Natural Product Biomolecules - Organic Chemistry Concepts: An EFL Approach - page 154

Hemiacetal formation gives a new chirality center which can have either of
the two possible configurations. Because of the other fixed chirality centers in
the sugar, two diastereomeric hemiacetals (anomers) are possible. As seen in
Figure 8.6 , these anomers are labeled α- or β- depending on the orientation of
the -OH group at the anomeric carbon of the hemiacetal.
FIGURE 8.6
Pyranose chair conformations of glucose.
Another feature of simple sugars is their conformation. The pyranose struc-
ture of glucose can be compared to the cyclohexane system in Section 3.3.2.
The favored chair forms of the hemiacetals in Figure 8.6 have different sta-
bilities because of the orientation of the -OH substituent at the anomeric
carbon.
In Section 7.7.1.4, we saw hemiacetals react with a second alcohol group to
give acetals. Complex carbohydrates are polymers of simple sugars which are
linked together as acetals. In carbohydrate chemistry, these are called gly-
cosides. As Figure 8.7 shows, in glycosides, the only difference in the acetal
formation is that the second alcohol function comes from a second sugar mol-
ecule ( Figure 8.7 ).
FIGURE 8.7
Sucrose glycoside ( acetal ) from glucose and fructose.
As Figure 8.8 shows, glycoside acetal bonds are easily broken by hydroly-
sis to give the simple sugar building blocks from the complex carbohydrate
polysaccharides.