Individual Enantiomers Research Articles

Total syntheses of cis-cyclopropane fatty acids: dihydromalvalic acid, dihydrosterculic acid, lactobacillic acid, and 9,10-methylenehexadecanoic acid.

cis-Cyclopropane fatty acids (cis-CFAs) are widespread constituents of the seed oils of subtropical plants, membrane components of bacteria and protozoa, and the fats and phospholipids of animals. We describe a systematic approach to the synthesis of enantiomeric pairs of four cis-CFAs: cis-9,10-methylenehexadecanoic acid, lactobacillic acid, dihydromalvalic acid, and dihydrosterculic acid. The approach commences with Rh2(OAc)4-catalyzed cyclopropenation of 1-octyne and 1-decyne, and hinges on the preparative scale chromatographic resolution of racemic 2-alkylcycloprop-2-ene-1-carboxylic acids using a homochiral Evan's auxiliary. Saturation of the individual diastereomeric N-cycloprop-2-ene-1-carbonylacyloxazolidines, followed by elaboration to alkylcyclopropylmethylsulfones, allowed Julia-Kocienski olefination with various ω-aldehyde-esters. Finally, saponification and diimide reduction afforded the individual cis-CFA enantiomers.

ChemInform Abstract: Quinap and Congeners: Atropos PN Ligands for Asymmetric Catalysis

AbstractReview: 75 refs.

Stereoselective quantitation of haloxyfop in environment samples and enantioselective degradation in soils

The chiral separation of haloxyfop enantiomers was first performed on (R, R) Whelk-O1 chiral column (pirkle type) by high-performance liquid chromatography (HPLC). Chromatographic conditions such as mobile phase composition and column temperature were optimized, and the best resolution was obtained using hexane/n-propanol (98/2) with Rs value of 3.43. Chiral residue analysis methods for haloxyfop enantiomers in environmental matrices, such as soil and water, were developed with recoveries ranging from 85.95% to 104.25%. The results showed that these methods were effective enough for detecting the residual enantiomers environmental matrices. The behavior of haloxyfop in four soils was studied and the enantioselective degradation was found with enantiomer fraction values ranging from 0.058 to 0.61. The research work was extremely useful for investigating the fate of individual enantiomers in environment, the mechanism of the stereoselective behaviors, and the risk assessment of chiral pesticide.

Synthesis, X-ray crystal structure and antimycobacterial activity of enantiomerically pure 1-ethyl-2,3-dicyano-5-(het)aryl-6-hetaryl-1,6-dihydropyrazines

The Petasis reaction of 6-alkoxy adducts of 1-alkyl-2,3-dicyano-5-arylpyrazinium salts with aromatic boronic acids, such as 2-thienylboronic, 2-furanylboronic and 3-thienylboronic acids, or their benzo analogs in dichloromethane proceeds smoothly at room temperature with the formation of the corresponding 5-aryl-6-hetaryl substituted 1,6-dihydropyrazine derivatives. All dihydropyrazines were separated as pure enantiomers by chiral HPLC, and their absolute configurations for each pair of enantiomers have been determined by X-ray analysis. Individual enantiomers were screened in vitro for their antimycobacterial activities against Mycobacterium tuberculosis H37Rv, avium, terrae and extensively drug-resistant and multi-drug-resistant strains isolated from tuberculosis patients in Ural region (Russia). It has been shown that several compounds exhibit a good level of antituberculosis activity compared to the reference drugs.

Revision of the classical dopamine D2 agonist pharmacophore based on an integrated medicinal chemistry, homology modelling and computational docking approach.

The scientific advances during the 1970ies and 1980ies within the field of dopaminergic neurotransmission enabled the development of a pharmacophore that became the template for design and synthesis of dopamine D2 agonists during the following four decades. A major drawback, however, is that this model fails to accommodate certain classes of restrained dopamine D2 agonists including ergoline structures. To accommodate these, a revision of the original model was required. The present study has addressed this by an extension of the original model without compromising its obvious qualities. The revised pharmacophore contains an additional hydrogen bond donor feature, which is required for it to accommodate ergoline structures in a low energy conformation and in accordance with the steric restrictions dictated by the original model. The additional pharmacophore feature suggests ambiguity in the binding mode for certain compounds, including a series of ergoline analogues, which was reported recently. The ambiguity was confirmed by docking to a homology model of the D2 receptor as well as by pharmacological characterization of individual enantiomers of one of the analogues. The present research also addresses the potential of designing ligands that interact with the receptor in a large, distal cavity of the dopamine D2 receptor that has not previously been studied systematically. The pharmacological data indicate that this area may be a major determinant for both the dopamine D2 affinity and efficacy, which remains to be explored in future studies.

