Enzymatic Resolutions of Heterocyclic Alcohols

The butyrates and acetates of heterocyclic alcohols like 3 - hydroxy tetrahydrofuran and - pyran, 3- and 4 - chromanol as well as the corresponding sulfur heterocycles were hydrolyzed using lipase from Candida rugosa (CRL) and from Pseudomonas cepacia, (PCL). Poor to excellent enantioselectivities were obtained depending on the structure of the substrates. An electrostatic amendment to the steric substrate model for PGL is proposed.     

A genomic search approach to identify carbonyl reductases in Gluconobacter oxydans for enantioselective reduction of ketones

The versatile carbonyl reductases from Gluconobacter oxydans in the enantioselective reduction of ketones to the corresponding alcohols were exploited by genome search approach. All purified enzymes showed activities toward the tested ketoesters with different activities. In the reduction of 4-phenyl-2-butanone with in situ NAD(P)H regeneration system, (S)-alcohol was obtained with an e.e. of up to 100% catalyzed by Gox0644. Under the same experimental condition, all enzymes catalyzed ethyl 4-chloroacetoacetate to give chiral products with an excellent e.e. of up to 99%, except Gox0644. Gox2036 had a strict requirement for NADH as the cofactor and showed excellent enantiospecificity in the synthesis of ethyl (R)-4-chloro-3-hydroxybutanoate. For the reduction of ethyl 2-oxo-4-phenylbutyrate, excellent e.e. (>99%) and high conversion (93.1%) were obtained by Gox0525, whereas the other enzymes showed relatively lower e.e. and conversions. Among them, Gox2036 and Gox0525 showed potentials in the synthesis of chiral alcohols as useful biocatalysts.     

Nonlinear stereoselective pharmacokinetics of ketoconazole in rat after administration of racemate

The stereoselective pharmacokinetics of ketoconazole (KTZ) enantiomers were studied in rat after i.v. and oral administration of (+/-)-KTZ. Sprague-Dawley rats were administered racemic KTZ as 10 mg/kg i.v. or orally over the range 10-80 mg/kg as single doses. Serial blood samples were collected over a 24-h period via surgically placed jugular vein cannulae. Plasma was assayed for KTZ enantiomer concentrations using stereospecific HPLC. Enantiomeric plasma protein binding was determined using an erythrocyte partitioning method at racemic concentrations of 10 and 40 mg/L. Stereoselective metabolism was tested by incubating the racemate (0.5-250 microM) with rat liver microsomes. In all rats, (+)-KTZ plasma concentrations were higher (up to 2.5-fold) than (-)-KTZ. The clearance and volume of distribution of the (-) enantiomer were approximately twofold higher than antipode. Half-life did not differ between the enantiomers. After oral doses the t(max) was not stereoselective. For both enantiomers with higher doses the respective half-life were found to increase. The mean unbound fraction of the (-) enantiomer was found to be up to threefold higher than that of the (+) enantiomer. At higher concentrations nonlinearity in plasma protein binding was observed for both enantiomers. There was no evidence of stereoselective metabolism by liver microsomes. Stereoselectivity in KTZ pharmacokinetics is attributable to plasma protein binding, although other processes such as transport or intestinal metabolism may also contribute.     

Lipase Catalyzed Resolution of (±)-2-Substituted 1-Propanol Derivatives Possessing an Aromatic Ring

For the purpose of the chiral synthesis of natural products, lipase-catalyzed kinetic resolutions of three types of 2-substituted 1-propanol derivatives (each having an aromatic ring) was investigated. In every case, chiral recognition of the primary alcohol unit took place to provide the corresponding alcohols and the acetates in high optical purity. The (2S,3S)-5-aryl-2-methyl-3-hydroxy-4E-pentenol 5 was formally converted into the antibiotic, (-)-oudemansin X.     

