Question

Draw a chair conformation for the \(\alpha\) form of a disaccharide in which two units of D-glucopyranose are joined by a \(\beta-1,3\)-glycosidic bond.

Short answer

Question: Draw a chair conformation of an α-anomer of a disaccharide composed of two D-glucopyranose units linked by a β-1,3-glycosidic bond. Answer: To draw the chair conformation, first sketch both D-glucopyranose units in chair conformation with α-anomers. Then, connect them through a β-1,3-glycosidic bond involving the hydroxyl group on carbon 1 of the first unit and the hydroxyl group on carbon 3 of the second unit, ensuring the bond's oxygen is equatorial relative to the pyranose rings.

Step-by-step solution

Draw the first D-glucopyranose unit in chair conformation

Begin by drawing a chair conformation of the first D-glucopyranose unit. The D-glucopyranose unit is a 6-membered ring with carbon atoms forming the basic structure. The first carbon has an axial hydroxyl, indicating the α-anomer. The oxygen atom links carbon 1 to carbon 4 to form the pyranose ring structure.

Identify the β-1,3-glycosidic bond

Next, we need to represent the β-1,3-glycosidic bond that links the two D-glucopyranose units. As the name implies, this bond involves the hydroxyl group on carbon 1 of the first unit and the hydroxyl group on carbon 3 of the second unit. The bond forms with a β-configuration, meaning the electronegative oxygen in the bond is equatorial in relation to the pyranose ring.

Draw the second D-glucopyranose unit in chair conformation

Now, draw the second D-glucopyranose unit in chair conformation, with an α-anomer as well. The first carbon of this unit should also have an axial hydroxyl group, and the oxygen atom should link carbon 1 to carbon 4, just like the first D-glucopyranose unit.

Draw the entire disaccharide by connecting the two D-glucopyranose units

Finally, connect the two D-glucopyranose units through the β-1,3-glycosidic bond identified in step 2. Ensure that the oxygen in the bond is equatorial relative to the corresponding pyranose rings. You should now have a complete chair conformation of the α form of the disaccharide with two D-glucopyranose units joined by a β-1,3-glycosidic bond.