Chiral Synthons Research Articles

Enantioselective synthesis of a chiral intermediate of himbacine analogs by Burkholderia cepacia lipase A.

The enantiomers of (4R/S)-4-hydroxy-N, N-diphenyl-2-pentynamide are key chiral synthons for the synthesis of thrombin receptor antagonists such as vorapaxar. In this paper, we report the enzymatic preparation of enantiomerically enriched (4R)-4-hydroxy-N, N-diphenyl-2-pentynamide using lipase A from Burkholderia cepacia ATCC 25416 as the catalyst. First, the lipase gene (lipA) and its chaperone gene (lipB) was cloned and expressed in Escherichia coli system. After purification, lipase A activation was performed with the assistance of foldase lipase B. Enzyme assay revealed that the activated lipase A showed the optimal catalytic activity at 60ºC and pH 7. The effects of various metals on the activity were investigated and results demonstrated that most of the metals inhibited the activity. To further improve the catalytic outcome, two-phase reaction was studied, and n-hexane proved to be a good organic solvent for the combination system. Using the optimize conditions, (4R)-4-hydroxy-N, N-diphenyl-2-pentynamide with 94.5% ee value and 48.93% conversion ratio was achieved. Our investigation on this lipase reveals lipase A as a promising biocatalyst for producing chiral propargyl alcohol for preparation of novel himbacine analogs.

One-pot two-step chemoenzymatic deracemization of allylic alcohols using laccases and alcohol dehydrogenases

A series of enantioenriched (hetero)aromatic secondary allylic alcohols has been synthesized through deracemization of the corresponding racemic mixtures combining a non-selective chemoenzymatic oxidation (laccase from Trametes versicolor and oxy-radical TEMPO) and a stereoselective biocatalyzed reduction (lyophilized cells of E. coli overexpressing an alcohol dehydrogenase, ADH). Both steps were performed in aqueous medium under very mild reaction conditions. After optimization, a sequential one-pot two-step protocol was set up, obtaining the corresponding chiral alcohols in moderate to high conversions (48–95%) and enantiomeric excess (65->99% ee). Depending on the ADH stereopreference, both antipodes from these valuable chiral synthons could be prepared, even at preparative scale (119−178 mg), in a straightforward manner.

Synthesis of Enantioenriched β-Aryl-β-aryloxy Esters via Sequential Photoisomerization and Enantioselective Hydrogenation.

A general method to prepare valuable chiral synthon β-aryl-β-aryloxy esters from enantioselective hydrogenation of (E)-β-aryl-β-aryloxy-α,β-unsaturated esters is described. The E-isomer was prepared via isomerization of the mixture of E- and Z-isomers using photocatalyst [Ir(ppy)2(dtbbpy)]PF6. A laser as the light source facilitated isomerization with only 0.05 mol % catalyst. The enantioselective hydrogenation was conducted with (NBD)2Rh(BF4) and a commercially available Josiphos ligand to provide the synthons in up to 95% yield with 97% ee.

High–throughput enantioseparation of Nα–fluorenylmethoxycarbonyl proteinogenic amino acids through fast chiral chromatography on zwitterionic-teicoplanin stationary phases

In this study, 31 racemates of Nα-FMOC (fluorenylmethoxycarbonyl) amino acids (AAs) with different chemico-physical characteristics (neutral nonpolar, neutral polar, acidic and basic) have been successfully resolved in fast enantioselective chromatography on recently-developed zwitterionic-teicoplanin chiral stationary phases (CSPs). The CSPs were prepared by covalently bonding the teicoplanin selector on fully-porous particles of narrow dispersion particle-size distribution (particle diameter 1.9 µm) and superficially-porous particles (2.0 µm). Both the zwitterionic-teicoplanin CSPs have proved to be ideal media for the separation of this important class of compounds. In particular, the zwitterionic CSP prepared on superficially-porous particles exhibited superior enantioselectivity and resolution, compared to that made of fully porous particles, in virtue of more favorable thermodynamics. The zwitterionic nature of these CSPs allowed avoiding the annoying effect of Donnan's exclusion of enantiomers from the stationary phase. This effect, on the opposite, was frequently observed on a commercial teicoplanin CSP (Teicoshell) employed for comparative purposes. Noticeably, on the zwitterionic-teicoplanin CSPs, by using either acetonitrile- or methanol-rich mobile phases (MPs), it was possible to favor speed over enantioresolution and vice versa. This work gives further replies to the request for rapid determination of enantiomeric excess of Nα-FMOC proteinogenic (and non–proteinogenic) AAs, typically used as preferred chiral synthons in the solid-phase synthesis of therapeutic peptides.