Lipase Catalyzed Resolution of (±)-2-Substituted 1-Propanol Derivatives Possessing an Aromatic Ring

For the purpose of the chiral synthesis of natural products, lipase-catalyzed kinetic resolutions of three types of 2-substituted 1-propanol derivatives (each having an aromatic ring) was investigated. In every case, chiral recognition of the primary alcohol unit took place to provide the corresponding alcohols and the acetates in high optical purity. The (2S,3S)-5-aryl-2-methyl-3-hydroxy-4E-pentenol 5 was formally converted into the antibiotic, (-)-oudemansin X.     

Lactones 29 . Enzymatic resolution of racemic γ-lactones

Studies on the application of commercially available enzymes to resolution of the racemic unsaturated γ-lactones: 5-(3-methylbutylidene)-4-methyl-tetrahydrofuran-2-one (1a) and 5-(3,3-dimethylbutylidene)-4-methyl-tetrahydrofuran-2-one (2a) are presented. Lipase PS, Rhizopus niveus lipase, Rhizopus arrhizus lipase, porcine pancreas lipase and hog liver esterase transformed substrates into their respective γ-keto acids with good efficiency (50-75%). Three of them hydrolysed the studied lactones with moderate enantioselectivity. Enantiomeric excesses determined by GC for the unreacted lactones were in the range of 20-60%. Lipase PS preferentially hydrolysed the (+) enantiomers of lactones 1a and 2a whereas R. niveus lipase hydrolysed the (-) enantiomers, respectively.     

产碱杆菌ECU0401催化拆分扁桃酸及其衍生物

从土壤中分离的1株产碱杆菌Alcaligenes sp.ECU0401具有扁桃酸脱氢酶活性,可以以扁桃酸、苯甲酰甲酸或苯甲酸为唯一C源生长,并且具有较高的脱氢酶活力。以外消旋扁桃酸为C源,采用分批补料策略培养（或反应）99h,扁桃酸累计投入量为30.4g/L,（S）-（＋）-扁桃酸被完全降解,（R）-（-）-扁桃酸回收产率为32.8%,对映体过量值（e.e.）〉99.9%。利用静息细胞作为催化剂不对称降解外消旋扁桃酸的氯代衍生物,制备获得光学活性的（R）-（-）-邻氯扁桃酸、（S）-（＋）-间氯扁桃酸和（S）-（＋）-对氯扁桃酸,光学纯度均超过99.9%e.e.。     

Stereoselective degradation kinetics of tebuconazole in rabbits

Tebuconazole[(RS)-1-p-chlorophenyl-4,4-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl)pentan-3-ol] is a potent triazole fungicide and consists of a pair of enantiomers. The enantioselective degradation kinetics of tebuconazole was investigated in rabbits by intravenous (iv) injection. The concentrations of (-)-(R)-tebuconazole and (+)-(S)-tebuconazole in plasma and tissues were determined by HPLC with a cellulose tris(3,5-dimethylphenylcarbamate)-based chiral stationary phase. Enantioselective analysis methods for this fungicide in plasma and tissues were developed and validated. Good linearities were obtained over the concentration range of 0.25-25 mg/l for both enantiomers. The degradation followed pseudo-first-order kinetics and the degradation of the (+)-(S)-tebuconazole was much faster than that of the (-)-(R)-tebuconazole in plasma after administration of racemic tebuconazole. This study also indicated that environmental assessment of enantiomeric degradation may be needed to fully evaluate risks of tebuconazole use.     

