Question

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

Short answer

(a)

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

(b)

α -D-fructofuranosyl-β -D-mannopyranoside

(c)

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

Step-by-step solution

Disaccharide

A sugar consisting of two monosaccharide unit is a disaccharide. Most common disaccharides are sucrose, lactose, maltose etc.

Pyranose and furanose

Pyranose is a six-membered cyclic hemiacetal while a furanose is a five-membered cyclic hemiacetal.

Anomers

The diastereomers resulting from cyclisation are known as anomers. They differ only in the configuration around first carbon (C1) which is referred to as the anomeric carbon (hemiacetal carbon atom).

Identifying  and  anomers of pyranoses α and β furanoses

For α -anomer, hydroxy group (-OH) on the anomeric carbon is down (axial position) while for β - anomer, hydroxy group (-OH) on the anomeric carbon is up (equatorial position). Again, for α -anomer, the anomeric hydroxy (-OH) group is placed trans to the terminal -CH₂OH group while for β -anomer, the anomeric hydroxy (-OH) group is placed cis to the terminal -CH₂OH group.

Structure of disaccharides

(a)

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

(b)

α -D-fructofuranosyl-β -D-mannopyranoside

(c)

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