The Methyl Ethers of Hexuronic Acids

Abstract

This chapter provides an overview of the methyl ethers of hexuronic acids. Hexuronic acid residues occur in a large number of different polysaccharides. The chapter focuses on the methyl ethers of the naturally occurring D-glucuronic, D-galacturonic, and D-mannuronic acids, and their derivatives. D-glucuronic acid (or its 4-methyl ether) occurs in combination with several different sugar residues: with D-galactose and D-mannose in plant gums, with D-xylose in wood and straw hemicelluloses, and with D-glucose, L-rhamnose, D-glucosamine, and D-galactosamine in various mucopolysaccharides. D-galacturonic acid occurs in pectic acid, and combined with L-rhamnose and D-galactose, in seed mucilages and plant gums. D-mannuronic acid is only known to occur as the sole constituent of the seaweed mucilage, alginic acid. The Haworth method is suitable for the direct methylation either of reducing uronic acids or their nonreducing methyl glycosides. In both cases, the product is the sodium salt of the methyl glycoside of the methylated uronic acid, from which the methyl glycoside methyl ester may be formed either by methanolysis or by esterification of the free acid with diazomethane.