Separation and aquatic toxicity of enantiomers of 1-(substituted phenoxyacetoxy)alkylphosphonate herbicides

A series of organophosphorous compounds (OPs), 1-(substituted phenoxyacetoxy)alkylphosphonates containing a chiral carbon atom, show notable herbicidal activities. In this study, the enantioselective separation and biological toxicity of all these compounds were investigated. The enantioselective separation on the columns of Chiralpak AD, Chiralpak AS, Chiralcel OD, and Chiralcel OJ were compared under various chromatographic conditions. All the analytes investigated obtained baseline resolution (R(s) > 1.5) on Chiralpak AD column, which showed best chiral separation capacity. Further investigation was carried out on Chiralpak AD to evaluate the influence of the mobile phase composition and column temperature. The effect of the structural features on discrimination was also examined. The resolved enantiomers were distinguished by their signs of circular dichroism. The acute aquatic toxicity of enantiomers and racemate to Daphnia magna (D. magna) were assessed. The in vivo assays showed that compound 3 was about 2-148.5 times more toxic than the other four analogues to D. magna. The racemates of compounds 3 and 5 showed intermediate toxicity compare to their enantiomers, while those of compounds 1, 2, and 4 showed synergistic or antagonistic effect. These results suggest that the biological toxicity of chiral OPs to nontarget organisms is enantioselective and therefore should be evaluated with their pure enantiomers.     

Candida antarctica lipase-B-catalyzed kinetic resolution of 1,3-dialkyl-3-hydroxymethyl oxindoles

Candida antarctica (CAL-B) lipase-catalyzed resolution of 1,3-dialkyl-3-hydroxymethyl oxindoles has been performed to obtain (R)-1,3-dialkyl-3-acetoxymethyl oxindoles with up to 99% ee and (S)-1,3-dialkyl-3-hydroxymethyl oxindoles with up to 78% ee using vinyl acetate as acylating agent and acetonitrile as solvent transforming (S)-3-allyl-3-hydroxymethyl oxindole to (3S)-1′-benzyl-5-(iodomethyl)-4,5-dihydro-2H-spiro[furan-3,3′-indolin]-2′-one. The optically active 3-substituted-3-hydroxymethyl oxindoles and spiro-oxindoles are among the key synthons in the synthesis of potentially biologically active molecules.     

New bulky chiral Lewis acid catalyst: 3,3'-di(2-mesitylethynyl)binaphthol-titanium(IV) complex

A new chiral Lewis acid catalyst 9 was prepared in situ from a 1:2 molar mixture of (R)-3,3'-di(2-mesitylethynyl)binaphthol (6) and titanium(IV) isopropoxide at ambient temperature. The 3- and 3'-substituents on 6 were effective for preventing undesired aggregation between Ti(IV) complexes and increasing the enantioselectivity (up to 82% ee) in the Diels-Alder reaction of methacrolein with cyclopentadiene.     

Enantioselective degradation of indoxacarb in cabbage and soil under field conditions

The enantioselective degradation of indoxacarb in cabbage and soil has been investigated in Beijing and Anhui under open conditions. Indoxacarb enantiomers in samples were extracted with acetonitrile, cleaned up by florisil SPE column, separated on high performance liquid chromatography with a cellulose-tris-(3, 5-dimethylphenylcarbamate)-based chiral stationary phase (CDMPC-CSP), and determined by a photodiode array detector. The validation of the developed method by fortification rac-indoxcarb in cabbage and soil showed good accuracy and precision. The results of field trials indicated that the dissipation of indoxacarb enantiomers followed pseudo-first-order kinetics or first-order kinetics in cabbage and soil at two locations. The half-lives of two enantiomers in cabbage ranged from 2.8 to 4.6 d which were shorter than those in soil ranging from 23 to 35 d. The changes of enantiomeric fraction values proved that enantioselective degradation of indoxacarb happened in cabbage and soil. The (-)-indoxacarb showed faster degradation in the Beijing cabbage, whereas in the Anhui cabbage, (+)-indoxacarb preferentially degraded. In soil, preferential degradation of (+)-indoxacarb was observed at two locations.     

