Microwave-assisted enzymatic resolution of (R,S)-2-octanol in ionic liquid

Combination use of microwave irradiation (MW) as heating mode and ionic liquid (IL) as reaction medium in enzymatic resolution of (R,S)-2-octanol with vinyl acetate as the acyl donor through transesterification by Novozym 435 was investigated. A synergistic effect of MW and IL [EMIM][NTf2], which was screened as the best reaction medium for this reaction, on improving enzyme activity and enantioselectivity was observed. The activity and enantioselectivity of Novozym 435 in [EMIM][NTf2] under MW were much higher than that in solvent free system under conventional heating, in solvent free system under MW, and in [EMIM][NTf2] under conventional heating, respectively. A systematic screening and optimization of the reaction parameters in [EMIM][NTf2] under MW were performed. Under the optimum conditions, 50% yield of (S)-2-octanol with 99% enantiomeric excess was obtained in 6 h. Furthermore, increased thermal stability and reusability of Novozym 435 under the combination use of MW and IL condition were also observed.     

Solvent effects on the enantioselectivity of the thermophilic lipase QLM in the resolution of (R,S)-2-octanol and (R,S)-2-pentanol

The thermophilic lipase QLM-catalyzed resolution of (R, S)-2-octanol and (R, S)-2-pentanol via transesterification was carried out in various organic solvents, and the solvent effects on the enzyme's enantioselectivity were investigated. A significant negative correlation between the enantiomeric ratio, E, and the size of the solvent molecules was observed. The highest E value, 21, was obtained in the small molecular-sized solvent dichloromethane when (R, S)-2-octanol was resolved with vinyl acetate as acyl donor. Thermodynamic analysis indicated that the difference in activation free energy between the two enantiomers was about 25.5% lower in dichloromethane than in the solvent-free system, and the change in the difference in activation entropy between the enantiomers was the main contributor to the changes in E values with the molecular size of solvents.     

Enantioselective preparation of (R) and (S)-3-hydroxycyclopentanone by kinetic resolution

A straightforward approach to enantiomerically enriched (R) and (S)-3-hydroxycyclopentanone is described. The key step involves a kinetic resolution of racemic 3-hydroxycyclopentanone using commercial Pseudomonas cepacia lipase immobilized on diatomite (Amano lipase PS-DI). The absolute stereochemistry of the product was determined by derivatization into (R)-3-(benzyloxy)cyclopentanone.     

Immobilized Candida antarctica lipase B on ZnO nanowires/macroporous silica composites for catalyzing chiral resolution of (R,S)-2-octanol

ZnO nanowires were successfully introduced into a macroporous SiO₂ by in situ hydrothermal growth in 3D pores. The obtained composites were characterized by SEM and XRD, and used as supports to immobilize Candida antarctica lipase B (CALB) through adsorption. The high specific surface area (233 m²/g) and strong electrostatic interaction resulted that the average loading amount of the composite supports (196.8 mg/g) was 3–4 times of that of macroporous SiO₂ and approximate to that of a silica-based mesoporous material. Both adsorption capacity and the activity of the CALB immobilized on the composite supports almost kept unchanged as the samples were soaked in buffer solution for 48 h. The chiral resolution of 2-octanol was catalyzed by immobilized CALB. A maximum molar conversion of 49.1% was achieved with 99% enantiomeric excess of (R)-2-octanol acetate under the optimal condition: a reaction using 1.0 mol/L (R,S)-2-octanol, 2.0 mol/L vinyl acetate and 4.0 wt.% water content at 60 °C for 8 h. After fifteen recycles the immobilized lipase could retain 96.9% of relative activity and 93.8% of relative enantioselectivity.     