Identification of neuropeptide S antagonists: structure-activity relationship studies, X-ray crystallography, and in vivo evaluation.

Modulation of the neuropeptide S (NPS) system has been linked to a variety of CNS disorders such as panic disorder, anxiety, sleeping disorders, asthma, obesity, PTSD, and substance abuse. In this study, a series of diphenyltetrahydro-1H-oxazolo[3,4-α]pyrazin-3(5H)-ones were synthesized and evaluated for antagonist activity at the neuropeptide S receptor. The absolute configuration was determined by chiral resolution of the key synthetic intermediate, followed by analysis of one of the individual enantiomers by X-ray crystallography. The R isomer was then converted to a biologically active compound (34) that had a Ke of 36 nM. The most potent compound displayed enhanced aqueous solubility compared with the prototypical antagonist SHA-68 and demonstrated favorable pharmacokinetic properties for behavioral assessment. In vivo analysis in mice indicated a significant blockade of NPS induced locomotor activity at an ip dose of 50 mg/kg. This suggests that analogs having improved drug-like properties will facilitate more detailed studies of the neuropeptide S receptor system.

Enantioselective HPLC determination and pharmacokinetic study of secnidazole enantiomers in rats.

Secnidazole is a long-lasting nitroimidazole antimicrobial agent that is used as racemic mixture in clinical settings. We developed and validated an enantioselective high-performance liquid chromatography method to determine secnidazole enantiomers in rat plasma. Secnidazole enantiomers and S-(-)-ornidazole (internal standard) were extracted from 50 μL of rat plasma using diethyl ether-dichloromethane (3:2, v/v). Baseline resolution (Rs=2.45) was achieved within 7.0 min on a Chiral-AGP column (150 mm × 4.0mm, 5 μm) at 20°C. The mobile phase consisted of 10mM ammonium acetate-methanol (96:4, v/v) and was delivered at a flow rate of 0.5 mL/min with ultraviolet detection at 318 nm. The method was linear over the concentration range 0.500-100 μg/mL for both enantiomers. The lower limit of quantification was 0.500 μg/mL for both enantiomers. The relative standard deviation values for intra- and inter-day precision were 0.8-8.6 and 1.8-8.2% for S-(+)-secnidazole and R-(-)-secnidazole, respectively. The relative error values of accuracy ranged from -7.8 to 1.1% for S-(+)-secnidazole and from -7.3 to -0.1% for R-(-)-secnidazole. The method was successfully used to determine the pharmacokinetic properties of secnidazole enantiomers in rats after administration of the racemate and individual enantiomers. The pharmacokinetic results indicate that the disposition of secnidazole enantiomers is not stereoselective and chiral inversion and enantiomer-enantiomer interaction do not occur in rats.

THE DETERMINATION OF THE STEREOISOMERS OF A MULTIFUNCTIONAL B-ADRENOCEPTOR AGENT BY CHIRAL DERIVATIZATION AND HPLC

The reaction of chiral methylbenzyl and napthylethyl isocyanates with a β-adrenoceptor agent having two stereogenic centers and multiple reaction sites was investigated as a means of ascertaining the stereochemical composition of an individual enantiomer during chemical process development. The products of the reaction were unequivocally assigned using mass spectrometry (MS) and nuclear magnetic resonance (NMR). Both one and two equivalents of the isocyanate were found to add to the target analyte. Normal phase high-performance liquid chromatography using the chiral methylbenzyl isocyanate as the derivatizing reagent was found to give acceptable selectivity and sensitivity for the diastereomeric derivatives.

Two Potent OXE-R Antagonists: Assignment of Stereochemistry

5-Oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE) is formed by the oxidation of 5-hydroxy-6E,8Z,11Z,14Z-eicosatetraenoic acid (5-HETE), which is a major metabolite of enzymatic oxidation of arachidonic acid (AA). 5-Oxo-ETE is the most potent lipid chemoattractant for human eosinophils. Its actions are mediated by the selective OXE receptor, which is therefore an attractive target in eosinophilic diseases such as allergic rhinitis and asthma. Recently, we have reported two excellent OXE receptor antagonists that have IC50 values at low nanomolar concentrations. Each of these antagonists has a chiral center, and the isolation of the individual enantiomers by chiral high-performance liquid chromatography (HPLC) revealed that in each case one enantiomer is over 300 times more potent than the other. To unambiguously assign the stereochemistry of these enantiomers and to provide access to larger amounts of the active compounds for biological testing, we report here their total synthesis.

