2009_Book_FoodChemistry

274 4 Carbohydrates
(4.57)
(4.58)
4.2.4.4.3 Secondary Products of 3-Deoxyosones
Formula 4.59 shows examples of products
obtained on the decomposition of 3-deoxyosones.
The best known compounds are
5-hydroxymethylfurfural from hexoses (HMF,
II in Formula 4.59) and furfural from pentoses
(I in Formula 4.59). Taking the furanoid
structures of 3-deoxyosone as a basis (Formula
4.55), 3,4-dideoxyosone is obtained after
ring opening, enolization, and water elimination
(Formula 4.60). Water elimination from the
hemiacetal form of 3,4-dideoxyosone directly
yields HMF. Taking into account the water
elimination required to form 3-deoxyosone
(cf. Formula 4.55), 5-hydroxymethylfurfural
is formed from hexose by the stoichiometric
elimination of 3 mols of water.
In the presence of higher concentrations of
ammonia, primary amines or amino acids,
3-deoxyosone preferentially gives rise to
2-formyl-5-hydroxymethylpyrrole (III in Formula
4.59) or the corresponding N-alkylated
derivatives rather than to HMF. The most important
reaction intermediate is 3,4-dideoxyosone
(cf. Formula 4.60), which can react with amino
compounds with the elimination of water to give
the corresponding pyrrole (Formula 4.61) or
pyridine derivatives (Formula 4.62). The reaction
with ammonia plays a role, especially in the
production of sugar couleur.
If pyrrole formation occurs with an amino acid,
this product can react further (Formula 4.63) to
yield a bicyclic lactone (V in Formula 4.59).
Other secondary products of 3-deoxyosone are
compounds with a pyranone structure. In fact,
β-pyranone (VI in Formula 4.59) is under discussion
as the most important intermediate. It can
be formed from the pyranose hemiacetal form of
3-deoxyosone (Formula 4.64). This compound
has been identified only in the full acetal form
(e. g., with carbohydrates on drying) because
only this structure makes a relatively stable end
product possible. The compounds mentioned
have acidic hydrogen atoms in position 4, easily
allowing condensation reactions with aldehydes
and polymerization or the formation of brown
dyes.
Another compound obtained from 3-deoxyosone
via a relatively complex reaction is maltoxazine
(VII in Formula 4.59), which has been identified
in malt and beer. This compound could be formed
from 3,4-dideoxyosone, which first undergoes
a Strecker reaction with the secondary amino
acid proline with decarboxylation to give the
1-pyrroline derivative (Formula 4.65). Enolization,
formation of a five-membered carbocyclic
compound and nucleophilic addition of the
hydroxymethyl group to the pyrroline cation
yields the tricyclic maltoxazine. In general, the
formation of such carbocyclic compounds is
favored in the presence of secondary amino acids
like proline.
3-Deoxyosones predominantly form pyrazines
and imidazoles with ammonia. The following
compounds were isolated from sugar coloring
(cf. Formula 4.66).