Refer to the cellobiose structure in Worked Example 20.5. How would you classify the link between the monosaccharides in cellobiose?

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>

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?

Draw the structures of a ketohexose.

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)

Are one or more of the disaccharides maltose, lactose, cellobiose, and sucrose part of the trisaccharide in Problem 20.23? If so, identify which disaccharide and its location. (Hint: Look for an α-1,4 link, ß-1,4 link, or 1,2 link, and then determine if the correct monosaccharides are present.)

Are the α and ß forms of the disaccharide lactose enantiomers of each other? Why or why not?

In solution, glucose exists predominantly in the cyclic hemiacetal form, which does not contain an aldehyde group. How is it possible for mild oxidizing agents to oxidize glucose?

How many chiral carbon atoms are present in each of the molecules shown in Problem 20.31?

How many chiral carbon atoms are present in each of the molecules shown in Problem 20.31?

Draw the open-chain structure of a 4-carbon deoxy sugar.

Name four important monosaccharides and tell where each occurs in nature.

Only three stereoisomers are possible for 2,3-dibromo-2, 3-dichlorobutane. Draw them, indicating which pair are enantiomers (optical isomers). Why does the other isomer not have an enantiomer?

In Section 15.6, you saw that aldehydes react with reducing agents to yield primary alcohols (RCH=O → RCH₂OH). The structures of two d-aldotetroses are shown. One of them can be reduced to yield a chiral product, but the other yields an achiral product. Explain. <IMAGE><IMAGE>

Sucrose and d-glucose rotate plane-polarized light to the right; d-fructose rotates light to the left. When sucrose is hydrolyzed, the glucose–fructose mixture rotates light to the left.

b. Why do you think the mixture is called “invert sugar”?

Aldoheptoses have five chiral carbon atoms. What is the maximum possible number of aldoheptose stereoisomers? Draw all of the aldoheptose stereoisomers.

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>

Treatment of d-glucose with a reducing agent yields sorbitol, a substance used as a sugar substitute by people with diabetes. Draw the structure of sorbitol.

Reduction of d-fructose with a reducing agent yields a mixture of d-sorbitol along with a second, isomeric product. What is the structure of the second product?

Treatment of an aldose with an oxidizing agent such as Tollens’ reagent (Section 15.5) yields a carboxylic acid. Gluconic acid, the product of glucose oxidation, is used as its magnesium salt for the treatment of magnesium deficiency. Draw the structure of gluconic acid.

Oxidation of the aldehyde group of ribose yields a carboxylic acid. Draw the structure of ribonic acid.

Draw a disaccharide of two cyclic mannose molecules attached by an α-1,4 glycosidic linkage. Explain why the glycosidic products in Problem 20.58 are not reducing sugars, but the product in this problem is a reducing sugar.

Draw the enantiomer of the following monosaccharides, and in each pair identify the d sugar and the l sugar.

a. <IMAGE>

Draw the enantiomer of the following monosaccharides, and in each pair identify the d sugar and the l sugar.

b. <IMAGE>

Lactose and maltose are reducing disaccharides, but sucrose is a nonreducing disaccharide. Explain.

Trehalose, a disaccharide found in the blood of insects, has the following structure. What simple sugars would you obtain on hydrolysis of trehalose? (Hint: Rotate one of the rings in your head or redraw it rotated.) <IMAGE>

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

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