Natural Product Biomolecules - Organic Chemistry Concepts: An EFL Approach - page 153

FIGURE 8.4
Ketose to aldose tautomerism.
The oxidation of the carbonyl group to a carboxylic acid causes the reduction
of the oxidizing agent. This is the reason that monosaccharides are often called
reducing sugars.
The carbonyl groups in sugars can be reduced by the hydride reagents shown in
Section 7.7.1.3. The alcohol groups react in the expected ways such as the ether
formation, seen in Section 7.6, or esterification, seen in Section 7.8.2.
Sugars can be in open chain or cyclic structures. As Figure 8.5 shows, these struc-
tures come from reversible nucleophilic addition of the alcohol and carbonyl
groups in the sugar to give hemiacetals. This reaction is an intramolecular ver-
sion of the earlier intermolecular one. The only difference from the earlier exam-
ples in Section 7.7.1.4 is that both reacting species are in the same molecule.
Six-membered pyranose and five-membered furanose cyclic hemiacetals are the
most easily made. The sugar molecules are in equilibrium between the open
chain and cyclic structures. Details are given in Program 25.
FIGURE 8.5