28. Carbohydrates

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(a) Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimers results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.

Name the epimers of D-glucose.

Use Figure 23-3 (the D family of aldoses) to name the following aldoses.

(a) the C2 epimer of D-arabinose

(b) the C3 epimer of D-mannose

(c) the C3 epimer of D-threose

Which of the following molecule pairs are epimers?

(a)

(b)

(c)

c. What sugar is the C-4 epimer of L-gulose?

d. What sugar is the C-4 epimer of D-lyxose?

What sugar is the C-3 epimer of D-fructose?

When glucose undergoes base-catalyzed isomerization in the absence of the enzyme, mannose is one

of the products that is formed (Section 20.5). Why is mannose not formed in the enzyme-catalyzed reaction?

a. Draw D-allose, the C3 epimer of glucose.

b. Draw D-talose, the C2 epimer of d-galactose