Quinap and congeners: atropos PN ligands for asymmetric catalysis.

Among the range of P,N-chelating ligands that have been employed in asymmetric catalysis, those relying on atropisomerism for the stability of individual enantiomers form a definable class. These APN (atropos P,N) ligands require a specific type of biaryl, with one component carrying a pendant phosphine unit, most commonly diaryl substituted, and the other bearing an sp(2)-nitrogen adjacent to the biaryl link. When substituents in the biaryl inhibit rotation about the linking bond, stable nonracemizing six-membered ring chelates can be formed. This Perspective relates the background to the initial synthesis in 1993 of Quinap, the original member of the series, and initial observations on its effectiveness in asymmetric catalysis. The current state of play in development of syntheses of this and other members of the APN ligand family is assessed, and their applications in asymmetric catalysis are presented. These include hydroboration and diboration of alkenes, 1,3-dipolar cycloadditions, alkynylation of iminium salts in a three-component (A(3)) condensation, and conjugate additions of Cu acetylides.

Enantioselective acute toxicity effects and bioaccumulation of furalaxyl in the earthworm (Eisenia foetida).

The enantioselectivities of individual enantiomers of furalaxyl in acute toxicity and bioaccumulation in the earthworm (Eisenia foetida) were studied. The acute toxicity was tested by filter paper contact test. After 48 h of exposure, the calculated LC50 values of the R-form, rac-form, and S-form were 2.27, 2.08, and 1.22 µg cm(-2), respectively. After 72 h of exposure, the calculated LC50 values were 1.90, 1.54, and 1.00 µg cm(-2), respectively. Therefore, the acute toxicity of furalaxyl enantiomers was enantioselective. During the bioaccumulation experiment, the enantiomer fraction of furalaxyl in earthworm tissue was observed to deviate from 0.50 and maintained a range of 0.55-0.60; in other words, the bioaccumulation of furalaxyl was enantioselective in earthworm tissue with a preferential accumulation of S-furalaxyl. The uptake kinetic of furalaxyl enantiomers fitted the first-order kinetics well and the calculated kinetic parameters were consistent with the low accumulation efficiency.

Contribution of carboxylesterase and cytochrome P450 to the bioactivation and detoxification of isocarbophos and its enantiomers in human liver microsomes.

Organophosphorus pesticides are the most widely used pesticides in modern agricultural systems to ensure good harvests. Isocarbophos (ICP), with a potent acetylcholinesterase inhibitory effect is widely utilized to control a variety of leaf-eating and soil insects. However, the characteristics of the bioactivation and detoxification of ICP in humans remain unclear. In this study, the oxidative metabolism, esterase hydrolysis, and chiral inversion of ICP in human liver microsomes (HLMs) were investigated with the aid of a stereoselective LC/MS/MS method. The depletion of ICP in HLMs was faster in the absence of carboxylesterase inhibitor (BNPP) than in the presence of NADPH and BNPP, with t1/2 of 5.2 and 90 min, respectively. Carboxylesterase was found to be responsible for the hydrolysis of ICP, the major metabolic pathway. CYP3A4, CYP1A2, CYP2D6, CYP2C9, and CYP2C19 were all involved in the secondary metabolism pathway of desulfuration of ICP. Flavin-containing monooxygenase (FMO) did not contribute to the clearance of ICP. The hydrolysis and desulfuration of (±)ICP, (+)ICP, and (-)ICP in HLMs follow Michaelis-Menten kinetics. Individual enantiomers of ICP and its oxidative desulfuration metabolite isocarbophos oxon (ICPO) were found to be inhibitors of acetylcholinesterases at different extents. For example, (±)ICPO is more potent than ICP (IC50 0.031μM vs. 192μM), whereas (+)ICPO is more potent than (-)ICPO (IC50 0.017μM vs. 1.55μM). Given the finding of rapid hydrolysis of ICP and low abundance of oxidative metabolites presence in human liver, the current study highlights that human liver has a greater capacity for detoxification of ICP.

