Epimers

In the previous post, we listed most of the naturally occurring D aldoses and ketoses together with their enantiomeric L isomers. As a reminder, all the stereogenic centers are inverted when comparing D and L isomers since they are enantiomers and any other pair of stereoisomers represents diastereomers. For example, while the D and L-Glucoses are enantiomers, D-Glucose and D-mannose are diastereomers since the configuration of only one stereogenic center (C2) is changed: 

 

 

Now, diastereomers that differ in the configuration of only one chiral center are called epimers. Of course, there should be more than one chiral center, otherwise, the change of one would indicate a pair of enantiomers.

So, D-Glucose and D-mannose are epimers and to specify, we can say that they are epimeric at carbon-2.

On the other hand, D-glucose and D-galactose are epimeric at carbon-4 since that is the only stereogenic center with an opposite configuration:

 

 

 

Anomers

Carbohydrates exist also in a cyclic form and this is especially favored when 5- 6-membered rings can be formed. These rings are classified as a furanose and a pyranose ring respectively which is a general nomenclature for oxygen-containing 5– 6-membered rings.

 

 

They are formed through an intramolecular hemiacetal formation. For example, the linear D-Glucose converts into a pyranose ring through the attack of C5-OH group on the carbonyl forming a new asymmetric center. Interestingly, this new asymmetric center is formed in both configurations:

 

 

Carbon 1 is the new stereogenic center shown with a wiggly line which means the formation of both configurations.

Now, the structure on the right is the cyclic form of D-Glucose which, as a six-membered ring adopts a chair conformation. I know that it is not evident that these two structures represent the same molecule, but for now, take it as a fact, and we will describe this process for converting between Fischer, Haworth, and Chair forms of Glucose in the next post.

 

 

Notice that all the initial chiral centers remain intact, and the two cyclic forms differ in the configuration of only one chiral center. We classify these as epimers, and in the case of sugars, they are said to be anomers. So, an anomer is a type of epimer characterized by the carbon in two possible configurations of a cyclic saccharide.

This carbon is called the anomeric carbon (carbon 1 in the picture above) and the configuration about it is denoted by prefixes ɑ – and β.

 

If the newly formed OH group on the asymmetric center is pointing down (trans to CH₂OH group at C-5), then it is ɑ-D-glucose. On the other hand, if the OH group is pointing up (cis to the CH₂OH group at C-5), then the hemiacetal is β-D-glucose.

 

 

Keep in mind that “up” and “down” is always ambiguous unless we specify what it is relative to. For example, if we flip the chairs 180^o (not a ring-flip), all the “up” and “down” notation will change. That is why the cis and trans orientation of the Oh group to the CH₂OH on carbon 5 is emphasized.

One statement that is true regardless of the way we draw the chair is that all the OH groups in β -D-glucose are equatorial.

 

Does Glucose Exist in forms of Enantiomers or Diastereomers?

Yes, we stated that D and L isomers are enantiomers since all the chiral centers have opposite configurations. This is true, however, when dealing with cyclic forms of sugars, you need to keep in mind epimers which are diastereomers that differ in the configuration of only one chiral center. And if these diastereomers are cyclic hemiacetals like sugars are, then they are classified as anomers. Therefore, D and L Glucose are enantiomers, while ɑ-D-glucose and β-D-glucose are diastereomers.

To summarize what we learned about epimers and anomers

• Epimers are diastereomers that differ in the configuration of only one chiral center.
• Anomers are epimers specifically applied to characterize cyclic carbohydrates.

 

Need some practice on carbohydrates?

Check this Multiple-Choice, summary quiz on the structure and reactions of carbohydrates with a 40-min video solution!

 

Check also in Carbohydrates

• Carbohydrates – Structure and Classification
• Erythro and Threo
• D and L Sugars
• Aldoses and Ketoses: Classification and Stereochemistry
• Epimers and Anomers
• Converting Fischer, Haworth, and Chair forms of Carbohydrates
• Mutarotation
• Glycosides
• Isomerization of Carbohydrates
• Ether and Ester Derivatives of Carbohydrates
• Oxidation of Monosaccharides
• Reduction of Monosaccharides
• Kiliani–Fischer Synthesis
• Wohl Degradation