Food Chemistry

250 4 Carbohydrates

Correspondingly,L-erythrose andL-threose are
obtained fromL-glyceraldehyde:

(4.5)

The nitriles can also be directly reduced to di-
astereomeric aldoses with PdO/BaSO 4 , by pass-
ing the lactone intermediate stage. Another reac-
tion for the formation of monosaccharides is the
nitroalkane synthesis. The epimeric nitro com-
pounds, obtained by the reaction of an aldose with
nitromethane as anions, are separated and con-
verted to the corresponding aldoses by an acini-
troalkane cleavage (Nef-reaction):

(4.6)

After repeated cyanhydrin reactions, four tetroses
will provide a total of eight pentoses (each tet-
rose provides a pair of new diastereomers with
one more chiral center), which can then yield six-
teen stereoisomeric hexoses. The compounds de-
rived fromD-glyceraldehyde are designated as
D-aldoses and those fromL-glyceraldehyde as
L-aldoses.
An important degradation reaction of aldoses pro-
ceeds via the disulfone of the dithioacetal:

(4.7)

Figure 4.1 shows the formulas and names for

D-aldoses using simplifiedFischerprojections.

The occurrence of aldoses of importance in food

is compiled in Table 4.1. Epimers are monosac-

charides which differ in configuration at only

one chiral C-atom. D-Glucose andD-mannose

are 2-epimers. D-glucose and D-galactose are

4-epimers.

The enantiomersD-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 givesD-2-ketoses

in their simplifiedFischerprojections.

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 monosaccha-

ride. Figure 4.1 gives the configuration prefix

derived from the trivial names, representing

a specified configuration applied in monosac-

charide classification. Thus, systematic names

forD-glucose andD-fructose areD-gluco-hexose

Table 4.1.Occurrence of aldoses

Name, structure Occurrence

Pentoses

D-Apiose (3-C-Hydroxy- Parsley, celery seed

methyl-D-glycero-

tetrose)

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 Ribonucleic acid

D-Xylose Xylanes, hemicelluloses

plant gums, glycosides

Hexoses

L-Fucose (6-Deoxy- Human milk, seaweed

L-galactose) (algae),

plant gums and mucilage

D-Galactose Widespread in oligo-

and polysaccharides

D-Glucose Widespread in plants and

animals

D-Mannose Widespread as polysac-

charide building blocks

L-Rhamnose (6-Deoxy- Plant gums and mucilage,

L-mannose) glycosides