Enantioselective toxicity, bioaccumulation and degradation of the chiral insecticide fipronil in earthworms (Eisenia feotida)

The enantioselective acute toxicity to earthworms of racemic fipronil and its individual enantiomers was studied. R-(−)-fipronil was approximately 1.5 times more toxic than the racemate and approximately 2 times more toxic than S-(+)-fipronil after 72 and 96h of exposure, respectively. Assays of fipronil enantiomer bioaccumulation and degradation in earthworms were conducted. The bio-concentration factors (BCFs) were slightly different between the two enantiomers. The enantiomeric fraction (EF) values in earthworms in the bioaccumulation period were approximately 0.5, which indicated there was no enantioselective bioaccumulation. In contrast, the degradation of fipronil in earthworms was enantioselective: the t1/2 values for R- and S-fipronil were 3.3 and 2.5days, respectively, in natural soil, and 2.1 and 1.4days, respectively, in artificial soil. The results of soil analyses showed that the degradation of fipronil was not enantioselective, which suggested that the enantioselectivity of fipronil in earthworms results from the organism's metabolism. The study also demonstrated that the presence of earthworms could accelerate the degradation of fipronil in soil.

Chiral HPLC for a study of the optical purity of new liquid crystalline materials derived from lactic acid

New liquid crystalline (LC) materials were prepared by derivatization of lactic acid. First compound possesses the lactic acid unit as the only chiral center and the second group of LC materials contains two chiral centers. Mesomorphic properties of both the newly synthesized LC materials were studied and the presence of the SmA*–SmC* or exhibit the twist grain boundary (TGB) phases, namely TGBA and TGBC, in a wide range of temperatures down to the room temperature was established. The potential of high-performance liquid chromatography (HPLC) applying chiral stationary phases to separate enantiomers or diastereoisomers of the synthesized LC compounds was evaluated. Two different brands of commercial chiral sorbents, Lux Amylose-2 and Chiralpak AD-3, both based on modified silica with derivatized polysaccharide, were employed in the development of separation procedures. The optimized chiral HPLC method provided a baseline separation of the individual enantiomers for the LC material containing one chiral center. In the case of the more complex compound with two asymmetric carbon atoms, where four isomers exist, partial separation was reached only using the current chiral HPLC.

Pharmacokinetics of the individual enantiomer S-(+)-ketoprofen after intravenous and oral administration in dogs at two dose levels

The pharmacokinetic of the individual S-(+)-enantiomer of ketoprofen, S-(+)-ketoprofen, after intravenous (IV) and oral (PO) administration was determined in six dogs at 1 and 3 mg/kg. Plasma concentrations were determined by high performance liquid chromatography with ultraviolet detection. The concentration–time curves were analyzed by non-compartmental methods. Steady-state volume of distribution (Vss) and clearance (Cl) of S-(+)-ketoprofen after IV administration were 0.22 ± 0.07 and 0.19 ± 0.03 L/kg, and 0.10 ± 0.02 and 0.09 ± 0.01 L/h/kg, at 1 and 3 mg/kg, respectively. Following PO administration, S-(+)-ketoprofen achieved maximum plasma concentrations of 4.91 ± 0.76 and 12.47 ± 0.62 μg/ml, at two dose levels, respectively. The absolute bioavailability after PO route was 88.66 ± 12.95% and 85.36 ± 13.90%, respectively.

Facile synthesis of enantiomerically pure 1-(5-bromo-2-chlorophenyl)-1-(4-ethoxyphenyl)ethane

A facile 7-step procedure for the synthesis of enantiomerically pure 1-(5-bromo-2-chlorophenyl)-1-(4-ethoxyphenyl)ethanes[(R)-2 and(S)-2] that started from (5-bromo-2-chlorophenyl)(4-ethoxyphenyl)methanone 3 was developed. The key step was the resolution of 2-(5-bromo-2-chlorophenyl)-2-(4-ethoxyphenyl)acetic acid 6 by crystallizations of its L- and D-menthyl esters 7 and 8 from petroleum ether to give optically pure enantiomers 9 and 10, respectively. The absolute configurations of the products were unambiguously determined by single-crystal X-ray diffractions of four key intermediates, 9, 10, 13 and 14. This procedure is characterized by inexpensiveness, scalability and ability to produce two individual enantiomers of a diarylethane with unambiguously determined absolute configurations and high enantiomeric purities.

Bioanalysis of Chiral Compounds During Drug Development Using A Tiered Approach

Significant differences in the pharmacodynamic activity and pharmacokinetic properties could exist for a pair of enantiomeric drugs. In order to evaluate the activity, toxicity, absorption, distribution, metabolism, and excretion properties of the individual enantiomers, and any potential for chiral inversion caused by the biotransformation process, chiral bioanalytical assays are necessary for individual enantiomers and/or their metabolites for in vivo samples. However, development and validation of chiral quantitative assays are highly challenging in comparison to typical nonchiral assays. Therefore, a tiered approach should be used to address specific needs arising in different scenarios of chiral drug development, including development of racemate or fixed-ratio (nonracemic) enantiomers, development of a single enantiomer, racemic switches, and quantitation of enantiomeric metabolites. The choice of a nonchiral quantitative assay, a chiral qualitative assay, or a chiral quantitative assay should be based on the development strategy and on the molecular properties of the drug candidate.

