28. Carbohydrates

Provide the mechanism for the cyclic hemiacetal formation of the following hydroxycarbonyl.

The carbonyl group in D-galactose may be isomerized from C1 to C2 by brief treatment with dilute base (by the enediol rearrangement). The product is the C4 epimer of fructose. Draw the furanose structure of the product.

4-Hydroxy- and 5-hydroxyaldehydes exist primarily as cyclic hemiacetals. Draw the structure of the cyclic hemiacetal formed by each of the following:

b. 4-hydroxypentanal

4-Hydroxy- and 5-hydroxyaldehydes exist primarily as cyclic hemiacetals. Draw the structure of the cyclic hemiacetal formed by each of the following:

c. 5-hydroxypentanal

4-Hydroxy- and 5-hydroxyaldehydes exist primarily as cyclic hemiacetals. Draw the structure of the cyclic hemiacetal formed by each of the following:

d. 4-hydroxyheptanal

Draw the structures of the compounds

(a) methyl a-D-galactopyranoside

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Draw the structures of the compounds

(d) ethyl b-D-ribofuranoside

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(a) Propose a mechanism for the cyclization, assuming a trace of acid is present.

(b) The cyclic hemiacetal is more stable than the open-chain form, so very little of the open-chain form is present at equilibrium. Will an aqueous solution of glucose reduce Tollens reagent and give a positive Tollens test? Explain.

Two structures for the sugar glucose are shown on page 914. Interconversion of the open-chain and cyclic hemiacetal forms is catalyzed by either acid or base.

(b) The cyclic hemiacetal is more stable than the open-chain form, so very little of the open-chain form is present at equilibrium. Will an aqueous solution of glucose reduce Tollens reagent and give a positive Tollens test? Explain.

Draw the structures of the compounds

(c) a-D-allopyranose

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

draw the chair conformations of

(a) b-D-mannopyranose (the C2 epimer of glucose).

Without referring to the chapter, draw the chair conformations of

(b) a-D-allopyranose (the C3 epimer of glucose).

Without referring to the chapter, draw the chair conformations of

(d) N-acetylglucosamine, glucose with the C2 oxygen atom replaced by an acetylated amino group.