Editing Epimer

{{Short description|One of a pair of diastereomers}}
In [[stereochemistry]], an '''epimer''' is one of a pair of [[diastereomer]]s.<ref name="Clayden">{{ cite book | last1 = Clayden | first1 = Jonathan | author-link1 = Jonathan Clayden | last2 = Greeves | first2 = Nick | last3 = Warren | first3 = Stuart | author-link3 = Stuart Warren | title = Organic Chemistry | edition = 2nd | publisher = Oxford University Press | date = 2012 | page =1112}}</ref> The two epimers have opposite [[absolute configuration|configuration]] at only one [[stereogenic center]] out of at least two.<ref>{{GoldBookRef|title=Epimers|file=E02167}}</ref> All other stereogenic centers in the molecules are the same in each. '''Epimerization''' is the interconversion of one epimer to the other epimer.

[[Doxorubicin]] and [[epirubicin]] are two epimers that are used as drugs. 
{| class="wikitable skin-invert-image" style="margin: 1em auto 1em auto"
|[[File:Doxorubicin–epirubicin comparison.svg|456px]] 
|- align="center"
|{{center|Doxorubicin–epirubicin comparison}}
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== Examples ==
The stereoisomers β-<small>D</small>-[[glucopyranose]] and β-<small>D</small>-[[Mannose|mannopyranose]] are epimers because they differ only in the stereochemistry at the C-2 position. The hydroxy group in β-D-glucopyranose is equatorial (in the "plane" of the ring), while in β-D-mannopyranose the C-2 hydroxy group is [[Cyclohexane conformation#Chair conformation|axial]] (up from the "plane" of the ring). These two molecules are epimers but, because they are not mirror images of each other, are not [[enantiomer]]s. (Enantiomers have the same name, but differ in '''<small>D</small>''' and '''<small>L</small>''' classification.) They are also not sugar [[Anomer|anomers]], since it is not the [[anomeric carbon]] involved in the stereochemistry. Similarly, β-<small>D</small>-glucopyranose and β-<small>D</small>-[[Galactose|galactopyranose]] are epimers that differ at the C-4 position, with the former being equatorial and the latter being axial.
{| class="wikitable skin-invert-image" style="margin: 1em auto 1em auto"
|[[File:Beta-D-glucopyranose.svg|200px]]||[[File:Beta-D-mannopyranose.svg|200px]]
|- align="center"
|{{center|β-<small>D</small>-glucopyranose }}||
{{center|β-<small>D</small>-mannopyranose }}
|}


In the case that the difference is the -OH groups on C-1, the anomeric carbon, such as in the case of α-<small>D</small>-glucopyranose and β-<small>D</small>-glucopyranose, the molecules are both epimers and anomers (as indicated by the '''α''' and '''β''' designation).<ref>[http://www.biotopics.co.uk/as/glucose2.html Structure of the glucose molecule<!-- Bot generated title -->]</ref>
{| class="wikitable skin-invert-image" style="margin: 1em auto 1em auto"
|[[Image:Alpha-D-glucopyranose-2D-skeletal.svg|220px]] || [[Image:Beta-D-glucopyranose-2D-skeletal.svg|220px]]
|- align="center"
|{{center|α-<small>D</small>-glucopyranose}} ||
{{center|β-<small>D</small>-glucopyranose}}
|}


Other closely related compounds are [[epi-inositol|''epi''-inositol]] and [[inositol]] and [[lipoxin]] and [[epilipoxin]].
{| class="wikitable skin-invert-image" style="margin: 1em auto 1em auto"
|[[File:Epi-inositol.svg|156px]] || [[File:Myo-inositol.svg|156px]] || [[File:Lipoxin B4.svg|200px]] || [[File:15-epi-lipoxin B4.svg|200px]]
|- align="center"
|{{center|''epi''-inositol}}
|{{center|Inositol}}  
|{{center|Lipoxin}}  
|{{center|Epilipoxin}}
|}

==Epimerization==
Epimerization is a chemical process where an epimer is converted to its diastereomeric counterpart.<ref name="Clayden"/> It can happen in [[condensed tannins depolymerization]] reactions. Epimerization can be spontaneous (generally a slow process), or catalysed by enzymes, e.g. the epimerization between the sugars [[N-acetylglucosamine|''N''-acetylglucosamine]] and [[N-acetylmannosamine|''N''-acetylmannosamine]], which is catalysed by [[RENBP|renin-binding protein]].

The penultimate step in Zhang & Trudell's classic [[epibatidine]] synthesis is an example of epimerization.<ref name="ZhangTrudell1996">{{cite journal|last1=Zhang|first1=Chunming|last2=Trudell|first2=Mark L.|title=A Short and Efficient Total Synthesis of (±)-Epibatidine|journal=The Journal of Organic Chemistry|volume=61|issue=20|year=1996|pages=7189–7191|issn=0022-3263|doi=10.1021/jo9608681|pmid=11667626}}</ref> Pharmaceutical examples include epimerization of the [[Diastereomer#Erythro_/_threo|erythro isomers]] of [[methylphenidate]] to the pharmacologically preferred and lower-energy threo isomers, and undesired ''in vivo'' epimerization of [[tesofensine]] to [[brasofensine]].

==References==
{{Reflist}}

[[Category:Stereochemistry]]