what is a diastereomer
The exact term for stereoisomers that are not optical isomers is diastereomers. A special kind of diastereomer is an epimer. Epimers are diastereomers that differ at one of several asymmetric carbon atoms. There is also something called an anomer, a special type of epimer. An anomer differs at a new asymmetric carbon atom when a ring is formed (in carbohydrate chemistry).