Two chemical compounds are epimers when they are diastereomers, and in exactly one asymmetric carbon atom the configuration is reversed.
The term epimer is mainly used in sugar chemistry.
For example, the monosaccharides D-mannose and D-galactose are the two naturally occurring D-glucose epimers. D-mannose differs from D-glucose in the configuration around C number 2, D-galactose differs from D-glucose in the configuration around C number 4. D-galactose and D-mannose are no epimers: they differ in 2 configurations .
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