Cyclic Structures of Monosaccharides

The representation of monosaccharide molecules as open-chain structures is only partially correct. In solution, the molecules with five- and six carbon atoms can also appear in cyclic forms which are in equilibrium with the open-chain structures. The cyclization process represents the intramolecular nucleophilic attack of the hydroxyl group on the aldehyde or keto group. 

Anomers also appear in cyclic form as five-membered rings, for instance in the monosaccharides fructose and ribose. These cyclic molecules are named furanoses because of their similarities to the cyclic ether furan. Starting from D-fructose and D-ribose, the cyclization yields anomers a- and p-D-fructofuranose, as well as a-and P-D-ribofuranose, respectively. 

In analogy with pyranoses, the furanoses are also in equilibrium with the open-chain forms. The new functionality which appears in the cyclic forms is hemiacetal, the structure already mentioned in our discussions on alcohol addition to the carbonyl group. The hemiacetals can easily undergo hydrolysis yielding the open chain monosaccharide molecule. 

Representation of the cyclic forms could be simplified by using the special shorthand notation called the Haworth formula (in the honor of Sir Norman Haworth). Fischer formulas can be easily transformed into Haworth formulas by the rules given in the following scheme. 

Chiral carbon atoms that do not contribute to the formation of the cyclic form have substituents attached labeled in blue. In D-glucopyranose these atoms are numbered as 2, 3 and 4 while in D-fructofuranose these atoms are 3 and 4. The rale for writing Haworth formulas says that the substiments below the plane of the ring in Haworth projections are equivalent to those on the right-hand side of the corresponding Fischer projection (shown in blue). 

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Haworth projection common way of representing the cyclic structure of monosaccharides using a three-dimensional perspective. 

Monosaccharide Stereoisomers Cyclic Structure of Monosaccharides Reactions of Monosaccharides Important Monosaccharides... 

Section 22.10 Cyclic Structure of Monosaccharides Hemiacetal Formation... 

Pyranose and Furanose Names Cyclic structures of monosaccharides are named according to their five- or six-membered rings. A six-membered cyclic hemiacetal is called a pyranose, derived from the name of the six-membered cyclic ether pyran. A five-membered cyclic hemiacetal is called a furanose, derived from the name of the five-membered cyclic ether ran. The ring is still numbered as it is in the sugar, not beginning with the heteroatom as it would be in the heterocyclic nomenclature. These structural names are incorporated into the systematic names of sugars. 

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