase (ANL@ZnGlu-MNPs) was employed for the preparation of 1,2-epoxycyclooctane by oxidation of cyclooctene. The chosen variables, including substrate concentration, reaction temperature, immobilized enzyme dose, and H2O2 dose, were optimized in the reaction system of ethyl acetate. The yield and the enantiomeric excess of the product were achieved at 56.8% and 84.1%, respectively, under the following optimum reaction conditions: the concentration of substrate (cyclooctene) was 150 mM, the dosages of immobilized enzyme (ANL@ZnGlu-MNPs) and hydrogen peroxide were respectively 100 mg and 4.4 mmol, and the reaction was carried out in the system of 4 mL ethyl acetate at 40 °C. Further study on the operational stability of ANL@ZnGlu-MNPs showed that more than 51.6% of product yield was obtained after reusing for ten batches. A novel immobilized lipase was prepared and applied to synthesize 1,2-epoxycyclooctane from cyclooctene. Although ANL@ZnGlu-MNPs performs well in operational stability and the reaction can achieve high enantiomeric purity of the product, the yield of the catalytic reaction needs to be further improved.

Enantioselective Synthesis of Clavaminol A, Xestoaminol C and their Stereoisomers Exhibiting Cytotoxic Activity

The simple preparation of the natural sphingoid bases possessing cytotoxic activity – the marine drugs Clavaminol A and Xestoaminol C and all their unnatural stereoisomers – in high enantiomeric purity (ca. 95 % of major enantiomer) was described. The individual enantiomers were obtained by the utilization of asymmetric Henry reaction. The diastereomers of target compounds were separated by column chromatography after transformation into corresponding 2‐phenyloxazoline derivatives. The individual stereoisomers of Clavaminol A and Xestoaminol C were evaluated for antiproliferative activity in cancer cell lines (A‐549; Jurkat; SH‐SY5Y, MG‐63). From the obtained IC50 values is obvious, that the stereoisomers of Xestoaminol C are more potent inhibitors of cell proliferation than the stereoisomers of Clavaminol A. Further, it was found, that stereoisomers with syn‐configuration exhibited larger antiproliferative effects in comparison with the stereoisomers having anti‐configuration, in both sphingoid bases. Nevertheless, the values of IC50 found for individual enantiomers in each of the enantiomeric pairs are rather comparable implying a possibility of using racemic mixtures for induction of cytotoxicity.

Organocatalytic Synthesis of Chiral Spirooxindoles with Quaternary Stereogenic Centers

The structure of spirooxindoles is present in a large number of natural compounds, with relevant and different biological activities. The organic synthesis of these compounds has been a synthetic challenge due to the fusion of two or more cycles with one or more contiguous stereogenic centers. In this review, it has been described, the feasibility of organocatalytic asymmetric synthesis for the achievement of chiral spirooxindoles through two‐ or three‐component reaction in the presence of a chiral organocatalyst producing the compounds with high yield, high diastereoselectivity and with high enantiomeric purity. This review covers literature from 2010 to 2019. The most explored reactions for this purpose have been 1,3‐dipolar cycloaddition, Diels‐Alder and nucleophilic heterocyclic carbenes reactions, where their reaction mechanisms showed in this review are an approach of a possible mechanism.

Transition-Metal-Free Stereospecific Oxidative Annulative Coupling of Indolines with Aziridines.

Tandem C-N bond formation for the oxidative annulation of indolines with aziridines is accomplished employing the combination of DDQ and NaOCl at ambient conditions. Optically active aziridine can be coupled with high enantiomeric purity (>99% ee). The substrate scope, stereocontrol with the enantioenriched substrate, and scale-up are the important practical advantages.

Rhodium(III)-Catalyzed Asymmetric Addition of Inert Arene C-H Bond to Aldehydes To Afford Enantioenriched Phthalides.

An asymmetric rhodium(III)-catalyzed Grignard-type addition of inert arene C-H bond to aldehydes is reported. It provides a new strategy for the synthesis of chiral 3-substituted phthalides in good yields (up to 87%) with high enantiomeric purity (up to 99% ee). Interestingly, a chiral-matching effect between substrate and catalyst was observed, which is crucial to accomplish satisfied reaction outcomes. Mechanistically, the reaction is assumed to proceed via consecutive C(sp2)-H activation of benzamide, addition to aldehyde, and lactonization.

Structure-activity relationship studies on the inhibition of the bacterial translation of novel Odilorhabdins analogues

A structure–activity relationship (SAR) study of NOSO-95179, a nonapeptide from the Odilorhabdin class of antibacterials, was performed by systematic variations of amino acids in positions 2 and 5 of the peptide. A series of non-proteinogenic amino acids was synthesized in high enantiomeric purity from Williams’ chiral diphenyloxazinone by highly diastereoselective alkylation or by aldol-type reaction. NOSO-95179 analogues for SAR studies were prepared using solid-phase peptide synthesis. Inhibition of bacterial translation by each of the synthesized Odilorhabdin analogues was measured using an in vitro test. For the most efficient analogues, antibacterial efficacy was measured against two wild-type Enterobacteriaceae (Escherichia coli and Klebsiella pneumoniae) and against an efflux defective E. coli strain (ΔtolC) to evaluate the impact of efflux on the antibacterial activity.

Asymmetric synthesis, biological activity and molecular docking studies of some unsaturated α-amino acids, derivatives of glycine, allylglycine and propargylglycine

New enantiomerically enriched unsaturated tailor-made amino acids have been obtained. As a starting amino acid synthon for the asymmetric synthesis of tailor-made unsaturated amino acids, Ni(II) square-planar complexes of Schiff’s bases of propargylglycine, allylglycine and glycine with chiral auxiliary (S)-2-N-(N’-benzylprolyl)-aminobenzophenone ((S)-BPB) were used. The Cα-alkylation of propargylglycine, allylglycine and glycine moieties resulted in the asymmetric synthesis of novel (S)-α-propargylglycine, (S)-α-allylglycine and glycine derivatives containing an aromatic group in the side chain (de 80–95,5%). After purification and cleavage of the metal complexes, the amino acids were isolated in high enantiomeric purity (ee >99%). Of the obtained seven tailor-made amino acids four showed inhibitory activity to collagenase G. The amino acid with an acetylene bond in the side chain (IC50 = 1.29 ± 0.02 mM) had the best result. Molecular docking showed that the amino acids with activity to collagenase G contained hydrogen and π-π bonds with the enzyme.

Dual Mechanoenzymatic Kinetic Resolution of (±)‐Ketorolac

AbstractRecently, biocatalysis mediated by mechanical energy has become a convenient strategy to obtain highly valuable products following the principles of green chemistry. In particular, mechanoenzymatic techniques have allowed the isolation of chiral drugs with high enantiomeric purity. In this regard, being aware of the preponderant anti‐inflammatory activity of Ketorolac's (S)‐isomer relative to its (R)‐enantiomer, we developed a mechanoenzymatic kinetic resolution protocol using Candida antarctica Lipase B to yield both enantiomers with high enantiopurity. The significant enantiopreference of CALB for the (R)‐isomer of Ketorolac allowed the isolation of the active (S)‐enantiomer, either by means of an enantioselective esterification of racemic Ketorolac or via enantioselective hydrolysis of suitable ester derivatives. Both techniques proceeded in short reaction times, with high enantiomeric excess (>83 %), remarkable enantiodiscrimination (E≫500) and rather good conversion (46 %). Furthermore, (R)‐Ketorolac, an effective drug for ovarian cancer management, was obtained in essentially enantiopure form (ee >99 %).