Download Carbohydrates

The stable form for the aldopentoses (ribose) and the ketohexoses (fructose) are the five membered furanose
form.
CH 2OH
Attack from above
C
O
HO
C
H
H
C
OH
H
C
OH
Attack from below
CH 2OH
CH2OH
C
CH2OH
O
CH 2OH
OH C
C
C
OH C
C
C
OH
OH
O
C
CH 2OH
OH
OH
These cyclic hemiacetals and hemiketals are semi stable compounds. A particular ring structure exists for a
brief period of time in solution and then opens to form the linear molecule only to react with itself again to
form one of the cyclic structures. In solution all forms exist in equilibrium. The rapid interconversion
between the open chain form and one of the cyclic forms is called MUTAROTATION.
When monosaccharides cyclize, the achiral carbonyl carbon (carbon 1 of aldose or carbon 2 of ketose) is
transformed into a chiral carbon. This new chiral carbon can exists in one of two configurations depending
upon the direction of attack by the hydroxyl group (from above or below the plane of the carbonyl group).
The resulting pair of stereoisomers are known as ANOMERS and the hemiacetal or hemiketal carbon atom
(formerly the carbonyl group carbon) is referred to as the ANOMERIC CARBON. α-D-Glucose and β-DGlucose are anomers of each other and carbon 1 of the ring structures is the anomeric carbon.
Converting the linear structure to the most stable cyclic structure is an easy task. Draw the pyran or furan
ring with the oxygen at the top of the ring structure for the furans and at the top right for the pyrans.
Glucose, galactose, and mannose become pyranoses; ribose and fructose become furanoses. The carbon in
the ring to the right of the oxygen is the anomeric carbon; carbon 1 for glucose, galactose, mannose, and
ribose, carbon 2 of fructose. By convention the last carbon of the D-monosaccharides is above the plane of
the ring attached to the carbon one carbon left of the ring oxygen. In the α-form of a D-sugar, the anomeric
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©Kevin R. Siebenlist, 2016