Page 83 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 391

SCHEME 17 Formation of disaccharides.

Benzylidene groups are also useful in the selective plified by the oxidation of a secondary hydroxyl group to
blocking of saccharide derivatives. They usually involve a keto group. This involves the elimination of two hydro-
the primary hydroxyl group of a pyranoside and the gen atoms and the formation of a double bond between the
4-hydroxyl group if it is suitably positioned. Benzyli- carbon atom and the oxygen atom of the alcohol function.
dene groups are also useful for introducing a halogen The first step takes place by breakage of a C H bond and
atom instead of the primary hydroxyl group by the elimination of a hydride ion. Because the hydride anion
Hanessian method using N-bromosuccinimide as shown is a very poor leaving group, this bond breakage is gener-
in Scheme 18. ally the slowest step of the oxidation (the rate-determining
step). What makes the reaction possible, despite this, is the
fact that the elimination is aided by the oxidant, which cap-
5. Oxidation
tures the pair of electrons and converts the hydride ion to a
The oxidation of a sugar involves the breaking of C H, proton. Another proton is concurrently liberated by disso-
C C, O H bonds and the transfer of electrons to the ox- ciation of the O H bond of the alcohol, and both protons
idant. Two types of oxidation are possible. The first in- react with a base (usually a hydroxyl group to form wa-
volves the transfer of two electrons from one atom to the ter). Such an oxidation usually occurs by a concerted or
oxidant and is referred to as heterotytic oxidation. The E2 type of mechanism, in which the oxidant is not bonded
other, known as homolytic oxidation, occurs in two steps, to the sugar, except possibly in the transition state.
each involving the transfer of one electron. Oxidants, for example, chromic acid, have been shown
to form intermediate esters, which subsequently decom-
a. Heterolytic oxidations. Heterolytic oxidations, pose by bimolecular eliminations. The difference between
that is, those involving two-electron transfers, are exem- this and the previous oxidation is that the leaving group is