20. Carbohydrates

Draw a Haworth projection for α-D-altrose.

D-ribose is an aldopentose sugar that is found in the DNA. It commonly exists as a five-membered β anomer. Draw D‑ribose in its cyclic hemiacetal form.

In the monosaccharide hemiacetal shown below number all the carbon atoms, identify the anomeric carbon atom, and identify it as the α or ß anomer. <IMAGE>

The cyclic structure of d-idose, an aldohexose, is shown in the margin. Convert this to the straight-chain Fischer projection structure. <IMAGE>

Identify the following as diastereomers, enantiomers, and/or anomers. (a) ß-d-fructose and

ß-d-fructose (b) d-galactose and l-galactose (c) l-allose and d-glucose (both aldohexoses)

d-Talose, a constituent of certain antibiotics, has the open-chain structure shown next. Draw d-talose in its cyclic hemiacetal form. <IMAGE>

l-Fucose is one of the naturally occurring l monosaccharides. It is present in the short chains of monosaccharides by which blood groups are classified (see the Chemistry in Action “Cell-Surface Carbohydrates and Blood Type” on p. 640). Compare the structure of l-fucose shown in the margin with the structures of α- and ß-d-galactose and answer the following questions.

d. Fucose” is a common name. Is 6-deoxy-l-galactose a correct name for fucose? Why or why not? <IMAGE>

Draw the structure of the α and ß anomers that result from the reaction of methanol and ribose. Are these compounds acetals or hemiacetals?

In its open-chain form, d-mannose, an aldohexose found in orange peels, has the structure shown here. Coil mannose around and draw it in the cyclic hemiacetal α and ß forms. <IMAGE>

d-Fructose can form a six-membered cyclic hemiacetal as well as the more prevalent five-membered cyclic form. Draw the α isomer of d-fructose in the six-membered ring.

What are the kind and number of atoms in the ring portion of the Haworth structure of glucose?

Draw the Haworth structures for α- and ß-D-glucose.

Identify each of the following as the α or ß isomer:

a. <IMAGE>

The sugar d-gulose is a sweet-tasting syrup. (13.3, 13.4)

<IMAGE>

b. Draw the Haworth structures for α− and β-d-gulose.

Identify the following carbohydrates as the α or ß anomer:

(b) <IMAGE>

d-Fructose can also form a six-membered ring. Draw the anomer of d-fructose in the six-membered ring form.

Identify the following carbohydrates as the α or ß anomer:

(a) <IMAGE>

Draw the α and ß anomer of d-talose in pyranose ring form:

<IMAGE>

d-Talose

Pentoses also exist in a ring form, but they most commonly occur as furanose rings. d-Ribose exists in its furanose ring form in the nucleic acid RNA. Using the structure of d-ribose from Table 6.1, draw the furanose form of ß-d-ribose.

Identify the following carbohydrates as the α or ß anomer:

(a) <IMAGE>