General Index


Cyclic Forms of Monosaccharides


The aldehyde group of aldoses can form an internal hemiacetal with one of the alcoholic groups -OH of the molecule. In general, this is the usual form of pentoses and hexoses. In D-Glucose the aldehyde form the hemiacetal with carbon 5, resulting in a structure called Pyranose due to its similarity to the cyclic ether Pyran.

When the cycle forms there appears a new asymmetrical center in carbon 1, giving two isomers called a, if C1 acquires the configuration of D-Glyceraldehyde and b if it acquires that of L-Glyceraldehyde. In this case, carbon 1 is known as Anomeric carbon.

The structure of a-D-Glucopyranose is:




In the Fischer projection, a-D-glucopyranose is represented as follows, with the -OH substituting the anomeric carbon to the right:

In general we prefer for the cyclic forms the Haworth perspective, in which the pyranose ring is represented as an hexagon in perspective. Those substituents that appear to the right in the Fischer projection are represented under the plane of the ring; and those that appear to the left, are represented above the plane of the ring. In the Haworth perspective, a-D-glucopyranose is represented as follows:

Note that the Haworth representation is a better reproduction than Fischer's, being closer to the real shape of the molecule. In general, Pentoses and Hexoses are always represented as Haworth perspectives of their cyclic forms.

The structure of b-D-Glucopyranose is:




In the Fischer projection, the -OH substituting the anomeric carbon appears to the left:

In the Haworth perspective, -OH appears above the plane of the pyranose ring:




Ketohexoses can also form an internal hemiacetal with the alcoholic groups of the same molecule. In the case of ketohexoses, the anomeric carbon will be carbon 2 and forms a hemiacetal bridge with the carbon 5. Then it results a cycle with five atoms, four carbons and one oxygen. These cyclic forms are called Furanoses because of their similarity with the cyclic ether Furan. As in the case of the aldoses, there appears a new asymmetric center in the anomeric carbon (C2), resulting in a and b forms. Let's see the structure of a-D-Fructofuranose:




In the Fischer projection, a-D-Fructofuranose is represented as follows:

Note that the -OH group of the anomeric carbon appears to the right:

And in the Haworth perspective (the usual representation):


As in the case of D-Glucopyranose, D-Fructofuranose presents a b anomer, b-D-Fructofuranose:




In the Fischer projection, the -OH of the anomeric carbon appears to the left:

And in the Haworth perspective, appears above the plane of the ring:



Aldopentoses present also cyclic forms of the furanose type, with the hemiacetal bridge established between carbons 1 (anomeric) and 4. They also present anomeric forms a and b. The structure of a-D-Ribofuranose is:




That in the Haworth perspective is represented as:


The structure of b-D-Ribofuranose, is:




In the Haworth perspective:





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