278 4 Carbohydrates
(4.70)re-formation of the reductone structure (For-
mula 4.74). Reductones reduce Ag⊕,Au^3 ⊕,Pt^4 ⊕
to the metals, Cu^2 ⊕to Cu⊕,Fe^3 ⊕to Fe^2 ⊕and
Br 2 or I 2 to Bror Irespectively. Reductones
are present as mono-anions at pH values<6. The
di-anion occurring under alkaline conditions is
easily oxidized in the presence of O 2.
(4.71)(4.72)(4.73)(4.74)The compound 3,5-dihydroxy-2-methyl-5,6-
dihydropyran-4-one (V in Formula 4.67) is
also formed from the pyranoid hemiacetals of
1-deoxy-2,3-hexodiulose (Formula 4.75). In
comparison, maltol is preferentially formed
from disaccharides like maltose or lactose (For-
mula 4.76) and not from dihydroxypyranone by
water elimination. The formation of maltol from
monosaccharides is negligible. A comparison of
the decomposition of 1-deoxyosones from the
corresponding cyclic pyranone structure clearly
shows (cf. Formula 4.75 and 4.76) that the glyco-
sidically bound carbohydrate in the disaccharide
directs the course of water elimination in another
direction (Formula 4.76). It is the stabilization of
the intermediates to quasi-aromatic maltol which
makes possible the cleavage of the glycosidic
bond with the formation of maltol. Parallel to
the formation of maltol, isomaltol derivatives
which still contain the second carbohydrate
molecule are also formed from disaccharides
(Formula 4.77). Indeed, the formation of free
isomaltol is possible by the hydrolysis of the