Chiral and Isotope Analyses for Assessing the Degradation of Organic Contaminants in the Environment: Rayleigh Dependence

The Rayleigh equation is frequently used to describe isotope fractionation as a function of conversion. In this article we propose to draw a parallel between isotope and enantiomeric enrichments and derive a set of conditions that allow the use of the Rayleigh approach to describe the enantiomeric enrichment-conversion dependencies. We demonstrate an implementation of the Rayleigh equation for the enantioselective enzymatic hydrolysis of Mecoprop-methyl, Dichlorprop-methyl, and dimethyl-methylsuccinate by lipases from Pseudomonas fluorescens, Pseudomonas cepacia, and Candida rugosa. The data obtained for all the studied reactions gave good fits to the Rayleigh equation, with a linear regression R(2) > 0.96. In addition to that, our analysis of four microcosm studies on the hydrolysis of the individual enantiomers of Dichloroprop methyl, Lactofen, Fenoxaprop-ethyl, and Metalaxyl reported in the literature by other research groups revealed a suitability of the Rayleigh dependence. Two dimensional plots describing the isotope fractionation versus enantiomeric enrichment are demonstrated for all studied cases. Processes not accompanied by enantiomeric enrichment (acid and base hydrolysis) and by isotope enrichment (transesterification) are demonstrated, their 2-D plots are either horizontal or vertical which can illuminate concealed degradation pathways.

Enantioselective analysis and fate of polycyclic musks in a water recycling plant in Sydney (Australia).

Synthetic polycyclic musks (PCMs) Galaxolide (HHCB), Tonalide (AHTN), Phantolide (AHDI), Traseolide (ATII) and Cashmeran (DPMI) are chiral chemicals that are commonly used in washing product industries as racemic mixtures. The major source of PCMs in municipal wastewater is from personal care and household products. Recent studies have shown that PCMs may enhance the relative toxicity of other environmental chemicals by inhibiting cellular xenobiotic defence systems. High sensitivity enantioselective analysis of these compounds enables improved characterisation of the environmental persistence and fate of PCMs, distinguishing between individual enantiomers so that a more complete understanding of environmental risks tributed by individual enantiomers may be obtained. Concentrations of PCMs through the various treatment stages of an advanced water recycling plant in Sydney were investigated to assess the removal of these chemicals. Average concentrations of HHCB, AHTN, AHDI, ATII and DPMI in influent were: 2,545, 301, 2, 5 and 33 ng L(-1), respectively. In the final effluent, AHDI, ATII and DPMI were not detected, while HHCB and AHTN were still measured at concentrations of 21 and 2 ng L(-1). No significant enantioselective transformation was detected during biological or advanced treatment processes.

Enantioselective disruption of the endocrine system by Cis‐Bifenthrin in the male mice

Bifenthrin (BF), as a chiral pyrethroid, is widely used to control field and household pests in China. At present, the commercial BF is a mixed compound containing cis isomers (cis-BF) including two enantiomers of 1R-cis-BF and 1S-cis-BF. In the present study, the two individual cis-BF enantiomers were separated by a preparative supercritical fluid chromatography. Then, four week-old adolescent male ICR mice were orally administered 1R-cis-BF and 1S-cis-BF separately daily for 3 weeks at doses of 0, 7.5 and 15 mg/kg/day, respectively. Results showed that the transcription status of some genes involved in cholesterol synthesis and transport as well as testosterone (T) synthesis in the testes were influenced by cis-BF enantiomers. Especially, we observed that the transcription status of key genes on the pathway of T synthesis including cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc) and cytochrome P450 17α-hydroxysteroid dehydrogenase (P45017α)) were selectively altered in the testis of mice when treated with 1S-cis-BF, suggesting that it is the possible reason to explain why the lower serum T concentration in 1S-cis-BF treated group. Taken together, it concluded that both of the cis-BF enantiomers have the endocrine disruption activities, while 1S-cis-BF was higher than 1R-cis-BF in mice when exposed during the puberty. The data was helpful to understand the toxicity of cis-BF in mammals under enantiomeric level.