2009_Book_FoodChemistry

4.2 Monosaccharides 251
Fig. 4.1. D-Aldoses in Fischer projection
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
Hexulose
D-Fructose
D-Psicose
Heptulose
D-manno-2-Heptulose
Octulose
D-glycero-D-manno-
2-Octulose
Nonulose
D-erythro-L-gluco-
2-Nonulose
Occurrence
Present in plants and
honey
Found in residue of
fermented molasses
Avocado fruit
Avocado fruit
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 portions
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 as α and
β-forms. Formula 4.9 illustrates the two anomeric
D-glucose molecules in Tollens ring formulas and
Haworth projections.