28. Carbohydrates

Draw the mechanism for the formation of β-lactose from ⍺-D-galactose and β-D-glucose in dilute HCl.

The disaccharide lactulose consists of a D-galactopyranose subunit and a D-fructofuranose subunit joined by a β-1,4′-glycosidic linkage. After treatment of lactulose with 1. excess CH₃I/Ag₂O, 2. HCl/H₂O, the d-galactopyranose subunit was found to have one nonmethylated OH group, whereas the D-fructofuranose subunit had two. Draw the structure of ⍺-lactulose.

In each of the disaccharides shown (a–c), (i) identify the reducing end of the sugar, (ii) tell what monosaccharides are used to make it, and (iii) tell what type of glycoside linkage is present.

(a)

In each of the disaccharides shown (a–c), (i) identify the reducing end of the sugar, (ii) tell what monosaccharides are used to make it, and (iii) tell what type of glycoside linkage is present.

(b)

Maltose, like cellobiose, is composed of two glucose units. What is the difference between maltose and cellobiose?

Trehalose, C₁₂H₂₂O₁₁, is a nonreducing sugar that is only 45% as sweet as sugar. When hydrolyzed by aqueous acid or the enzyme maltase, it forms only D-glucose. When it is treated with excess methyl iodide in the presence of Ag₂O and then hydrolyzed with water under acidic conditions, only 2,3,4,6-tetra-O-methyl-d-glucose is formed. Draw the structure of trehalose.

Trehalose is a nonreducing disaccharide (C₁₂H₂₂O₁₁) isolated from the poisonous mushroom Amanita muscaria. Treatment with an α-glucosidase converts trehalose to two molecules of glucose, but no reaction occurs when trehalose is treated with a β-glucosidase. When trehalose is methylated by dimethyl sulfate in mild base and then hydrolyzed, the only product is 2,3,4,6-tetra-O-methylglucose. Propose a complete structure and systematic name for trehalose.

An unknown reducing disaccharide is found to be unaffected by invertase enzymes. Treatment with an α-galactosidase cleaves the disaccharide to give one molecule of D-fructose and one molecule of D-galactose. When the disaccharide is treated with excess iodomethane and silver oxide and then hydrolyzed in dilute acid, the products are 2,3,4,6-tetra-O-methylgalactose and 1,3,4-tri-O-methylfructose. Propose a structure for this disaccharide, and give its complete systematic name.

Give an equation to show the reduction of Tollens reagent by maltose.

Draw the structures (using chair conformations of pyranoses) of the following disaccharides.

(a) 4-O-(α-D-glucopyranosyl)-D-galactopyranose

(b) α-D-fructofuranosyl-β-D-mannopyranoside

(c) 6-O-(β-D-galactopyranosyl)-D-glucopyranose

Draw the structures of the individual mutarotating α and β anomers of maltose.

Does lactose mutarotate? Is it a reducing sugar? Explain. Draw the two anomeric forms of lactose

Cellulose is converted to cellulose acetate by treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibers. Show the structure of cellulose acetate.