Catalytic fast pyrolysis of biomass with Ni-P-MCM-41 to selectively produce levoglucosenone

Levoglucosenone (LGO) is a valuable chiral synthon and can be produced from pyrolysis of cellulose and biomass. In the present study, Ni-P-MCM-41 catalysts with various P-to-Ni ratios were prepared and employed for selective production of LGO from biomass. Both analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and lab-scale experiments were conducted to explore the effects of several factors on the formation of LGO, including P-to-Ni ratio in Ni-P-MCM-41, catalyst/sample ratio and pyrolysis temperature. The results revealed that all the Ni-P-MCM-41 catalysts, especially the one with a P-to-Ni ratio of 5, exhibited excellent catalytic capability and selectivity for LGO production. For Py-GC/MS, the maximal LGO yields of 27.34 wt% and 14.30 wt% were achieved from cellulose and pine at 350 °C with catalyst/cellulose ratio of 3 and catalyst/pine ratio of 2, respectively. While in lab-scale experiments, the highest LGO yields from cellulose and pine were 21.37 wt% and 10.71 wt%, with the corresponding selectivity of 78.31% and 53.75%, respectively. The Ni-P-MCM-41 catalyst possessed much better catalytic efficiency on the LGO production than previously reported catalysts, including H2SO4, H3PO4, solid superacid, solid phosphoric acid and activated carbon.

Furan-Derived Chiral Bicycloaziridino Lactone Synthon: Collective Syntheses of Oseltamivir Phosphate (Tamiflu), (S)-Pipecolic acid and its 3-Hydroxy Derivatives.

A unified synthetic strategy for oseltamivir phosphate (tamiflu), (S)-pipecolic acid, and its 3-hydroxy derivatives from furan derived common chiral bicycloaziridino lactone synthon is described here. Key features are the short (4-steps), enantiopure, and decagram-scale synthesis of common chiral synthon from furan and its first-ever application in the total synthesis of biologically active compounds by taking the advantages of high functionalization ability of chiral synthon.

Stereoselective synthesis of the key intermediate of ticagrelor and its diverse analogs using a new alcohol dehydrogenase from Rhodococcus kyotonensis

Bioreduction catalyzed by alcohol dehydrogenase/reductase is one of the most valuable biotransformation processes widely used in industry. The (S)-2-Chloro-1-(3, 4-difluorophenyl) ethanol is a key chiral synthon for synthesizing the antithrombotic agent ticagrelor. Herein, a new alcohol dehydrogenase (named Rhky-ADH) identified from Rhodococcus kyotonensis by an enzyme promiscuity-based genome mining method was successfully cloned and functionally expressed in Escherichia coli. The whole cell biocatalyst harboring Rhky-ADH was biochemically characterized and was shown to be able to convert 2-Chloro-1-(3, 4-difluorophenyl) ethanone to (S)-2-Chloro-1-(3, 4-difluorophenyl) ethanol with more than 99 % enantiomeric excess (ee) and 99 % conversion. Our data showed that the optimum temperature and pH for Rhky-ADH were 25 °C and pH 8.0, respectively. The addition of NADH and an appropriate concentration of isopropanol enhanced the activity of Rhky-ADH, and 1 mM Mn2+ increased the enzyme activity by about 8 %. Substrate specificity experiments showed that Rhky-ADH had notable enzyme promiscuity and could reduce several ketones with high stereoselectivity. Our investigation on this novel enzyme adds another rare biocatalyst to the toolbox for producing chiral alcohols, which are widely used in the pharmaceutical industry.

Strategy for identification of cis-dihydrodiendiol-degrading dehydrogenases in E. coli BW25113

cis-Dihydrodiendiols are valuable compounds, finding multiple application as chiral synthons in organic chemistry. The biotechnological route for the generation of cis-dihydrodiendiols involves the dihydroxylation of aromatic compounds, catalyzed by Rieske non-heme iron dioxygenases. To date, numerous examples of recombinant E. coli, harboring such dioxygenases, can be found in the literature. Nevertheless, there is only a minor number of publications, addressing the E. coli catalyzed degradation of cis-dihydrodiendiols into catechols via dehydrogenases. Identification and elimination of such dehydrogenase catalyzed degradation is key for the establishment of enhanced recombinant E. coli platforms pursuing the production of cis-dihydrodiendiols. Here, we provide a fast and easy strategy for the identification of promiscuous alcohol dehydrogenases in E. coli BW25113, catalyzing the degradation of cis-dihydrodiendiols into catechols. This approach is based on the screening of dehydrogenase deficient KEIO strains, regarding their incapability of degrading a cis-dihydrodiendiol of choice.•Novel screening strategy for E. coli BW25113 dehydrogenase knock-outs, incapable of degrading cis-dihydrodiendiols was validated for cis-1,2-dihydrocatechol as substrate•Corresponding knock-outs can be used for recombinant production of cis-dihydrodiendiols•Simple analysis based on liquid chromatography with diode array detector (HPLC-DAD)

