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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/04 04:23:10」(JST)

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This article is about the concept in chemistry. For a discussion of enantiomers in mathematics, see Chirality (mathematics).

(S)-(+)-lactic acid (left) and (R)-(–)-lactic acid (right) are nonsuperimposable mirror images of each other

In chemistry, an enantiomer (/ɨˈnæntɪ.ɵmər/ ə-NAN-tee-ə-mər; from the Greek ἐνάντιος, opposite, and μέρος, part or portion) is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are the same except for opposite orientation.^[1]

Organic compounds that contain a chiral carbon usually have two non-superposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form), hence this structural property is now commonly referred to as enantiomerism.

Enantiopure compounds refer to samples having, within the limits of detection, molecules of only one chirality.^[2]

Enantiomers have, when present in a symmetric environment, identical chemical and physical properties except for their ability to rotate plane-polarized light (+/−) by equal amounts but in opposite directions (although the polarized light can be considered an asymmetric medium). A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic and has zero net rotation of plane-polarized light because the positive rotation of each (+) form is exactly counteracted by the negative rotation of a (−) one.

Enantiomers of each other often show different chemical reactions with other substances that are also enantiomers. Since many molecules in the bodies of living beings are enantiomers themselves, there is often a marked difference in the effects of two enantiomers on living beings. In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiologic effects, while the other enantiomer is less active, inactive, or sometimes even responsible for adverse effects.

Owing to this discovery, drugs composed of only one enantiomer ("enantiopure") can be developed to enhance the pharmacological efficacy and sometimes do away with some side effects. An example of this kind of drug is eszopiclone (Lunesta), which is enantiopure and therefore is given in doses that are exactly 1/2 of the older, racemic mixture called zopiclone. In the case of eszopiclone, the S enantiomer is responsible for all the desired effects, though the other enantiomer seems to be inactive; while an individual must take 2 mg of zopiclone to get the same therapeutic benefit as they would receive from 1 mg of eszopiclone, that appears to be the only difference between the two drugs.

Naming conventions[edit]

Main article: Chirality (chemistry)#Naming conventions

Criterion of enantiomerism[edit]

Fischer projection of meso-tartaric acid

Most compounds that contain one or more asymmetric carbon atoms show enantiomerism. But this is not always true.

There are a few known compounds that do have asymmetric carbons, but, being non-dissymmetric with respect to the whole molecule, do not show enantiomerism. Thus, meso tartaric acid has two asymmetric carbons but samples still exhibit optically inactivity because the two halves of the molecule are equal and opposite to each other, and thus is superimposable on its geometric mirror image.

Examples[edit]

Enantiomers of mecoprop (2-(4-chloro-2-methylphenoxy)propanoic acid)

Enantiomers of citalopram. The top is (R)-citalopram and the bottom is (S)-citalopram.

An example of such an enantiomer is the sedative thalidomide. It was sold in a number of countries across the world from 1957 until 1961, when it was withdrawn from the market after being found to be a cause of birth defects.

In the herbicide mecoprop, the carboxyl group and the hydrogen atom on the central C-atom are exchanged (with the screen as plane of symmetry). After rotating one of the isomers 180 degrees (in the same plane), the two are still mirror images of each other. The mirror image of each enantiomer is superposable on the other enantiomer.

Another example is the antidepressant drugs escitalopram and citalopram. Citalopram is a racemate [1:1 mixture of (S)-citalopram and (R)-citalopram]; escitalopram [(S)-citalopram] is a pure enantiomer. The dosages for escitalopram are typically 1/2 of those for citalopram.

Enantioselective preparations[edit]

See also: chiral resolution and asymmetric synthesis

There are two main strategies for the preparation of enantiopure compounds. The first is known as chiral resolution. This method involves preparing the compound in racemic form, and separating it into its isomers. In his pioneering work, Louis Pasteur was able to isolate the isomers of tartaric acid because they crystallize from solution as crystals each with a different symmetry. A less common method is by enantiomer self-disproportionation.

