28. Carbohydrates

Draw the mechanism for the elimination step in the Wohl degredation.

What product or products are obtained when d-galactose reacts with each of the following?

g.

1. hydroxylamine/trace acid

2. acetic anhydride/∆

3. HO^-/H₂O

A D-aldopentose is oxidized by nitric acid to an optically active aldaric acid. A Wohl degradation of the aldopentose leads to a monosaccharide that is oxidized by nitric acid to an optically inactive aldaric acid. Identify the D-aldopentose.

The Wohl degradation, an alternative to the Ruff degradation, is nearly the reverse of the Kiliani–Fischer synthesis. The aldose carbonyl group is converted to the oxime, which is dehydrated by acetic anhydride to the nitrile (a cyanohydrin). Cyanohydrin formation is reversible, and a basic hydrolysis allows the cyanohydrin to lose HCN. Using the following sequence of reagents, give equations for the individual reactions in the Wohl degradation of D-arabinose to D-erythrose. Mechanisms are not required.

a. hydroxylamine hydrochloride

b. acetic anhydride

c. ^–OH, H₂O

What two monosaccharides can be degraded to

a. D-ribose? 

What two monosaccharides can be degraded to

b. D-arabinose? 

Treatment with sodium borohydride converts aldose A to an optically inactive alditol. Wohl degradation of A forms B, whose alditol is optically inactive. Wohl degradation of B forms D-glyceraldehyde. Identify A and B.

A hexose was obtained after (+)-glyceraldehyde underwent three successive Kiliani–Fischer syntheses. Identify the hexose from the following experimental information: oxidation with nitric acid forms an optically active aldaric acid; a Wohl degradation followed by oxidation with nitric acid forms an optically inactive aldaric acid; and a second Wohl degradation forms erythrose.