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Carbohydrates 393

Ketoses are generally resistant to bromine oxidation, elimination of the C-1 hydrogen atom, cleavage of the
and the latter may be useful in the removal of aldose con- O H bond, and formation of the corresponding 2-O-
taminants from ketoses. However, by extending the pe- glycosylaldonic acid or by a minor reaction involving
riod of oxidation, D-fructose affords D-lyxo-5-hexulosonic cleavage of the C-1 C-2 bond with formation of the
acid. O-formyl acetal of the next lower sugar.
b. Homolytic oxidations. Most homolytic oxida-
6. Reduction of Carbohydrates
tions occur in two successive steps, each of which involves
a one-electron transfer. The most difficult (and slowest) Numerous reagents have been employed to reduce carbo-
step in the homolytic oxidation of a sugar is the first, hydrates and their derivatives. These include such inor-
which involves the abstraction of a hydrogen atom from a ganic hydrides as those of aluminum and boron; reactive
C H group, followed by the transfer of an electron from metals, for example, sodium in the presence of proton
this hydrogen atom to the oxidant, to afford a proton. In donors; and metal catalysts, such as palladium, platinum,
this process, the sugar that lost a hydrogen atom is con- or nickel in the presence of molecular hydrogen.
verted to a radical, which is stabilized by resonance (the
unshared pair of electrons on the adjacent oxygen atom a. Lithium aluminum hydride. Reductions of car-
can migrate to the carbon atom from which hydrogen had bohydrate derivatives with lithium aluminum hydride are
been abstracted). This explains why the initial abstraction usually performed in nonaqueous solutions (such as ben-
of hydrogen usually takes place by the removal of a hy- zene, ether, or tetrahydrofuran), because the reagent re-
drogen atom attached to carbon (H C O H), rather acts violently with water. The boric acid formed during
than from the one attached to oxygen (H C O H). It the borohydride reduction is usually removed as volatile
is also in agreement with the fact that compounds having methyl borate. Lithium aluminum hydride converts car-
equatorial hydrogen atoms linked to carbon atoms (i.e., bonyl groups to hydroxyl groups and ether lactones to
carbon-linked hydrogen atoms, which are more accessible alditols. For example, 2,3,5,6-tetra-O-acetyl-D-glucono-
to the oxidant than the axially oriented hydroxyl groups 1,4-lactone yields D-glucitol. (The acetyl groups are re-
attachedtothesamecarbonatom)undergomorefacilecat- ductively cleaved by the reagent.)
alytic oxidation than axially oriented C H groups (which If less reactive hydrides are desired, to avoid reducing
are linked to equatorial hydroxyl groups). The final step the aldehydes first formed to hydroxyls, then lithium tri-
of the oxidation is a fast reaction involving the homolysis ter-butoxyaluminum hydride or diisobutylaluminum hy-
of the O H bond of the sugar radical, to afford a carbonyl dride (DIBAL-H) may be used at low temperature. The
group plus a second hydrogen radical which, like the first, first will reduce an aldonic acid chloride into an aldose and
is converted by the oxidant to a proton. the second an ester or a nitrile to an aldehyde (Scheme 19).
Degradation of a carbohydrate having a carbonyl group
or potential carbonyl group begins with nucleophilic ad- b. Borohydrides and diborane. In contrast to
dition of hydroperoxide anion to the group, forming a hy- lithium aluminum hydride, sodium borohydride and
droperoxide hydrin called the hydroperoxide adduct. lithium borohydride are relatively stable in water and can
The adducts of substances having a hydroxyl group be used to reduce lactones to sugars in acid media or to
attached to the carbon atom adjacent to the carbonyl the alditol in basic media. Thus, at pH 5, D-glucono-1,5-
group usually decompose by a process that is called lactone affords D-glucose and, at pH 9, D-glucitol.
the β-hydroxy hydroperoxide cleavage reaction. With al- Sodium borohydride (or sodium borodeuteride) is also
doses, this affords formic acid and the next lower aldose. usedtoconvertaldosesoroligosacchridestoalditolderiva-
α,β-Diketones react rapidly with alkaline hydrogen tives for examination by gas–liquid chromatography–
peroxide with rupture of the C C bond and produc- mass spectrometry. Unlike hydride ions, which are
tion of the two carboxyl groups. Degradation of an α,β- electron-rich nucleophiles, diborane is the dimer of an
unsaturated ketone takes place by addition of hydroper- electron-deficient electrophilic species (BH 3 ). It is formed
oxide anion to the double bond. The adduct decomposes, by reaction of a borohydride ion with strong acids and
producing an epoxide which may react further, as in the is used in 2.2 -dimethoxydiethyl ether (diglyme) as the

oxidation of the enolic form of diketomyoinositol. solvent. Diborane reacts faster with carboxylic acids, re-
The degradation of a reducing disaccharide proceeds ducing them to primary alcohols, than with esters, be-
relatively rapidly by the β-hydroxy hydroperoxide cleav- cause the carbonyl character of the adduct of the first is
age reaction until the process is interrupted by the gly- stabilized by resonance, and that of the second is desta-
cosidic linkage. At this point, the hydroperoxide adduct, bilized by resonance. Diisoamylborane, which does not
lacking an α-hydroxyl group, decomposes largely by cleave ester groups (as does diborane), reduces acetylated