Instereochemistry, anepimer is one of a pair ofdiastereomers.[1] The two epimers have oppositeconfiguration at only onestereogenic center out of at least two.[2] All other stereogenic centers in the molecules are the same in each.Epimerization is the interconversion of one epimer to the other epimer.
The stereoisomers β-D-glucopyranose and β-D-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 isaxial (up from the "plane" of the ring). These two molecules are epimers but, because they are not mirror images of each other, are notenantiomers. (Enantiomers have the same name, but differ inD andL classification.) They are also not sugaranomers, since it is not theanomeric carbon involved in the stereochemistry. Similarly, β-D-glucopyranose and β-D-galactopyranose are epimers that differ at the C-4 position, with the former being equatorial and the latter being axial.
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β-D-glucopyranose | β-D-mannopyranose |
In the case that the difference is the -OH groups on C-1, the anomeric carbon, such as in the case of α-D-glucopyranose and β-D-glucopyranose, the molecules are both epimers and anomers (as indicated by theα andβ designation).[3]
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α-D-glucopyranose | β-D-glucopyranose |
Other closely related compounds areepi-inositol andinositol, andlipoxin andepilipoxin.
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epi-inositol | Inositol | Lipoxin | Epilipoxin |
Doxorubicin andepirubicin are two epimers that are used as drugs.
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Doxorubicin–epirubicin comparison |
Epimerization is a chemical process where an epimer is converted to its diastereomeric counterpart.[1] It can happen incondensed tannins depolymerization reactions. Epimerization can be spontaneous (generally a slow process), or catalysed by enzymes, e.g. the epimerization between the sugarsN-acetylglucosamine andN-acetylmannosamine, which is catalysed byrenin-binding protein.
The penultimate step in Zhang & Trudell's classicepibatidine synthesis is an example of epimerization.[4] Pharmaceutical examples include epimerization of theerythro isomers ofmethylphenidate to the pharmacologically preferred and lower-energy threo isomers, and undesiredin vivo epimerization oftesofensine tobrasofensine.