Access to a Catalytically Generated Umpolung Reagent through the Use of Cu-Catalyzed Reductive Coupling of Ketones and Allenes for the Synthesis of Chiral Vicinal Aminoalcohol Synthons.

We report the development of a stereoselective method for the allylation of ketones utilizing N-substituted allyl equivalents generated from a chiral allenamide. By employing N-heterocyclic carbenes as ligands for the Cu catalyst, good branched selectivity can be obtained with high diastereocontrol. This methodology allows access to a catalytically generated, polarity-reversed (umpolung) allyl nucleophile to enable the preparation of chiral 1,2-aminoalcohol synthons containing a dissonant functional group relationship.

Engineered C–N Lyase: Enantioselective Synthesis of Chiral Synthons for Artificial Dipeptide Sweeteners

AbstractAspartic acid derivatives with branched N‐alkyl or N‐arylalkyl substituents are valuable precursors to artificial dipeptide sweeteners such as neotame and advantame. The development of a biocatalyst to synthesize these compounds in a single asymmetric step is an as yet unmet challenge. Reported here is an enantioselective biocatalytic synthesis of various difficult N‐substituted aspartic acids, including N‐(3,3‐dimethylbutyl)‐l‐aspartic acid and N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)propyl]‐l‐aspartic acid, precursors to neotame and advantame, respectively, using an engineered variant of ethylenediamine‐N,N′‐disuccinic acid (EDDS) lyase from Chelativorans sp. BNC1. This engineered C–N lyase (mutant D290M/Y320M) displayed a remarkable 1140‐fold increase in activity for the selective hydroamination of fumarate compared to that of the wild‐type enzyme. These results present new opportunities to develop practical multienzymatic processes for the more sustainable and step‐economic synthesis of an important class of food additives.

Engineered C-N Lyase: Enantioselective Synthesis of Chiral Synthons for Artificial Dipeptide Sweeteners.

Aspartic acid derivatives with branched N‐alkyl or N‐arylalkyl substituents are valuable precursors to artificial dipeptide sweeteners such as neotame and advantame. The development of a biocatalyst to synthesize these compounds in a single asymmetric step is an as yet unmet challenge. Reported here is an enantioselective biocatalytic synthesis of various difficult N‐substituted aspartic acids, including N‐(3,3‐dimethylbutyl)‐l‐aspartic acid and N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)propyl]‐l‐aspartic acid, precursors to neotame and advantame, respectively, using an engineered variant of ethylenediamine‐N,N′‐disuccinic acid (EDDS) lyase from Chelativorans sp. BNC1. This engineered C–N lyase (mutant D290M/Y320M) displayed a remarkable 1140‐fold increase in activity for the selective hydroamination of fumarate compared to that of the wild‐type enzyme. These results present new opportunities to develop practical multienzymatic processes for the more sustainable and step‐economic synthesis of an important class of food additives.

MnFe2O4 magnetic nanoparticles modified with chitosan polymeric and phosphotungstic acid as a novel and highly effective green nanocatalyst for regio- and stereoselective synthesis of functionalized oxazolidin-2-ones.

A novel magnetically recoverable acid nanocatalyst, MnFe2O4/chitosan/phosphotungstic acid (MnFe2O4@CS@PTA), was successfully synthesized. The synthesized nanoparticle was studied as the heterogenous nanocatalyst to prepare the functionalized oxazolidin-2-ones as versatile chiral synthons in asymmetrically synthesizing the compounds with biological activity through the reaction of α-epoxyketones with urea and thiourea. This new procedure has notable advantages such as excellent yields, green reaction conditions, and short reaction time. In addition, the excellent catalytic performance in protic solvent and the easy preparation, thermal stability and separation of the catalyst make it a good heterogeneous system and a useful alternative to other heterogeneous catalysts. Perhaps, the synthesized MnFe2O4@CS@PTA is a hopeful catalytic substance for mass fine chemicals production. The structure and composition of the nanocomposite was performed by different methods and analyzed by Infrared Spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM), Thermo-Gravimetric Analysis (TGA), X-ray Diffraction (XRD) Transmission Electron Microscopy (TEM), Vibrating Sample Magnetometer (VSM) and Brunauere Emmette Teller (BET).

