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In sterochemistry, epimer refers to one of a pair of stereoisomers. The two isomers differ in configuration at only one stereogenic center. All other stereocenters in the molecules, if any, are the same in each.

Doxorubicin and epirubicin are two closely related drugs and epimers.

Doxorubicin–epirubicin comparison

Examples

The sugars glucose and galactose are epimers. In glucose, the -OH group on the first carbon is in the axial position, the direction opposite the -OH group on carbon C-4. In galactose, the -OH group is oriented in the same direction, the equatorial position.^[1] In cyclical compounds like these, the -OH group on C-1 may lie in opposite directions as well. This structural difference distinguishes two anomers. The two molecules pictured are both epimers and anomers (as indicated by the α-glucose and β designation).


α-D-glucopyranose	β-D-glucopyranose

The stereoisomers β-D-glucopyranose and β-D-mannopyranose are epimers because they differ only in the stereochemistry at the C-2 position. The hydroxyl group in β-D-glucopyranose is equatorial (in the "plane" of the ring) while in β-D-mannopyranose the C-2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but, because not mirror images of each other, are also not enantiomers (enantiomers have the same name but differ in D and L classification). They are also not sugar anomers, since the wrong carbon is involved in the stereochemistry.


β-D-glucopyranose	β-D-mannopyranose

Other closely related compounds are epi-inositol and inositol and lipoxin and epilipoxin.


Epi-inositol	Inositol	Lipoxin	Epilipoxin

Epimerisation

Epimerisation is a chemical process where an epimer is transformed into its chiral counterpart. It can happen in condensed tannins depolymerisation reactions. Epimerisation can be spontaneous (generally a slow process), or catalyzed by enzymes, e.g. the epimerization between the sugars N-acetylglucosamine and N-acetylmannosamine, which is catalyzed by Renin-Binding Protein.

References

^ Structure of the glucose molecule

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1. アスリートにおけるアンドロゲンおよびその他のホルモンの使用 use of androgens and other hormones by athletes

English Journal

Synthesis and evaluation of 2,3-dinorprostaglandins: Dinor-PGD1 and 13-epi-dinor-PGD1 are peroxisome proliferator-activated receptor α/γ dual agonists.

Sato A, Dodo K, Makishima M, Hashimoto Y, Sodeoka M.SourceSodeoka Live Cell Chemistry Project, ERATO, JST, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan; RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
Bioorganic & medicinal chemistry letters.Bioorg Med Chem Lett.2013 May 15;23(10):3013-7. doi: 10.1016/j.bmcl.2013.03.024. Epub 2013 Mar 21.
2,3-Dinorprostaglandins (dinor-PGs) have been regarded as β-oxidation products of arachidonic-acid-derived prostaglandins, but their biological activities in mammalian cells remain unclear. On the other hand, C18 polyunsaturated fatty acids (PUFAs), such as γ-linolenic acid (GLA), have various bio
PMID 23566516

Highly stereoselective directed reactions and an efficient synthesis of azafuranoses from a chiral aziridine.

Lee H, Kim JH, Lee WK, Cho J, Nam W, Lee J, Ha HJ.SourceDepartment of Chemistry, Sogang University, Seoul 121-742, Korea. wonkoo@sogang.ac.kr.
Organic & biomolecular chemistry.Org Biomol Chem.2013 May 14;11(22):3629-34. doi: 10.1039/c3ob27390c.
Polyhydroxylated pyrrolidines, such as biologically important azafuranoses represented by the natural product (+)-2,5-imino-2,5,6-trideoxy-gulo-heptitol and its C(3)-epimer, were elaborated from a commercially available enantiomerically pure (2R)-hydroxymethylaziridine by highly stereoselective dire
PMID 23538672

Phosphorylation of D-allose by hexokinase involved in regulation of OsABF1 expression for growth inhibition in Oryza sativa L.

Fukumoto T, Kano A, Ohtani K, Inoue M, Yoshihara A, Izumori K, Tajima S, Shigematsu Y, Tanaka K, Ohkouchi T, Ishida Y, Nishizawa Y, Tada Y, Ichimura K, Gomi K, Yoo SD, Sheen J, Akimitsu K.SourceFaculty of Agriculture, Rare Sugar Research Center, and Gene Research Center, Kagawa University, Miki, Kagawa, 761-0795, Japan.
Planta.Planta.2013 May;237(5):1379-91. doi: 10.1007/s00425-013-1853-9. Epub 2013 Feb 9.
We previously reported that a rare sugar D-allose, which is the D-glucose epimer at C3, inhibits the gibberellin-dependent responses such as elongation of the second leaf sheath and induction of α-amylase in embryo-less half seeds in rice (Fukumoto et al. 2011). D-Allose suppresses expressions of g
PMID 23397192

Japanese Journal

Structures of enzymatic oxidation products of epigallocatechin

Matsuo Yosuke,Hayashi Toshimi,Saito Yoshinori,Kouno Isao,Tanaka Takashi
Tetrahedron 69(42), 8952-8958, 2013-10-21
… Isotheasinensin E (6) is a C-2 epimer of 5, and compounds 9 and 10 are oxidation products of 12. …
NAID 120005341848

Synthesis and Biological Activities of 1α,4α,25- and 1α,4β,25-Trihydroxyvitamin D3 and Their Metabolism by Human CYP24A1 and UDP-Glucuronosyltransferase

Sawada Daisuke,Tsukuda Yuya,Yasuda Kaori [他],Sakaki Toshiyuki,Saito Hiroshi,Takagi Ken-ichiro,Takenouchi Kazuya,Chen Tai C.,Reddy G. Satyanarayana,Kittaka Atsushi
Chemical and Pharmaceutical Bulletin 60(10), 1343-1346, 2012
A previous report has demonstrated the existence of a C4-hydroxylated vitamin D2 metabolite in serum of rats treated with pharmacological doses of vitamin D2. However, the biological significance and …
NAID 130001852369

New 6-Hydroxyeunicellins from a Soft Coral Cladiella sp.

Chen Yung-Husan,Hwang Tsong-Long,Su Yin-Di [他],Chang Yu-Chia,Chen Yu-Hsin,Hong Pei-Han,Hu Li-Chung,Yen Wei-Hsuan,Hsu Hsin-Yin,Huang Shu-Jung,Kuo Yueh-Hsiung,Sung Ping-Jyun
Chemical and Pharmaceutical Bulletin 60(1), 160-163, 2012
… The structures of eunicellins 1 and 2 were established by spectroscopic methods and 2 was found to be an epimer of the known eunicellin cladieunicellin F (3). …
NAID 130001852341