2009_Book_FoodChemistry

4.2 Monosaccharides 249
tetroses) and ketoses (starting from 2-pentuloses)
undergo intramolecular cyclization with hemiacetal
formation to form lactols (Formula 4.3). With
the exception of erythrose, monosaccharides are
crystallized in these cyclic forms and, even in
solution, there is an equilibrium between the
open chain carbonyl form and cyclic hemiacetals,
with the latter predominating. The tendency to
cyclize is, compared to hydroxyaldehydes, even
more pronounced in monosaccharides, as shown
by ΔG ◦ -values and equilibrium concentrations
in 75% aqueous ethanol (cf. Formula 4.3).
4.2.1.2 Configuration
Glyceraldehyde, a triose, has one chiral center,
so it exists as an enantiomer pair, i. e. in D-
and L-forms. According to the definition, the
secondary hydroxyl group is on the right in
D-glyceraldehyde and on the left in L-glyceraldehyde.
Although this assignment was at first
arbitrarily made, it later proved to be correct. It is
possible by cyanhydrin synthesis to obtain from
each enantiomer a pair of diastereomeric tetroses
(Kiliani–Fischer synthesis):
(4.2)
Lactols can be considered as tetrahydrofuran or
tetrahydropyran derivatives, hence, they are also
denoted as furanoses or pyranoses.
(4.4)
(4.3)