Experiment and simulation on kinetic resolution of (R,S)-2-chloromandelic acid by enzymatic transesterification

Optically pure 2-chloromandelic acid (ClMA) is a very important chiral drug intermediate for synthesis of (S)-clopidogrel, belonging to the platelet aggregation inhibitor. Enantioselective resolution of (R,S)-2-chloromandelic acid was carried out in organic solvent through irreversible transesterification catalyzed by lipase AK with vinyl acetate acting as the acyl donor. Effects of various conditions on enantioselectivity and activity of lipase were investigated, including organic solvents, temperature, water content, substrate ratio, enzyme loading, and reaction time. Based on homogeneous reaction and Ping-Pong bi-bi mechanism, a quantitative model was constructed to simulate and optimize the reaction process. Under the optimal conditions, excellent results were obtained with high conversion of (R)-2-ClMA (c _R, ≥98.85%) and large enantiomeric excess of substrate (ee _s, ≥98.15%). There is a good agreement between predicted values and experiment data, which indicates that the established method is a powerful tool for optimization of the enantioselective transesterification process for enantiomers separation.     

A two-step enzymatic resolution process for large-scale production of (S)- and (R)-ethyl-3-hydroxybutyrate

An efficient two-step enzymatic process for production of (R)- and (S)-ethyl-3-hydroxybutyrate (HEB), two important chiral intermediates for the pharmaceutical market, was developed and scaled-up to a multikilogram scale. Both enantiomers were obtained at 99% chemical purity and over 96% enantiomeric excess, with a total process yield of 73%. The first reaction involved a solvent-free acetylation of racemic HEB with vinylacetate for the production of (S)-HEB. In the second reaction, (R)-enriched ethyl-3-acetoxybutyrate (AEB) was subjected to alcoholysis with ethanol to derive optically pure (R)-HEB. Immobilized Candida antarctica lipase B (CALB) was employed in both stages, with high productivity and selectivity. The type of butyric acid ester influenced the enantioselectivity of the enzyme. Thus, extending the ester alkyl chain from ethyl to octyl resulted in a decrease in enantiomeric excess, whereas using bulky groups such as benzyl or t-butyl, improved the enantioselectivity of the enzyme. A stirred reactor was found unsuitable for large-scale production due to attrition of the enzyme particles and, therefore, a batchwise loop reactor system was used for bench-scale production. The immobilized enzyme was confined to a column and the reactants were circulated through the enzyme bed until the targeted conversion was reached. The desired products were separated from the reaction mixture in each of the two stages by fractional distillation. The main features of the process are the exclusion of solvent (thus ensuring high process throughput), and the use of the same enzyme for both the acetylation and the alcoholysis steps. Kilogram quantities of (S)-HEB and (R)-HEB were effectively prepared using this unit, which can be easily scaled-up to produce industrial quantities.     

Lipase-catalyzed kinetic resolution of (R,S)-1-phenylethyl propionate in an enzyme membrane reactor

Enzymatic stereoselective hydrolysis of (R,S)-1-phenylethyl propionate was performed in a stirred tank and in a biphasic enzyme membrane reactor. Lipase from Pseudomonas sp. was proved to be a good enantioselective catalyst for this reaction. The enzyme was covalently immobilized in a porous polyamide membrane (flat sheet as well as hollow-fibres) via glutaraldehyde. An influence of membrane hydrophobicity on reactor performance was observed. Initial lipase activity and productivity in the processes were equal to 1.05 × 10^?4, 1.3 × 10^?5 and 1.0 × 10^?5 mole/(h × mg of enzyme) in the case of native lipase, in the aromatic polyamide hydrophobic membrane reactor and in the hydrophilic polyamide-6 membrane reactor, respectively. The influence of some factors such as temperature, pH, buffer concentration, initial substrate concentration and addition of β-cyclodextrin derivatives on reaction rate and enantioselectivity was investigated and discussed. In the enzyme membrane reactor both organic and aqueous phases circulated countercurrently on both sides of the membrane. At a conversion degree of under 55–60%, pure enantiomer of the remaining ester (i.e. > 98%) was obtained.     

