What is "epimer"

n. (context chemistry English) any diastereoisomer that has the opposite configuration at only one of the stereogenic centres

In stereochemistry, 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 epimers that are used as drugs.

Doxorubicin–epirubicin comparison.svg

Doxorubicin–epirubicin comparison