Sweetness -

Alitame


Key Terms -

- aldaric acid
furanose
- alditol
- aldonic acid
- aldose
- amino sugar
- $\alpha$ anomer, $\beta$ anomer
- anomeric center
- anomer
- carbohydrate
- complex carbohydrate
- D sugar
- deoxy sugar
- disaccharide
- Fischer projection
- glycoside
- ketose
- Kiliani-Fischer synthesis
- Koenigs-Knorr reaction
- L sugar
- monosaccharide
- mutarotation
- polysaccharide
- pyranose
- reducing sugar
- simple sugar
- uronic acid
- Wohl degradation


Key Terms -


Summary -

Now that we've now seen all the common functional groups and reaction types, our focus has changed to looking at the major classes of biological molecules. Carbohydrates are polyhydroxy aldehydes and ketones. They are classified according to the number of carbon atoms and the kind of carbonyl group they contain. Glucose, for example, is an aldohexose, a six-carbon aldehydo sugar. Monosaccharides are further classified as either d sugars or l sugars, depending on the stereochemistry of the chirality center farthest from the carbonyl group. Carbohydrate stereochemistry is frequently depicted using Fischer projections, which represent a chirality center as the intersection of two crossed lines.


Summary -

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbonyl group with an -OH group three or four carbon atoms away. A five-membered cyclic hemiacetal is called a furanose, and a six-membered cyclic hemiacetal is called a pyranose. Cyclization leads to the formation of a new chirality center called the anomeric center and the production of two diastereomeric hemiacetals called alpha ( $\boldsymbol{\alpha}$ ) and beta ( $\boldsymbol{\beta}$ ) anomers.


Summary -

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with $\mathrm{NaBH}_{4}$ to form an alditol, oxidized with aqueous $\mathrm{Br}_{2}$ to form an aldonic acid, oxidized with $\mathrm{HNO}_{3}$ to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by Wohl degradation.


Summary -

Disaccharides are complex carbohydrates in which simple sugars are linked by a glycoside bond between the anomeric center of one unit and a hydroxyl of the second unit. The sugars can be the same, as in maltose and cellobiose, or different, as in lactose and sucrose. The glycosidic bond can be either $\alpha$ (maltose) or $\beta$ (cellobiose, lactose) and can involve any hydroxyl of the second sugar. A $1 \rightarrow 4$ link is most common (cellobiose, maltose), but others such as $1 \rightarrow 2$ (sucrose) are also known. Polysaccharides, such as cellulose, starch, and glycogen, are used in nature as structural materials, as a means of long-term energy storage, and as cell-surface markers.


Summary of Reactions -


Additional Problems -


Visualizing Chemistry -

PROBLEM Identify the following aldoses, and tell whether each is a D or L sugar:


Visualizing Chemistry -


Visualizing Chemistry -

PROBLEM Draw Fischer projections of the following molecules, placing the carbonyl group at the top in the 25-27 usual way. Identify each as a D or L sugar.
(a)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0930.jpg?height=264&width=386&top_left_y=893&top_left_x=482)
(b)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0930.jpg?height=321&width=481&top_left_y=894&top_left_x=947)


Visualizing Chemistry -

PROBLEM The following structure is that of an laldohexose in its pyranose form. Identify it, and tell whether
25-28 it is an $\alpha$ or $\beta$ anomer.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0930.jpg?height=353&width=451&top_left_y=1344&top_left_x=414)


Visualizing Chemistry -

PROBLEM The following model is that of an aldohexose:
25-29
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0930.jpg?height=288&width=603&top_left_y=1778&top_left_x=414)
(a) Draw Fischer projections of the sugar, its enantiomer, and a diastereomer.
(b) Is this a D sugar or an L sugar? Explain.
(c) Draw the $\beta$ anomer of the sugar in its furanose form.


Mechanism Problems -

PROBLEM Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-
25-30 glucose by galactose 4-epimerase, where UDP $=$ uridylyl diphosphate (a ribonucleotide diphosphate; Section 28.1). The enzyme requires NAD ${ }^{+}$for activity (Section 17.7), but it is not a stoichiometric reactant, and NADH is not a final reaction product. Propose a mechanism.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0931.jpg?height=348&width=1470&top_left_y=189&top_left_x=403)


Mechanism Problems -

PROBLEM Mannose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its
25-31 6-phosphate derivative from fructose 6-phosphate. No enzyme cofactor is required. Propose a mechanism.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0931.jpg?height=329&width=899&top_left_y=700&top_left_x=410)

PROBLEM Glucosamine, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its 25-32 6-phosphate derivative from fructose 6-phosphate by reaction with ammonia. Propose a mechanism.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0931.jpg?height=327&width=947&top_left_y=1189&top_left_x=405)


Mechanism Problems -

PROBLEM D-Glucose reacts with acetone in the presence of acid to yield the nonreducing 1,2:
25-33 5,6-diisopropylidene-D-glucofuranose. Propose a mechanism.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0931.jpg?height=408&width=871&top_left_y=1628&top_left_x=410)

1,2:5,6-Diisopropylidene-
D-glucofuranose
PROBLEM One of the steps in the biological pathway for carbohydrate metabolism is the conversion of fructose
25-34 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Propose a mechanism for the transformation.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0932.jpg?height=442&width=860&top_left_y=191&top_left_x=405)


Mechanism Problems -

PROBLEM L-Fucose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from 25-35 GDP-D-mannose by the following three-step reaction sequence, where GDP = guanosine diphosphate (a ribonucleoside diphosphate; Section 28.1):
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0932.jpg?height=269&width=1460&top_left_y=798&top_left_x=411)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0932.jpg?height=332&width=1386&top_left_y=1100&top_left_x=413)
(a) Step 1 involves an oxidation to a ketone, a dehydration to an enone, and a conjugate reduction. The step requires $\mathrm{NADP}^{+}$, but no NADPH is formed as a final reaction product. Propose a mechanism.
(b) Step 2 accomplishes two epimerizations and utilizes acidic and basic sites in the enzyme but does not require a coenzyme. Propose a mechanism.
(c) Step 3 requires NADPH as coenzyme. Show the mechanism.


Carbohydrate Structures -


Carbohydrate Structures -

PROBLEM Write open-chain structures for the following:
25-37 (a) A ketotetrose
(b) A ketopentose
(c) A deoxyaldohexose
(d) A five-carbon amino sugar


Carbohydrate Structures -

PROBLEM What is the stereochemical relationship of D-ribose to L-xylose? What generalizations can you make 25-38 about the following properties of the two sugars?
(a) Melting point
(b) Solubility in water
(c) Specific rotation
(d) Density


Carbohydrate Structures -

Ascorbic acid


Carbohydrate Structures -

PROBLEM Draw the three-dimensional furanose form of ascorbic acid (Problem 25-39), and assign $R$ or $S$ 25-40 stereochemistry to each chirality center.


Carbohydrate Structures -

PROBLEM Assign $R$ or $S$ configuration to each chirality center in the following molecules:


Carbohydrate Structures -

PROBLEM Draw Fischer projections of the following molecules:
25-42 (a) The $S$ enantiomer of 2-bromobutane (b) The $R$ enantiomer of alanine, $\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{CO}_{2} \mathrm{H}$
(c) The $R$ enantiomer of 2-hydroxypropanoic acid
(d) The $S$ enantiomer of 3-methylhexane