Efficient kinetic resolution of (R,S)-1-trimethylsilylethanol via lipase-mediated enantioselective acylation in ionic liquids

Efficient enantioselective acylation of (R,S)-1-trimethylsilylethanol {(R,S)-1-TMSE} with vinyl acetate catalyzed by immobilized lipase from Candida antarctica B (i.e., Novozym 435) was successfully conducted in ionic liquids (ILs). A remarkable enhancement in the initial rate and the enantioselectivity of the acylation was observed by using ILs as the reaction media when compared to the organic solvents tested. Also, the activity, enantioselectivity, and thermostability of Novozym 435 increased with increasing hydrophobicity of ILs. Of the six ILs examined, the IL C4MIm.PF6 gave the fastest initial rate and the highest enantioselectivity, and was consequently chosen as the favorable medium for the reaction. The optimal molar ratio of vinyl acetate to (R,S)-1-TMSE, water activity, and reaction temperature range were 4:1, 0.75, and 40 -50 degrees C, respectively, under which the initial rate and the enantioselectivity (E value) were 27.6 mM/h and 149, respectively. After a reaction time of 6 h, the ee of the remaining (S)-1-TMSE reached 97.1% at the substrate conversion of 50.7%. Additionally, Novozym 435 was effectively recycled and reused in C4MIm.PF6 for five consecutive runs without substantial lose in activity and enantioselectivity. The preparative scale kinetic resolution of (R,S)-1-TMSE in C4MIm.PF6 is shown to be very promising and useful for the industrial production of enantiopure (S)-1-TMSE.     

(R,S)-2-chlorophenoxyl pyrazolides as novel substrates for improving lipase-catalyzed hydrolytic resolution

The best reaction condition of Candida antartica lipase B as biocatalyst, 3-(2-pyridyl)pyrazole as leaving azole, and water-saturated methyl t-butyl ether as reaction medium at 45°C were first selected for performing the hydrolytic resolution of (R,S)-2-(4-chlorophenoxyl) azolides (1-4). In comparison with the kinetic resolution of (R,S)-2-phenylpropionyl 3-(2-pyridyl)pyrazolide or (R,S)-α-methoxyphenylacetyl 3-(2-pyridyl)pyrazolide at the same reaction condition, excellent enantioselectivity with more than two order-of-magnitudes higher activity for each enantiomer was obtained. The resolution was then extended to other (R,S)-3-(2-pyridyl)pyrazolides (5-7) containing 2-chloro, 3-chloro, or 2,4-dichloro substituent, giving good (E > 48) to excellent (E > 100) enantioselectivity. The thermodynamic analysis for 1, 2, and 4-7 demonstrates profound effects of the acyl or leaving moiety on varying enthalpic and entropic contributions to the difference of Gibbs free energies. A thorough kinetic analysis further indicates that on the basis of 6, the excellent enantiomeric ratio for 4 and 7 is due to the higher reactivity of (S)-4 and lower reactivity of (R)-7, respectively.     

Improving performance of Yarrowia lipolytica lipase lip2-catalyzed kinetic resolution of (R,S)-1-phenylethanol by solvent engineering

Extracellular Yarrowia lipolytica lipase Lip2 (YLIP2) demonstrated an (R)-enantiopreference for efficient resolution of (R,S)-1-phenylethanol by solvent engineering with different kinds of binary solvent. The enantioselectivity was significantly improved by the addition of 1, 4-dioxane. The reaction parameters including co-solvent concentration, reaction temperature, and the reaction time were optimized. When the reaction was carried out with n-hexane in the presence of 0.8% 1,4-dioxane at 50°C for 72 h, the enantiomeric excess of product markedly increased to 99.1% from 66% in pure n-hexane; the enantiomeric ratio was higher than 200, which was 500-fold compared with that in pure n-hexane. The results indicated that it is very important to design the proper co-solvents, especially to create appropriate micro-environment for YLIP2 for catalyzing the resolution of (R,S)-1-phenylethanol.     

Chiral recyclable dimeric and polymeric Cr(III) salen complexes catalyzed aminolytic kinetic resolution of trans-aromatic epoxides under microwave irradiation

Aminolytic kinetic resolution (AKR) of trans-stilbene oxide and trans-beta-methyl styrene oxide proceeded smoothly under microwave irradiation using chiral dimeric and polymeric Cr(III) salen complexes as efficient catalysts, giving regio-, diastereo-, and enantioselective anti-beta-amino alcohols in high yields (49%) and chiral purity (ee up to 94%) in case of 4-methylaniline within 2 min. The kinetic resolution system is approximately five times faster than traditional oil bath heating at 70 degrees C and 420 times faster than the reaction conducted at room temperature with concomitant recovery of respective chirally enriched epoxides (ee, 92%) in excellent yields (up to 48%). The catalyst 1 worked well in terms of enantioselectivity than the catalyst 2, but both the catalysts were easily recovered and reused five times with the retention of its efficiency.     

