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EPIMERS

IUPAC definition: Diastereoisomers that have the opposite configuration at only one of two
or more tetrahedral stereogenic centres present in the respective molecular
entities.

wikipedia definition:In chemistry, an epimer is a stereoisomer of another compound that has a different configuration at only one of several stereogenic centers. Stereoisomers include enantiomers and diastereomers, both which contain a stereogenic center (excluding geometric isomers, which is a class of diastereomers).

For example, the sugars ?-glucose and ?-glucose are epimers. In ?-glucose, the -OH group on the first (anomeric) carbon is in the direction opposite the methylene group. In ?-glucose, the -OH group is oriented in the same direction as the methylene group ^[1]. These two molecules are both epimers and anomers.

In this case ?-D-glucopyranose and ?-D-mannopyranose are epimers because the differ only in the stereochemistry at the 2 position. The hydroxyl group in ?-D-glucopyranose is equatorial (in the "plane" of the ring) while in ?-D-mannopyranose the 2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but not anomers.

In chemical nomenclature one of the epimeric pairs is given the prefix epi- for example in quinine and epi-quinine. When the pairs are enantiomers the prefix becomes ent-.

ANOMERS

In sugar chemistry, an anomer is a special type of epimer. It is a stereoisomer (diastereomer, more exactly) of a saccharide (in the cyclic form) that differs only in its configuration at the hemiacetal (or hemiketal) carbon, also called the anomeric carbon. If the structure is analogous to one with the hydroxyl group on the anomeric carbon in the axial position of glucose, then the sugar is an alpha anomer. If, however, that hydroxyl is equatorial, the sugar is a beta anomer. For example, ?-D-glucopyranose and ?-D-glucopyranose, the two cyclic forms of glucose, are anomers.

The ?- and ?-anomers of D-glucopyranose.

The anomeric effect helps stabilize the ?-anomer. The term for interconversion between the two anomers is mutarotation.

The two different anomers are two distinct chemical structures, and thus have different physical and chemical properties, notably optical rotation. For example, ?-D-glucose has an optical rotation of +112 degrees and its anomer, ?-D-glucose, has an optical rotation of +19 degrees.

SOURCE:WIKIPEDIA

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