Question

The disaccharide lactulose consists of a d-galactose pyranose subunit and a d-fructo furanose subunit joined by a b-1,4′-glycosidic linkage. After treatment of lactulose with

1. excess ${\mathbf{CH}}_{\mathbf{3}}\mathbf{I}\mathbf{,}{\mathbf{Ag}}_{\mathbf{2}}\mathbf{O}\mathbf{;}\mathbf{2}\mathbf{HCl}\mathbf{,}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$, the d-galactose pyranose subunit was found to have one nonmethylated OH group, whereas the d-fructo furanose subunit had two. Draw the structure of $\mathbf{\alpha }$-lactulose.

Short answer

Structure of $\mathrm{\alpha }$-lactulose

Step-by-step solution

Disaccharide lactulose

Lactulose is a disaccharide made up of two monosaccharide units named d-glacto pyranose and d- fructo furanose.

As given when these units show reaction with 1. ${\mathbf{CH}}_{\mathbf{3}}\mathbf{I}\mathbf{,}{\mathbf{Ag}}_{\mathbf{2}}\mathbf{O}\mathbf{;}\mathbf{2}\mathbf{HCl}\mathbf{,}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$that is hydroxyl group is replaced by methoxy ion show the arrangement os hydroxyl must be as:

Disaccharide glucose

Structure of lactulose

As also given both monosaccharide unit d-galactose pyranose and d-fructo furanose are joined by beta 1,4 glycosidic linkages, so the structure must be;

Structure of lactulose