Page 73 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: LLL/LLL P2: FJU Final Pages
Encyclopedia of Physical Science and Technology EN002C-80 May 25, 2001 20:18

Carbohydrates 381

mixtures. For example, solutions of hexoses contain less
than 1% of the acyclic form, and yet they readily afford
carbonyl group derivatives in high yields. This is because
the addition of a nucleophile to the carbonyl group of an
acyclic form will immediately shift the equilibrium in fa-
vor of this form, allowing more of it to be formed and to
react with the nucleophile.
Concerning the hydroxyl groups, it should be recog-
nized that the hydroxyl groups can play a dual role. Such
groups can act as leaving groups when stronger nucle-
ophiles attack the carbon atom to which they are attached.
These nucleophilic substitution reactions may be of the
S N 1orS N 2 type. Alternatively, the oxygen of the hydroxyl
group acts as a nucleophile, adding to carbonyl groups and
carbonium ions or displacing good leaving groups to af-
ford esters, acetals, ketals and ethers, and other species.
The hydroxyl group of the hemiacetal function at C-1 of an
aldose or C-2 of a ketose is the most reactive of all the hy-
droxyl groups found in a monosaccharide. The next most
reactive hydroxyl group is the primary hydroxyl group at
the terminal position. This is followed in reactivity by the
secondary hydroxyl groups. The oxidation of carbonyl and
hydroxyl groups has important applications in industrial
processes.

1. Reactions of the Carbonyl Group
The reactions of the carbonyl group of a monosaccharide
include the nucleophilic addition of a carbon, nitrogen,
oxygen, or sulfur atom. It should be noted that, although
these additions afford acyclic products, the latter may cy-
clize, so that the product may ultimately be acyclic or
cyclic. On the other hand, intramolecular nucleophilic ad-
dition, by a hydroxyl group attached to the sugar chain on
the carbonyl carbon atom, can afford only cyclic products.
Note also that, in both types of additions, the reaction may
occur with or without subsequent loss of water.
The addition of carbon nucleophiles to the carbonyl
group of aldoses has been widely used to extend the carbon
chains of saccharides, that is, to ascend the series. Similar
additions to the keto group of glyculoses have been used
to prepare branched sugars. Of particular value in form-
ing C C bonds are the nucleophiles − CN, − CH 2 NO 2 ,
and CH 2 N 2 , as well as the ylides and organometallic
−
nucleophiles involved in the Wittig and Grignard reac-
SCHEME 3 Addition of carbon nucleophiles to carbonyl groups.
tions (Scheme 3).
The nitrogen nucleophiles commonly used with
monosaccharides contain a primary amino group attached pathway for the reaction between a nitrogen nucleophile
to a carbon, a nitrogen, or an oxygen atom. The products and a free sugar is via the acyclic form of the monosac-
obtained from the first type of nucleophile are rarely of charide, which usually affords an acyclic addition product.
the acyclic, Schiff base type, because they readily cyclize. The latter may subsequently lose water to yield an acyclic
The other two types of nucleophile afford carbonyl-group condensation product, or it may cyclize. The same reac-
derivatives that usually exist in acyclic forms. The major tion product may be formed by nucleophilic substitution