Practical access to the proline analogs (S,S,S)- and (R,R,R)-2-methyloctahydroindole-2-carboxylic acids by HPLC enantioseparation

An efficient methodology for the preparation of the α‐tetrasubstituted proline analog (S,S,S)‐2‐methyloctahydroindole‐2‐carboxylic acid, (S,S,S)‐(αMe)Oic, and its enantiomer, (R,R,R)‐(αMe)Oic, has been developed. Starting from easily available substrates and through simple transformations, a racemic precursor has been synthesized in excellent yield and further subjected to HPLC resolution using a cellulose‐derived chiral stationary phase. Specifically, a semipreparative (250 mm × 20 mm ID) Chiralpak® IC column has allowed the efficient resolution of more than 4 g of racemate using a mixture of n‐hexane/tert‐butyl methyl ether/2‐propanol as the eluent. Multigram quantities of the target amino acids have been isolated in enantiomerically pure form and suitably protected for incorporation into peptides. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

(2R,3S)-(+)- and (2S,3R)-(-)-Halofuginone lactate: synthesis, absolute configuration, and activity against Cryptosporidium parvum

The trans-enantiomers of the commercially important anti-protozoal compound Halofuginone have been prepared and characterized, and the absolute configuration was assigned by X-ray crystallography. The activity of both enantiomers against Cryptosporidium parvum was determined in vitro and related to acute toxicity in vivo. It was shown that both the activity and the toxicity are properties of the (2R,3S)-enantiomer. We conclude that with respect to broadening the therapeutic window there is no advantage in application of one enantiomer over the application of the racemic mixture in the treatment of C. parvum infections.     

A novel lipase from Aspergillus oryzae catalyzed resolution of (R,S)-ethyl 2-bromoisovalerate

In this study, a novel lipase M5 derived from Aspergillus oryzae WZ007 was prone to exhibit high hydrolytic activity and stereoselectivity towards racemic substrate (R,S)-ethyl 2-bromoisovalerate. (R)-ethyl 2-bromoisovalerate was obtained by enzymatic resolution, which is the key chiral intermediate for highly efficient enantiomerically fluvalinate. The results showed that the enzymatic reaction was carried out in 120mM racemic substrate for 3 hours, the enantiomeric excess reached 98.6%, the conversion was 51.7%, and E value above 120. Therefore, the novel lipase M5 has the ability to efficiently produce (R)-ethyl 2-bromoisovalerate, which greatly reduces the industrial production cost of the highly efficient counterpart of fluvalinate.     

A straightforward stereoselective synthesis of meso-, (S,S)- and (R,R)-2,6-diaminopimelic acids from cis-1,4-diacetoxycyclohept-2-ene

A straightforward synthesis of meso-2,6-diaminopimelic acid (DAP) meso-1 was developed from 1,4-diacetoxycyclohept-2-ene (2) via an oxidative ring cleavage. Subsequently, an enantio-divergent synthesis of (S,S)- and (R,R)-1 was performed using a homochiral monoacetate 7 available from 2 by enzymatic desymmetrization.     

Synthesis, resolution, stereochemistry, and molecular modeling of (R)- and (S)-2-acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline AMPAR antagonists

Recently we identified (R,S)-2-acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (6) as a potent non-competitive AMPA receptor antagonist able to prevent epileptic seizures. We report here the optimized synthesis of compound 6, its resolution by chiral preparative HPLC, and the absolute configuration of (R)-enantiomer established by X-ray diffractometry. The biological tests of the single enantiomers revealed that higher anticonvulsant and antagonistic effects reside in (R)-enantiomer as also suggested by molecular modeling studies.     