Enantiomeric impurities in chiral catalysts, auxiliaries, and synthons used in enantioselective syntheses. Part 5.

The enantiomeric excess of chiral starting materials is one of the important factors determining the enantiopurity of products in asymmetric synthesis. Fifty-one commercially available chiral reagents used as building blocks, catalysts, and auxiliaries in various enantioselective syntheses were assayed for their enantiomeric purity. The test results were classified within five impurities level (ie, <0.01%, 0.01%-0.1%, 0.1%-1%, 1%-10%, >10%). Previously from 1998 to 2013, several reports have been published on the enantiomeric composition of more than 300 chiral reagents. This series of papers is necessitated by the fact that new reagents are forthcoming and that the enantiomeric purity of the same reagent can vary from batch to batch and/or from supplier to supplier. This report presents chiral liquid chromatography (LC) and gas chromatography (GC) methods to separate enantiomers of chiral compounds and evaluate their enantiomeric purities. The accurate and efficient LC analysis was done using newly introduced superficially porous particle (SPP 2.7μm) based chiral stationary phases (TeicoShell, VancoShell, LarihcShell-P, and NicoShell).

Improving the catalytic efficiency of aldo-keto reductase KmAKR towards t-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate via semi-rational design

t-Butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-2) is an important chiral diol synthon of atorvastatin calcium. Previously, we constructed a variant KmAKR-W297H (M1) of Kluyveromyces marxianus aldo-keto reductase (KmAKR, designated as M0), possessing excellent diastereoselectivity but moderate activity towards t-butyl 6-cyano-(5R)-hydroxy-3-oxohexanoate ((5R)-1). In this work, KmAKR-W297H/Y296W/K29H (M3) was developed via semi-rational design. It exhibited much improved catalytic efficiency towards (5R)-1. The Km values of M3 for NADPH and (5R)-1 were 0.15 mmol/L and 1.41 mmol/L, and the maximal reaction rate vmax was 55.56 μmol/min/mg. Compared with M1, the catalytic efficiency kcat/Km of M3 was increased 2.64-fold. Coupled with Exiguobacterium sibiricum glucose dehydrogenase (EsGDH) for nicotinamide adenine dinucleotide phosphate (NADPH) regeneration, M3 took 3.5 h to completely reduce (5R)-1 at up to 100.0 g/L, producing 237.4 mmol/L (3R,5R)-2 in d.e.P value above 99.5%. The space-time yield (STY) of M3-catalyzed (3R,5R)-2 synthesis was 372.8 g/L/d.

Laboratory evolution of an alcohol dehydrogenase towards enantioselective reduction of difficult-to-reduce ketones

BackgroundA thermostable alcohol dehydrogenase from Thermoanaerobacter brockii (TbSADH) has been repurposed to perform asymmetric reduction of a series of prochiral ketones with the formation of enantio-pure secondary alcohols, which are crucial chiral synthons needed in the preparation of various pharmaceuticals. However, it is incapable of asymmetric reduction when applied to bulky ketones. Recently, mutations at two key residues A85 and I86 were shown to be crucial for reshaping the substrate binding pocket. Increased flexibility of the active site loop appears to be beneficial in the directed evolution of TbSADH towards difficult-to-reduce ketones.MethodsUsing the reported mutant A85G/I86A as template, double-code saturation mutagenesis (DCSM) was applied at selected residues lining the substrate binding pocket with a 2-membered reduced amino acid alphabet.Results and conclusionsThe mutant A85G/I86A was first tested for activity in the reaction of the model substrate (4-chlorophenyl)-(pyridin-2-yl)methanone, which showed a total turnover number (TTN) of 3071. In order to further improve the turnovers, a small and smart mutant library covering a set of mutations at Q101, W110, L294, and C295 was created. Eventually, a triple-mutant A85G/I86A/Q101A was identified to be a superior catalyst that gave S-selective product with 99% ee and 6555 TTN. Docking computations explain the source of enhanced activity. Some of the best variants are also excellent catalysts in the reduction of other difficult-to-reduce ketones.