The second strategy is asymmetric synthesis: the use of various techniques to prepare the desired compound in high enantiomeric excess. Techniques encompassed include the use of chiral starting materials (chiral pool synthesis), the use of chiral auxiliaries and chiral catalysts, and the application of asymmetric induction. The use of enzymes (biocatalysis) may also produce the desired compound.

Enantioconvergent synthesis is the synthesis of one enantiomer from a racemic precursor molecule utilizing both enantiomers. Thus, the two enantiomers of the reactant produce a single enantiomer of product.

Enantiopure medications[edit]

Main article: Enantiopure drug

Advances in industrial chemical processes have made it economical for pharmaceutical manufacturers to take drugs that were originally marketed as a racemic mixture and market the individual enantiomers. In some cases, the enantiomers have genuinely different effects. In other cases, there may be no clinical benefit to the patient. In some jurisdictions, single-enantiomer drugs are separately patentable from the racemic mixture.^[3] It is possible that both enantiomers are active. Or, it may be that only one is active, in which case separating the mixture has no objective benefits, but extends the drug's patentability.^[4]

Quasi-enantiomers[edit]

These molecular species are not strictly enantiomers, but they behave as if they are. Quasi-enantiomers have applications in parallel kinetic resolution (see kinetic resolution).^[5]

References[edit]

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "enantiomer".
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "enantiomerically pure (enantiopure)".
^ http://www.ema.europa.eu/ema/index.jsp?curl=pages/news_and_events/news/2009/11/news_detail_000083.jsp&jsenabled=true
^ Merrill Goozner (2004). The $800 Million Pill: The Truth Behind the Cost of New Drugs (excerpt). University of California Press. ISBN 0-520-23945-8.
^ G.S. Coumbarides, M. Dingjan, J. Eames, A. Flinn, J. Northen and Y. Yohannes, Tetrahedron Lett. 46 (2005), p. 2897er

External links[edit]

chemwiki:stereoisomerism

English Journal

Profiling of enantiopure drugs towards aryl hydrocarbon (AhR), glucocorticoid (GR) and pregnane X (PXR) receptors in human reporter cell lines.

Novotna A1, Kamenickova A1, Pecova M1, Korhonova M1, Bartonkova I1, Dvorak Z2.Author information 1Department of Cell Biology and Genetics, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic.2Department of Cell Biology and Genetics, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic. Electronic address: moulin@email.cz.AbstractIn the past decade, a large number of enantiopure drugs were introduced to clinical practice, since improved therapeutic effects were demonstrated for one of the enantiomers from originally racemic drug. While the therapeutic effects and safety of enantiopure drugs were tested prior to their approval, various biological enantiospecific activities of these, often "old" drugs, remain to be elucidated. In the current paper, we examined enantiospecific effects of clinically used enantiopure drugs containing one chiral center in the structure (i.e. zopiclone, tamsulosin, tolterodine, modafinil, citalopram) towards aryl hydrocarbon (AhR), glucocorticoid (GR) and pregnane X (PXR) receptors in human reporter cell lines. The cytotoxicity (IC50), agonist (EC50) and antagonist effects (IC50) of R-form, S-form and racemic mixture for each tested drugs were determined and compared in AhR-, GR- and PXR-gene reporter cell lines. Since AhR, GR and PXR are key regulators of drug metabolism, energy metabolism, immunity and play many other physiological functions, the data presented here might be of toxicological significance.
Chemico-biological interactions.Chem Biol Interact.2014 Feb 5;208:64-76. doi: 10.1016/j.cbi.2013.11.018. Epub 2013 Dec 6.
In the past decade, a large number of enantiopure drugs were introduced to clinical practice, since improved therapeutic effects were demonstrated for one of the enantiomers from originally racemic drug. While the therapeutic effects and safety of enantiopure drugs were tested prior to their approva
PMID 24316275

Synthesis of Bioinspired Carbohydrate Amphiphiles that Promote and Inhibit Biofilms.