Direct conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone in zinc chloride solution under microwave irradiation

Conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone has been demonstrated in concentrated zinc chloride solution under microwave irradiation. Compared with the conventional oil-bath heating mode, microwave irradiation significantly reduced the reaction time and increased the yield of 1-(furan-2-yl)-2-hydroxyethanone. A typical degradation reaction with cellulose produced 1-(furan-2-yl)-2-hydroxyethanone in 12.0% molar yield in ZnCl₂ solution (ZnCl₂–H₂O ratio = 2.25:1, w/w) with microwave irradiation at 600 W for 5 minutes at 135 °C.     

Synthesis of (+)-(2R,3S,4R)-2,3,4-trihydroxycyclohexanone from D-glucose

We have described the synthesis of (+)-(2R,3S,4R)-2,3,4-trihydroxycyclohexanone by the reduction of a keto-conduritol derivative, the latter being prepared in five steps from (-)-(2S,3R,4S,5S)-2,3,4-tribenzyloxy-5-hydroxycyclohexanone, which is in turn readily synthesized from D-glucose.     

Syntheses of 2-Deoxy-2-[(2R,3S)-2-fluoro-3-hydroxytetradecanamido]-3-O-[(3R)-3-hydroxytetradecanoyl]-4-O-phosphono-D-glucopyranose and Its (2S,3R)-Isomer

2-Deoxy-2-[(2R,3S)-2-fluoro-3-hydroxytetradecanamido]-3-O-[(3R)-3-hydroxytetradecanoyl]-4-O-phosphono-D-glucopyranose and its (2S,3R)-isomer were respectively synthesized from allyl 2-[(2R,3S)-3-(benzyloxycarbonyloxy)-2-fluorotetradecanamido]-2-deoxy-4,6-O-isopropylidene-β-D-glucopyranoside and its corresponding (2S,3R)-isomer. Both target compounds did not activate macrophage, but the (2S,3R)-analogue strongly inhibited the binding of LPS to macrophage.     

Synthesis and antioxidant evaluation of (S,S)- and (R,R)-secoisolariciresinol diglucosides (SDGs)

Secoisolariciresinol diglucosides (SDGs) (S,S)-SDG-1 (major isomer in flaxseed) and (R,R)-SDG-2 (minor isomer in flaxseed) were synthesized from vanillin via secoisolariciresinol (6) and glucosyl donor 7 through a concise route that involved chromatographic separation of diastereomeric diglucoside derivatives (S,S)-8 and (R,R)-9. Synthetic (S,S)-SDG-1 and (R,R)-SDG-2 exhibited potent antioxidant properties (EC₅₀ = 292.17 ± 27.71 μM and 331.94 ± 21.21 μM, respectively), which compared well with that of natural (S,S)-SDG-1 (EC₅₀ = 275.24 ± 13.15 μM). These values are significantly lower than those of ascorbic acid (EC₅₀ = 1129.32 ± 88.79 μM) and α-tocopherol (EC₅₀ = 944.62 ± 148.00 μM). Compounds (S,S)-SDG-1 and (R,R)-SDG-2 also demonstrated powerful scavenging activities against hydroxyl [natural (S,S)-SDG-1: 3.68 ± 0.27; synthetic (S,S)-SDG-1: 2.09 ± 0.16; synthetic (R,R)-SDG-2: 1.96 ± 0.27], peroxyl [natural (S,S)-SDG-1: 2.55 ± 0.11; synthetic (S,S)-SDG-1: 2.20 ± 0.10; synthetic (R,R)-SDG-2: 3.03 ± 0.04] and DPPH [natural (S,S)-SDG-1: EC₅₀ = 83.94 ± 2.80 μM; synthetic (S,S)-SDG-1: EC₅₀ = 157.54 ± 21.30 μM; synthetic (R,R)-SDG-2: EC₅₀ = 123.63 ± 8.67 μM] radicals. These results confirm previous studies with naturally occurring (S,S)-SDG-1 and establish both (S,S)-SDG-1 and (R,R)-SDG-2 as potent antioxidants and free radical scavengers for potential in vivo use.