4.2 Monosaccharides 277(4.67)(4.68)(4.69)The compound 4-hydroxy-2,5-dimethyl-3(2H)-
furanone (furaneol, I in Formula 4.67) is the
corresponding degradation product from the
6-deoxy-L-mannose (rhamnose) (Formula 4.69).
Furaneol can also be formed from hexose phos-
phates under reducing conditions (cf. 4.2.4.4.6)
and from C-3 fragments (cf. Formula 4.47).
With a relatively low odor threshold value,
furaneol has an intensive caramel-like odor. It is
interesting that furaneol is also biosynthesized in
plants, e. g., in strawberries (cf. 18.1.2.6.9) and
pineapples (cf. 18.1.2.6.10).
The formation of another degradation prod-
uct of 1-deoxyosone, acetylformoin (III in
Formula 4.67) is shown in Formula 4.70. In
comparison with the formation of furanone in the
synthesis of acetylformoin (cf. Formula 4.68),
the water elimination at C-6 of the carbohy-
drate skeleton occurs before cyclization to
the furan derivative. Although further water
elimination is no longer easy, it is suggested to
explain the formation of methylene reductic acid
(Formula 4.70).
As a result of the presence of an enediol structure
element in theα-position to the oxo function in
the open-chain structures of acetylformoin, this
compound belongs to the group of substances
called reductones. Substances of this type, e. g.,
also vitamin C (ascorbic acid), are weakly acidic
(Formula 4.71), reductive (Formula 4.72) and
exhibit antioxidative properties. The latter are
attributed to the possible formation of resonance-
stabilized radicals (Formula 4.73) and also to
the disproportionation of two radicals with