Occurrence of Enantioselectivity in Nature: The Case of (S)‐Norcoclaurine

This review article is aimed at providing a monographic overview on (S)‐norcoclaurine (NC) alkaloid from three diverse points of view, collected all together for the first time: 1) the synthetic one, where the compound is seen as a target chiral molecule to be obtained in the highest optical purity and as a starting point for the development of biocatalytic asymmetric syntheses of tetrahydroisoquinoline alkaloids; 2) the chromatographic one, which addresses the HPLC separation of the two NC enantiomers; and 3) the biochemical one, for which a thorough understanding of the topology and mechanism of action of norcoclaurine synthase (NCS) enzyme is still a matter of debate. Special emphasis on the most recent studies in the field is given by discussing the results published by the main research groups who are working on NC and NCS. Chirality 28:169–180, 2016. © 2016 Wiley Periodicals, Inc.     

Elucidation of the enantioselective recognition mechanism of a penicillin G acylase-based chiral stationary phase towards a series of 2-aryloxy-2-arylacetic acids

A series of structurally related 2-aryloxy-2-arylacetic acids (1-3, 5-16) together with a thioisostere derivative (4) have been synthesized and characterized by GC-MS and 1H NMR. The designed compounds were analyzed on a Penicillin G Acylase chiral stationary phase (PGA-CSP) and the influence of the structure variations on retention and enantioselectivity was investigated. The chromatographic study includes the direct separation of the enantiomers of the synthesized compounds and the determination of the elution order of selected racemic mixtures. 10 out of 16 racemates were separated; high chromatographic enantioseparation factors (alpha > 2) were achieved for some compounds. For the enantiomers of four compounds whose absolute configuration was known (1, 3, 12, 16), the elution order was R:S with the exception of 2-(4-chloro-phenoxy)phenylacetic acid (1), for which the elution order was reversed. Preliminary molecular modeling studies suggest that both polar and charge-transfer interactions as well as steric effects play an important role in determining the retention factors and the enantioselectivities observed.     

Structure Elucidation of Gibberellin A32 and Its Acetonide

The structures of two gibberellin-like substances isolated from the immature seeds of Prunus persica, tentatively named PG–I and PG–II, were elucidated. PG–I was an ammonium salt of a novel gibberellin, ent-3α,10,12β,13,15α-pentahydroxy-20-norgibberella-l,16-diene-7,19-dioic-19,10-lactone (1), to which gibberellin number A₃₂ was allocated. PG–II was shown to be gibberellin A₃₂ acetonide (7), and concluded to be an artifact produced from gibberellin A₃₂ in the isolation process.     

S‐perindopril assay using a potentiometric,enantioselective membrane electrode

A potentiometric, enantioselective membrane electrode based on graphite paste (graphite powder and paraffin oil) has been constructed. The graphite paste is impregnated with a 10⁻³ mol/L 2‐hydroxy‐3‐trimethylammoniopropyl‐β‐cyclodextrin (as chloride salt) solution. The potentiometric, enantioselective membrane electrode can be used reliably for enantiopurity tests of S‐perindopril using a chronopotentiometric (zero current) technique, in the 10⁻⁵–10⁻² mol/L concentration range (detection limit 5 × 10⁻⁶ mol/L), with an average recovery of 99.58% (RSD = 0.33%). The enantioselectivity was determined over R‐perindopril and d ‐proline. The response characteristics of the enantioselective, potentiometric membrane electrode were also determined for R‐perindopril. It was shown that l ‐proline is the main interfering compound. The surface of the electrode can be regenerated simply by polishing, obtaining a fresh surface ready to be used in a new assay. Chirality 11:631–634, 1999. © 1999 Wiley‐Liss, Inc.     

(S)-3-Pentyn-2-ol production through microbial enzyme-catalyzed, highly enantioselective hydrolysis of racemic 3-pentyn-2-ol esters

Air-dried cells of Hansenula nonfermentans AKU 4332 catalyzed the production of (S)-3-pentyn-2-ol from (RS)-3- pentyn-2-ol acetate ester at 10% (v/v). The product was formed at 96.6% e.e. with a molar yield of 45% in 24 h. © Rapid Science Ltd. 1998     

Newly isolated Streptomyces spp. as enantioselective biocatalysts: hydrolysis of 1,2-O-isopropylidene glycerol racemic esters

