Chemistry Reference
In-Depth Information
as aqueous acetic acid/methanol, pyridine, pyridine/chloroform, trichloroacetic
acid/dichloroethane, ethanol/methanol, methanol/formamide, or DMA-LiCl [
47
].
Due to the fairly different reactivity of the two hydroxyl groups (at C
3
and C
6
)
and the amino group (at C
2
) of the repeating unit of Ch, acylation can be controlled
at the expected sites, that is, on either amino [
48
], hydroxyls [
49
], or on both
groups [
50
].
N
-acylation may be prepared by a number of ways, not all of which are
applicable to the preparation of all
N
-acyl derivatives. The simplest procedure is
the reaction between a carboxylic acid and Ch to produce
N
-formyl Ch by heating a
solution of Ch in 100 % HCOOH acid at 90
C with gradual addition of pyridine
[
51
], while Aiba [
52
] proceeds
N
-acetylation of Ch in 20 wt% acetic acid. This
latter method was much slower and the extent of
N
-acetylation being about 50 %
after 300 h at 80
C.
The most common reagents for
N
-acylation of Ch are the acyl anhydrides and
may be used under heterogeneous and homogeneous conditions, principally the
latter.
N
-acylation [
53
] under heterogeneous conditions has been studied using
three systems: they were (a) acetic anhydride-glacial acetic acid and HClO
4
acid;
(b) acetic anhydride at room temperature for 120 h followed by refluxing in acetic
anhydride for 2 h; (c) acetic anhydride-methanol at room temperature. Of these, the
last method was found to be the most efficacious. Acid chlorides have been found to
have little application in the selective
N
-acylation of Ch because of their high
reactivity. Kurita et al. [
54
] attempted
N
-acetylated water-soluble Ch, but only a
very limited amount of
N
-acetylation was obtained, and this is due to the steric
arrangement of the Ch molecules at the liquid interface limiting the accessibility of
the amino groups to the acetyl chloride molecules.
4.4.2.2
O
-Acyl Derivatives of Chitosan
Introducing a hydrophobic moiety with an ester linkage into Ch has two benefits:
(1) hydrophobic groups contribute organosolubility and (2) the ester linkage is
hydrolyzed by enzymes such as lipase. Moreover, the glycoside linkage of Ch
derivatives is also degraded by glycosidase. Therefore, Ch derivatives with
O
-acyl
groups are designed as biodegradable coating materials. The successful preparation
of
N
,
O-
acyl Ch in methyl sulfonic acid (MeSO
3
H) as solvent was performed [
49
].
The preparation of
O
,
O
-didecanoyl Ch,
O
-succinyl Ch was also reported through
protected
N
-phthaloyl Ch as an intermediate [
55
]. This method, however, needs
several steps as the protection of the amino group by phthaloylation,
O
-acylation,
and finally removal of protecting group by suitable method as by using hydrazine
hydrate.
Another method for the protection of the amino group is through the formation
of Schiff's base, followed by
O
-acetylation using acetic anhydride-pyridine to
prevent acid hydrolysis of the Schiff's base [
56
]. The protection of the amino
groups is necessary in this case due to the high reactivity of the amino groups as
compared with the hydroxyl groups. Using a Ch sample, it was found that with
