Monosaccharide acid-catalyzed reaction

As a typical acetal bond, a glycosidic bond readily hydrolyzes in an acid-catalyzed reaction. In this manner, di- and oligosaccharides can be split into monosaccharides. It is a common method for manufacturing of invert sugar, a mixture of a-D-glucose and P-D-fructose, from sucrose. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

Alternately, the importance of the hemiacetal group can be studied by using a compound that is an analog for a monosaccharide without the rest of the hydroxyl groups. Tetrahydro-2H-pyran-2-ol (THP) is such an analog for xylose. THP was reacted with an equimolar amount of DMU, in the absence of phenol, to follow the course of the hemiacetal-urea reactions. The acid-catalyzed mixture was heated between 81 and 116 °C over a 2-hour period. Samples were frozen to prevent further reaction prior to analysis. 

Experiments on reaction mechanisms showed that urea enhanced the dehydration (weight loss) reactions of monosaccharides in nonacidic environments. Addition of acid to a glucose-urea mixture slowed its dehydration at 108 °C. The addition of sufficient urea to an acid-catalyzed polyol solution appears to prevent, or at least delay considerably, the normal acid dehydration reaction of the polyol. 

If not utilized in the pulp industry, hemicelluloses are hydrolyzed in the acid-catalyzed process, mainly to monosaccharides and to furan-2-aldehyde (pen-tosanes) and 5-hydroxymethylfuran-2-aldehyde (5.62) (hexosanes). Monosaccharide-containing syrups, after purification, are either fermented or utilized as wood molasses for feeding ruminants. In another approach, xylose, the least soluble component of syrup, is allowed to crystallize. Separated xylose is then hydrogenated over an Ni/Al catalyst at 120°C under 6 x 106 Pa into xylitol. Hemicelluloses, together with proteins, are capable of the Maillard reaction and may contribute to the overall secondary aroma of processed foodstuffs (Tomasik and Zawadzki, 1998). 

Monosaccharides can be modified by reaction with alcohols and amines to form adducts. For example, D-glucose will react with methanol in an acid-catalyzed process the anomeric carbon atom reacts with the hydroxyl group... 

A more stereospecific course of glycoside formation by reaction of the polymer-bound C-1 bromide with the second monosaccharide hydroxyl component involves the initial formation of a c -l 2-orthoester. This undergoes an acid-catalyzed stereospecific isomerization (Kochetkov et al., 1967) to the corresponding glycoside (disaccharide). 

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