Anomers are stereoisomers of cyclic sugars that differ in configuration only at the hemiacetal or hemiketal carbon.

Glucose is both an aldehyde at C-1 and an alcohol at C-5. These two groups can react with each other to form a cyclic hemiacetal (glucopyranose).

In the hemiacetal, C-1 has become chiral. It can have the OH group either “down” or “up”. These two isomers are anomers, and C-1 (the original carbonyl carbon) is the anomeric carbon.

Fructose is both a ketone at C-2 and an alcohol at C-5. These two groups can react with each other to form a cyclic hemiketal (fructofuranose).

In the hemiketal, the anomeric carbon, C-2, has become chiral. It can have the OH group either “down” or “up”. These two isomers are also anomers.