Homopolysaccharides interaction with the dimethyl sulphoxide–paraformaldehyde cellulose solvent system. Selective oxidation of amylose and cellulose at secondary alcohol groups

Abstract

A ¹³C n.m.r. study of the interaction properties of the cellulose solvent dimethyl sulphoxide–paraformaldehyde with amylose suggests the reversible formation of hemiacetals between formaldehyde or its oligomers and hydroxy-groups at C-6 and C-2 of the polysaccharide. Such selectivity is, however, not found for the model compound methyl 4-O-methyl-α-D-glucopyranoside, which shows O-3 substitution as well. In agreement with this result, oxidation of amylose in this solvent system by adding acetic anhydride to the homogeneous solution leads to selective formation of carbonyl groups at C-3 in excellent yield. A similar result is obtained when acetic anhydride is added to a cellulose solution in dimethyl sulphoxide–paraformaldehyde. This suggests that the interaction mechanisms of cellulose and amylose with this solvent system must be similar. As a matter of fact, oxidation of a 6-substituted cellulose, such as 6-O-acetylcellulose, mainly takes place at C-2. The 6-O-acetylcellulose was prepared by the action of acetic acid–acetic anhydride in pyridine–tetrachloromethane on a pertrimethylsilyl-cellulose followed by hydrolysis of the remaining 2,3-bis(trimethylsilyl) ether. Previous results on the oxidation of 6-O-triphenylmethyl-cellulose or -amylose have been confirmed and corroborated. Determination of the site and degree of oxidation with the partly oxidized homopolysaccharides was achieved through their reduction with sodium borodeuteride, hydrolysis, and study by g.l.c.–mass spectrometry of the resulting hexoses in the form of their trimethylsilyl O-methyl oximes, or alditol acetate derivatives.