11.11 Disaccharides

If the glycoside or acetol is formed by reaction of the anomeric carbon of a monosaccharide with OH group of another monosaccharide molecule, then the glycoside product is a disaccharide (Scheme 10).

Scheme 10

The anomeric carbon can react with any of the hydroxyl groups of another monosaccharide unit to form a disaccharide. Disaccharides can be categorized by the position of the hydroxyl group of another monosaccharide making up the glycoside.

Disaccharides have three naturally occurring glycosidic linkages

• 1-4' link: The anomeric carbon is bonded to oxygen on C-4 of second monosaccharide.
• 1-6' link: The anomeric carbon is bonded to oxygen on C-6 of second monosaccharide.
• 1-2' link: The anomeric carbons of the two monosaccharide unit are bonded through an oxygen.

The “prime” superscript indicates that –OH group bonded carbon position of the second monosaccharide unit, α - and β -configuration given by based on the configuration at the anomeric carbon of the first monosaccharide unit.

1-4' Glycosides : These represent the most common naturally occurring disaccharides. The linkage is between C-1 of one sugar subunit and C-4 of the other. For example, maltose is a disaccharide with two D-glucose units bearing 1,4'-glycosidic linkage. The stereochemistry of this linkage is α . So, the glycosidic linkage is called α -1,4'-glycosidic linkage.