2009_Book_FoodChemistry

250 4 Carbohydrates
Correspondingly, L-erythrose and L-threose are
obtained from L-glyceraldehyde:
(4.5)
The nitriles can also be directly reduced to diastereomeric
aldoses with PdO/BaSO 4 , by passing
the lactone intermediate stage. Another reaction
for the formation of monosaccharides is the
nitroalkane synthesis. The epimeric nitro compounds,
obtained by the reaction of an aldose with
nitromethane as anions, are separated and converted
to the corresponding aldoses by an acinitroalkane
cleavage (Nef -reaction):
Figure 4.1 shows the formulas and names for
D-aldoses using simplified Fischer projections.
The occurrence of aldoses of importance in food
is compiled in Table 4.1. Epimers are monosaccharides
which differ in configuration at only
one chiral C-atom. D-Glucose and D-mannose
are 2-epimers. D-glucose and D-galactose are
4-epimers.
The enantiomers D- andL-tetrulose, by formally
inserting additional CHOH-groups between the
keto and existing CHOH-groups, form a series of
D- andL-2-ketoses. Figure 4.2 gives D-2-ketoses
in their simplified Fischer projections.
Data are provided in Table 4.2 on the occurrence
of ketoses of interest in food.
For the simplified presentation of structures,
abbreviations are used which usually consist of
the first letters of the name of the monosaccharide.
Figure 4.1 gives the configuration prefix
derived from the trivial names, representing
a specified configuration applied in monosaccharide
classification. Thus, systematic names
for D-glucose and D-fructose are D-gluco-hexose
Table 4.1. Occurrence of aldoses
Name, structure
Occurrence
(4.6)
After repeated cyanhydrin reactions, four tetroses
will provide a total of eight pentoses (each tetrose
provides a pair of new diastereomers with
one more chiral center), which can then yield sixteen
stereoisomeric hexoses. The compounds derived
from D-glyceraldehyde are designated as
D-aldoses and those from L-glyceraldehyde as
L-aldoses.
An important degradation reaction of aldoses proceeds
via the disulfone of the dithioacetal:
(4.7)
Pentoses
D-Apiose (3-C-Hydroxy- Parsley, celery seed
methyl-D-glycerotetrose)
L-Arabinose
Plant gums, hemicelluloses,
pectins, glycosides
2-Deoxy-D-ribose Deoxyribonucleic acid
D-Lyxose
Yeast-nucleic acid
2-O-Methyl-D-xylose Hemicelluloses
D-Ribose
D-Xylose
Ribonucleic acid
Xylanes, hemicelluloses
plant gums, glycosides
Hexoses
L-Fucose (6-Deoxy- Human milk, seaweed
L-galactose) (algae),
plant gums and mucilage
D-Galactose
Widespread in oligoand
polysaccharides
D-Glucose
Widespread in plants and
animals
D-Mannose
Widespread as polysaccharide
building blocks
L-Rhamnose (6-Deoxy- Plant gums and mucilage,
L-mannose) glycosides