Dane EL1, Ballok AE2, O'Toole GA2, Grinstaff MW1.Author information 1Departments of Chemistry and Biomedical Engineering, Boston University, Boston, MA.2Department of Microbiology & Immunology, Geisel School of Medicine at Dartmouth, Hanover, NH.AbstractThe synthesis and characterization of a new class of bioinspired carbohydrate amphiphiles that modulate Pseudomonas aeruginosa biofilm formation are reported. The carbohydrate head is an enantiopure poly-amido-saccharide (PAS) prepared by a controlled anionic polymerization of β-lactam monomers derived from either glucose or galactose. The supramolecular assemblies formed by PAS amphiphiles are investigated in solution using fluorescence assays and dynamic light scattering. Dried samples are investigated using X-ray, infrared spectroscopy, and transmission electron microscopy. Additionally, the amphiphiles are evaluated for their ability to modulate biofilm formation by the Gram-negative bacterium Pseudomonas aeruginosa. Remarkably, from a library of eight amphiphiles, we identify a structure that promotes biofilm formation and two structures that inhibit biofilm formation. Using biological assays and electron microscopy, we relate the chemical structure of the amphiphiles to the observed activity. Materials that modulate the formation of biofilms by bacteria are important both as research tools for microbiologists to study the process of biofilm formation and for their potential to provide new drug candidates for treating biofilm-associated infections.
Chemical science (Royal Society of Chemistry : 2010).Chem Sci.2014 Feb 1;5(2). doi: 10.1039/C3SC52777H.
The synthesis and characterization of a new class of bioinspired carbohydrate amphiphiles that modulate Pseudomonas aeruginosa biofilm formation are reported. The carbohydrate head is an enantiopure poly-amido-saccharide (PAS) prepared by a controlled anionic polymerization of β-lactam monomers der
PMID 24376911

The supramolecular interaction mediated chiral 1D cyanide-bridged metamagnet: synthesis, crystal structures and magnetic properties.

Zhang D, Bian Y, Qin J, Wang P, Chen X.Author information College of Chemical Engineering, Shandong University of Technology, Zibo 255049, China. dpzhang@sdut.edu.cn.AbstractTwo cyanide-bridged enantiopure one-dimensional single chain complexes, [Mn((R,R)-Salcy)Fe(pcq)(CN)3]2n·1.5nDMF (1) and [Mn((S,S)-Salcy)Fe(pcq)(CN)3]2n·1.5nDMF (2), have been synthesized and structurally characterized. Systematically magnetic investigations show the antiferromagnetic coupling between the cyanide-bridged Mn(iii)-Fe(iii) centers and the interesting metamagnetic behavior at about 5.0 K resulted from the intermolecular π-π interaction.
Dalton transactions (Cambridge, England : 2003).Dalton Trans.2014 Jan 21;43(3):945-9. doi: 10.1039/c3dt52996g. Epub 2013 Nov 25.
Two cyanide-bridged enantiopure one-dimensional single chain complexes, [Mn((R,R)-Salcy)Fe(pcq)(CN)3]2n·1.5nDMF (1) and [Mn((S,S)-Salcy)Fe(pcq)(CN)3]2n·1.5nDMF (2), have been synthesized and structurally characterized. Systematically magnetic investigations show the antiferromagnetic coupling betw
PMID 24270263

Japanese Journal

スチレンモノオキシゲナーゼを利用した光学活性エポキシアルカン化合物の生産

戸田弘,伊藤伸哉
バイオサイエンスとインダストリー 73(2), 121-125, 2015
NAID 40020391031

Influence of Surface Protonation–Deprotonation Stimuli on the Chiroptical Responses of (R)-/(S)-Mercaptosuccinic Acid-protected Gold Nanoclusters

,
Chemistry Letters 44(2), 171-173, 2015
… Racemic mixtures of mercaptosuccinic acid (MSA) are optically resolved to obtain enantiopure (R)-/(S)-MSA, and the (R)-/(S)-MSA-protected gold nanoclusters are synthesized. …
NAID 130004868335