7 junio, 2024

Anomeric carbon: what it is, hemiacetals, characteristics, examples

What is anomeric carbon?

He anomeric carbon it is a stereocenter present in the cyclic structures of carbohydrates (mono or polysaccharides). Being a stereocenter, more precisely an epimer, two diastereoisomers derive from it, designated with the letters α and β, which are the anomers, and are part of the extensive nomenclature in the world of sugars.

Each anomer, α or β, differs in the position of the OH group of the anomeric carbon with respect to the ring, but in both, the anomeric carbon is the same, and it is located in the same place in the molecule. Anomers are cyclic hemiacetals, the product of an intramolecular reaction in the open chain of sugars, be they aldoses (aldehydes) or ketoses (ketones).

The upper image shows the chair conformation for β-D-glucopyranose. As can be seen, it consists of a six-membered ring, including an oxygen atom between carbons 5 and 1. The latter, or rather the first, is the anomeric carbon, which forms two single bonds with two oxygen atoms. .

If you look closely, the OH group attached to carbon 1 is oriented above the hexagonal ring, as is the CH2OH group (carbon 6). This is the β anomer. The α-anomer, on the other hand, would differ only in this OH group, which would sit down the ring, as if it were a trans diastereoisomer.

hemiacetals

It is necessary to delve a little deeper into the concept of hemiacetals to better understand and distinguish the anomeric carbon. Hemiacetals are the product of a chemical reaction between an alcohol and an aldehyde (aldoses) or a ketone (ketoses).

This reaction can be represented by the following general chemical equation:

ROH + R’CHO => ROCH(OH)R’

As can be seen, an alcohol reacts with an aldehyde to form the hemiacetal. In the event that both R and R’ belong to the same chain, we would have a cyclic hemiacetal, and the only possible way that it can be formed is that both functional groups -OH and -CHO are present in the molecular structure.

In addition, the structure must consist of a flexible chain, and with bonds capable of facilitating the nucleophilic attack of the OH towards the carbonyl carbon of the CHO group. When this happens, the structure closes into a ring of five or six members.

cyclic hemiacetal

An example of the formation of a cyclic hemiacetal for glucose monosaccharide is shown in the top image. It can be seen that it consists of an aldose, with an aldehyde group CHO (carbon 1). This is attacked by the OH group on carbon 5, as indicated by the red arrow.

The structure goes from being an open chain (glucose), to a pyranous ring (glucopyranose). At first there may be no relationship between this reaction and the one just explained for the hemiacetal. But if the ring is carefully observed, specifically in the C5-O-C1(OH)-C2 section, it will be appreciated that this corresponds to the expected skeleton for a hemiacetal.

Carbons 5 and 2 come to represent R and R’ of the general equation, respectively. Since these are part of the same structure, it is then a cyclic hemiacetal (and the ring is enough to be evident).

Characteristics of anomeric carbon and how to recognize it

In glucose, this is the CHO group, which can be nucleophilically attacked by OH, either from below or from above. Depending on the orientation of the attack, two different anomers are formed: the α and β, as already mentioned.

A first characteristic that this carbon possesses is that in the open chain of the sugar it is the one that suffers the nucleophilic attack, that is, it is the CHO group, for aldoses, or the R2C=O group, for ketoses. However, once the cyclic hemiacetal or ring is formed, this carbon may appear to have disappeared.

It is here where there are other more specific characteristics to locate it in any pyranous or furanous ring of any carbohydrate:

The anomeric carbon is always to the right or left of the oxygen atom that makes up the ring.
Even more important, it is linked not only to this oxygen atom, but also to the OH group, coming from CHO or R2C=O.
It is asymmetric, that is, it has four different substituents.

With these characteristics, it is easy to recognize anomeric carbon by observing any “sweet structure”.

examples

Example 1

Above is β-D-fructofuranose, a cyclic hemiacetal with a five-membered ring.

To identify the anomeric carbon, one must first look at the carbons to the left and right of the oxygen atom that makes up the ring. Then, the one that is linked to the OH group is the anomeric carbon, which in this case is already circled in red.

This is the β anomer because the OH of the anomeric carbon is above the ring, as is the CH2OH group.

Example 2

Now, we try to explain which are the anomeric carbons in the structure of sucrose. As can be seen, it consists of two monosaccharides covalently linked by a glycosidic bond, -O-.

The ring on the right is exactly the same as just mentioned: β-D-fructofuranose, only that it is «flipped» to the left. The anomeric carbon remains the same for the previous case, and meets all the characteristics that would be expected of it.

On the other hand, the ring on the left is α-D-glucopyranose.

Repeating the same procedure of recognition of the anomeric carbon, looking at the two carbons on the left and right side of the oxygen atom, it is found that the right carbon is the one that is linked to the OH group, which participates in the glycosidic bond.

Therefore, both anomeric carbons are connected by the —O— bond, and that is why they are enclosed in red circles.

Example 3

Finally, it is proposed to identify the anomeric carbons of two glucose units in cellulose. Again, the carbons around the oxygen within the ring are observed, and it is found that in the glucose ring on the left the anomeric carbon participates in the glycosidic bond (circled in red).

In the glucose ring on the right, however, the anomeric carbon is to the right of the oxygen, and is easily identified because it is linked to the oxygen of the glycosidic bond. Thus, both anomeric carbons are fully identified.

References

Carey, F. Organic Chemistry. Mc Graw Hill.
Chang, S. A guide to the anomeric carbon: What is an anomeric carbon? Retrieved from chem.ucla.edu.
Gunawardena, G. Anomeric carbon. Retrieved from chem.libretexts.org.

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