Chitosan, acylated aldehydes

Acylation of the amine groups of chitosan can be readily carried out using acyl anhydride as the reactant. During the acylation process of a chitosan solution, the chitosan slowly loses the solubility and a gel is formed. Muzzarelli et al. (Muzzarelli et al, 1986) modified chitosan with aldehyde-acids and keto-acids, from which they obtained a series of polymers possessing such groups as carboxylic acids, primary and secondary amines and primary and secondary hydroxyl groups. The product showed excellent metal chelating properties (Muzzarelli, 1973). 

Chitosan is a strong nucleophile because of the presence of a nonbonding pair of electrons on its primary amino groups. Chitosan reacts readily with most aldehydes to produce imines (Kurita et al., 1988). It also reacts with acyl chlorides to form the corresponding acylated derivatives (Hirano et al., 1976) (Figure 2). 

FIG. 2 Reactions of chitosan with an aldehyde or acyl chloride. 

Chemical. A large number of chitin and chitosan derivatives have been synthesized through modification of the primary (C-6) and secondary (C-3) hydroxyl groups present on each repeat imit, including amine (C-2) functionality existing on deacetylated imits (31). Reactions typical of hydroxyl and amine groups (such as acylations with acid chlorides and anhydrides) including urethane and urea formation respectively, are feasible with isocyanates. The primary amine can be quatemized by alkyl iodides or converted to an imine with a variety of aldehydes and ketones that can subsequently be reduced to an N-alkylated derivative. Chitin and chitosan are reactive with a variety of alkyl chlorides after treatment with concentrated NaOH. Important derivatives such as carboxymethylated chitin and chitosan are commonly produced in this manner with the addition of sodium chloroacetate. 

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