264 4 Carbohydrates
(4.34)Dehydrating Reactions. The heating of monosac-
charides under acidic conditions, e. g., during
pasteurization of fruit juices and baking of rye
bread, gives rise to a large number of furan and
pyran compounds (examples in Formula 4.35).
The formation of these compounds can be
explained by enolizations and dehydrating
reactions of the carbohydrates. It is noticeable
that in some compounds, the aldehyde group of
an aldose is formally retained at C-1 (furfural,
5-hydroxymethyl furfural, 5-methylfurfural) and
in other components, the aldehyde group is re-
duced to a methyl group. As explained later, this
indicates the course of formation in each case.
(4.35)The reaction pathway in acid starts slowly with
enolization to important intermediates called ene-
diols. Glucose gives rise to 1,2-enediol, and fruc-
tose to 2,3-enediol as well (Formula 4.36). Start-
ing with the enediols, the further course of the re-
action is shown below.
The steps in the formation of 5-hydroxymethyl
furfural (HMF) from 1,2-enediol is shown in
Formula 4.37. HMF is also used as an indicator
for the heating of carbohydrate containing food,
e. g., honey. The (retro-Michaeladdition) water
elimination at C-3 and subsequently at C-4 leads
to a 1,2-diulose (3,4-dideoxyosone), which after
cyclization to a hemiacetal, a dihy-drofuran,
releases another molecule of water, producing
HMF. In the same way, e. g., furfural can be made
from pentoses and 5-methylfurfural from the 6-
methylpentose rhamnose. 2-Hydroxyacetylfuran,
which is preferentially formed from fructose,
can be obtained starting from the corresponding
2,3-enediol by water elimination at C-4, followed
by C-5 (Formula 4.38).