FabG: from a core to circumstantial catalyst

Core biochemical pathways such as Fatty-acid synthesis II (FAS II) is ascribed to the synthesis of fatty-acids, biotin and lipoic acid in prokaryotes. It has two dehydrogenases namely, FabG and FabI which interact with the fatty-acid chain bound to Acyl-carrier protein (ACP), a well-studied enzyme which binds to substrates of varying lengths. This protein-protein interaction 'broadens' the active site of these dehydrogenases thus, contributing to their flexible nature. This property is exploited for catalysing numerous chiral synthons, alkanes, long-chain alcohols and secondary metabolites in industries especially with FabG. FASI relegates FASII in eukaryotes making it a 'relic gene pool' and an antibacterial drug target with diverse inhibitor and substrate markush. FabG often substitutes other dehydrogenases for producing secondary metabolites in nature. This redundancy is probably due to gene duplication or addition events possibly making FabG, a progenitor to some of the complex short-chain dehydrogenases used in organisms and industries today.

Biocatalytic Asymmetric Michael Additions of Nitromethane to α,β-Unsaturated Aldehydes via Enzyme-bound Iminium Ion Intermediates.

The enzyme 4-oxalocrotonate tautomerase (4-OT) exploits an N-terminal proline as main catalytic residue to facilitate several promiscuous C–C bond-forming reactions via enzyme-bound enamine intermediates. Here we show that the active site of this enzyme can give rise to further synthetically useful catalytic promiscuity. Specifically, the F50A mutant of 4-OT was found to efficiently promote asymmetric Michael additions of nitromethane to various α,β-unsaturated aldehydes to give γ-nitroaldehydes, important precursors to biologically active γ-aminobutyric acids. High conversions, high enantiocontrol, and good isolated product yields were achieved. The reactions likely proceed via iminium ion intermediates formed between the catalytic Pro-1 residue and the α,β-unsaturated aldehydes. In addition, a cascade of three 4-OT(F50A)-catalyzed reactions followed by an enzymatic oxidation step enables assembly of γ-nitrocarboxylic acids from three simple building blocks in one pot. Our results bridge organo- and biocatalysis, and they emphasize the potential of enzyme promiscuity for the preparation of important chiral synthons.

Formal synthesis of Cladospolide C &epi-Cladospolide C using R-(+)-γ-valerolactone as a chiral synthon

The formal synthesis of Cladospolide-C and its analog is achieved by using enantiopure (R)-γ–valerolactone 10. The significant points of this synthesis are the stereoselective dihydroxylation of α, β-unsaturated ester 16 using Sharpless protocol, Wittig olefination of γ –valerolactol 6 with triphenylphosphonium iodide salt 7, one pot selective oxidation of 22 and subsequent C2-homologation with good E/Z ratio.

Stereoselective Synthesis of Molecular Square and Granny Knots.

We report on the stereoselective synthesis of both molecular granny and square knots through the use of lanthanide-complexed overhand knots of specific handedness as three-crossing “entanglement synthons”. The composite knots are assembled by combining two entanglement synthons (of the same chirality for a granny knot; of opposite handedness for a square knot) in three synthetic steps: first, a CuAAC reaction joins together one end of each overhand knot. Ring-closing olefin metathesis (RCM) then affords the closed-loop knot, locking the topology. This allows the lanthanide ions necessary for stabilizing the entangled conformation of the synthons to subsequently be removed. The composite knots were characterized by 1H and 13C NMR spectroscopy and mass spectrometry and the chirality of the knot stereoisomers compared by circular dichroism. The synthetic strategy of combining building blocks of defined stereochemistry (here overhand knots of Λ- or Δ-handed entanglement) is reminiscent of the chiron approach of using minimalist chiral synthons in the stereoselective synthesis of molecules with multiple asymmetric centers.

Highly Diastereo- and Enantioselective Access to syn-α-Amido β-Hydroxy Esters via Ruthenium-Catalyzed Dynamic Kinetic Resolution-Asymmetric Hydrogenation.

Dynamic kinetic resolution (DKR) of racemic aryl α-amino β-ketoesters via Ru-diphosphine-catalyzed asymmetric hydrogenation was realized at 70 °C under 50 atm of hydrogen, affording syn α-amido β-hydroxy esters in high yields (up to 96%) with high reactivity (TON up to 940) and diastereo- and enantioselectivities (up to 99:1 dr, 98% ee). These hydrogenation products provide valuable chiral synthons in many natural products and pharmaceuticals. Gram-scale DKR asymmetric hydrogenation (DKR-AH) was also performed with retained reactivity and stereoselectivity, revealing the synthetic utility of this method.