AIMS: To identify microbial strains with esterase activity able to enantioselectively hydrolyse esters of (R,S)-1,2-O-isopropylidene glycerol. METHODS AND RESULTS: The microbial hydrolysis of various racemic esters of 1,2-O-isopropylidene glycerol (IPG) was attempted by screening among Streptomyces spp. previously selected on the basis of their carboxylesterase activity. The best results were observed in the hydrolysis of butyrate ester and two strains appeared promising as they showed opposite enantioselectivity: Streptomyces sp. 90852 gave predominantly (S)-IPG, while strain 90930 mostly gave the R-alcohol. Streptomyces sp. 90930 was identified as Streptomyces violaceusniger, whereas Streptomyces sp. 90852 is a new species belonging to the Streptomyces violaceus taxon. The carboxylesterase belonging to strain 90852 gave a maximum value of enantiomeric ratio (E) of 14-16. This strain was lyophilized and used as dry mycelium for catalysing the synthesis of isopropylidene glycerol butyrate in heptane showing reaction rate and enantioselectivity (E = 6.6) lower than what observed for the hydrolysis. CONCLUSIONS: A new esterase with enantioselective activity towards (R,S)-IPG butyrate has been selected. The best enantioselectivity is similar or even better than the highest reported value in the literature with commercial enzymes. The enzyme is produced by a new species belonging to the S. violaceus taxon. SIGNIFICANCE AND IMPACT OF THE STUDY: New esterases from streptomycetes can be employed for the enantioselective hydrolysis of chiral esters derived from primary alcohols, not efficiently resolved with commercial enzymes.     

Interconversion and tissue distribution of pentoxifylline and lisofylline in mice

The aim of this study was to assess the interconversion pharmacokinetics and tissue distribution of pentoxifylline and the active (R)-enantiomer of its metabolite M1, lisofylline in male CD-1 mice. Both compounds were administered intravenously at a dose of 50 mg/kg on two separate occasions. Serum and tissues were collected at different time points following drug administration. In addition, the (S)-enantiomer of M1 was administered to a group of mice and serum samples were obtained. Analyte concentrations were measured by chiral HPLC. All serum concentration versus time data were fitted simultaneously to a pharmacokinetic model incorporating interconversion processes of parent drug and metabolites. The estimated conversion clearance of (-)-(R)-M1 to pentoxifylline (CL21) was six times greater than that for the reverse process (CL12). The interconversion of pentoxifylline and (+)-(S)-M1 was faster as reflected by the values of conversion clearances CL13 and CL31 which were approximately 16 and 7 times greater in comparison with the corresponding clearances for the interconversion of pentoxifylline and (-)-(R)-M1. When fitting pharmacokinetic data of both parent compounds to a one-compartment model, the values of elimination clearances assessed were close to those obtained on the basis of the interconversion model. After administration of pentoxifylline, tissue-to-serum AUC ratios ranged from 0.1 for liver and lungs to 0.32 for brain tissue. Serum levels of its metabolite, (-)-(R)-M1 were very low, whereas its tissue levels exceeded serum concentrations. The highest value of metabolite-to-parent AUC ratio (4.98) was observed in lungs. When (-)-(R)-M1 was given as a parent drug, tissue-to-serum AUC ratios in liver, kidney, and lungs were very close and ranged from 0.64 to 0.72. At the same time, levels of its metabolite, pentoxifylline were relatively low both in serum and all tissues studied. In consequence, metabolite-to-parent AUC ratios did not exceed the value of 0.27. In conclusion, reversible metabolism plays a modest role in the disposition of pentoxifylline and (-)-(R)-M1. It seems that pentoxifylline has less favourable pharmacokinetic properties than (-)-(R)-M1 due to lower concentrations attained in target organs. High levels of (-)-(R)-M1 observed after pentoxifylline administration in certain tissues such as liver or lungs suggest that pentoxifylline may constitute an effective prodrug for (-)-(R)-M1 in these organs.