28. Carbohydrates

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

Two structures of the sugar fructose are shown next. The cyclic structure predominates in aqueous solution.

(a) Number the carbon atoms in the cyclic structure. What is the functional group at C2 in the cyclic form?

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

Draw the mechanism for the interconversion of α-D-glucose and β-D-glucose in dilute HCl.

Allose is the C3 epimer of glucose. Draw the cyclic hemiacetal form of D-allose, first in the chair conformation and then in the Haworth projection.

Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.

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 compound methyl α-D-galactopyranoside.

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