Food Chemistry

4.2 Monosaccharides 251

Fig. 4.1.D-Aldoses inFischerprojection

and D-arabino-2-hexulose. Such nomenclature
makes it possible to systematically denote all
monosaccharides that contain more than four

Table 4.2.Occurrence of ketoses

Name, structure Occurrence

Hexulose
D-Fructose Present in plants and
honey
D-Psicose Found in residue of
fermented molasses
Heptulose
D-manno-2-Heptulose Avocado fruit
Octulose
D-glycero-D-manno-
2-Octulose Avocado fruit
Nonulose
D-erythro-L-gluco-
2-Nonulose Avocado fruit

chiral centers. According to this procedure the

portion of the molecule adjacent to the carbonyl

group is given the maximal possible prefix, while

the portion furthest from the carbonyl group is

denoted first. In the case of ketoses, the two por-

tions of the molecule separated by the keto group

are given. In a combined prefix designation, as

with aldoses, the portion which has the C-atom

furthest from the keto group is mentioned first.

However, when a monosaccharide does not have

more than four chiral centers, a designation in the

ketose series may omit the two units separated

by the keto group. The examples in Formula 4.8

illustrate the rule.

Lactol formation provides a new chiral center.

Thus, there are two additional diastereomers

for each pyranose and furanose. These isomers

are called anomers and are denoted asαand

β-forms. Formula 4.9 illustrates the two anomeric

D-glucose molecules inTollensring formulas and

Haworthprojections.