Question

An unknown disaccharide gives a positive Tollens’ test. A glycosidase hydrolyzes it to d-galactose and d-mannose. When the disaccharide is treated with methyl iodide and Ag_2O and then hydrolyzed with dilute HCl, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-O-methylmannose. Propose

a structure for the disaccharide.

Short answer

Disaccharides are sugars created when two monosaccharides are connected by glycosidic linkage. Condensation reactions are also involved in the generation of disaccharides. Some examples are sucrose and lactose.

Step-by-step solution

Disaacharide

Disaccharides are sugars created when two monosaccharides are connected by glycosidic linkage. Condensation reactions are also involved in the generation of disaccharides. Some examples are sucrose and lactose.

Proposing a structure for the disaccharide

The silver oxide raises the leaving tendency of the iodide ion from the methyl iodide, thereby allowing the nucleophilic substitution to happen with the poor nucleophilic alcohol groups.

As a mannose is missing in the methyl substituent on the oxygen at C-6, the disaccharide must be formed using the C-6 OH group of the mannose and the anomeric carbon of the galactose.

Structure of the disaacharide