Furanose sugars, anomeric carbon

Five-carbon sugars, such as D-ribose (Topic Gl) and D-deoxyribose (Topic FI), and six-carbon ketose sugars (ketohexoses), such as D-fructose, form rings called furanoses (Fig. 6a) by comparison with the compound furan (Fig. 6b). Again furanoses can exists in both a and (5 forms (Fig. 6a) except here the nomenclature refers to the hydroxyl group attached to C-2 which is the anomeric carbon atom. 

There is an anomeric carbon, resulting from the equilibrium between the acychc form and the four cyclic forms of a sugar monomer in solution. These cychc forms are comprised of a five-membered furanose ring and the generally more abundant six-membered pyranose ring. Mutorotation around the anomeric centre converts the a-form into the P-form. 

The anomeric carbon atom, C-l, in pyranose and furanose sugars is unique in that it is the only carbon atom in these molecules which is bonded to two atoms which are more electronegative. These more-electronegative atoms necessarily have lone-pair electrons. It is the electronic structure which arises from the electronegativity differences and the presence of these lone-pair electrons that gives rise to the special reactivity and structural properties of the anomeric carbon center. 

When sugars cyclize, they typically form furanose or pyranose structures (Figure 9.10). These are molecules with five-membered or six-membered rings, respectively. Cyclization creates a carbon with two possible orientations of the hydroxyl around it. We refer to this carbon as the anomeric carbon and the two possible forms as anomers. The two possible configurations of the hydroxyl group are called ot and which correspond to the hydroxyl being in the "down" and "up" positions, respectively, in standard projections (see here)... 

The cyclic hemiacetal (or hemiketal) can react with an alcohol to form an acetal (or ketal), called a glycoside. If the name pyranose or furanose is used, the acetal is called a pyranoside or a furanoside. The bond between the anomeric carbon and the alkoxy oxygen is called a glycosidic bond. The preference for the axial position by certain substituents bonded to the anomeric carbon is called the anomeric effect. If a sugar has an aldehyde, ketone, hemiacetal, or hemiketal group, it is a reducing sugar. 

The anomeric carbon of each sugar is bonded to a nitrogen of a heterocyclic compound in a j8-glycosidic linkage. (Recall from Section 21.10 that a j8-linkage is one in which the substituents at C-1 and C-4 are on the same side of the furanose ring.) Because the heterocyclic compounds are amines, they are commonly referred to as bases. 

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