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25.16: Additional Problems

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    25 • Additional Problems 25 • Additional Problems

    Visualizing Chemistry

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

    The ball-and-stick model of a four-carbon aldose. The gray, black, and red spheres represent hydrogen, carbon, and oxygen atoms, respectively. (b)

    The ball-and-stick model of a pyranose. The gray, black, and red spheres represent hydrogen, carbon, and oxygen atoms, respectively.

    D or L sugar. (a)

    The ball-and-stick model of a three carbon aldose. The gray, black, and red spheres represent hydrogen, carbon, and oxygen atoms, respectively. (b)

    The ball-and-stick model of a four carbon aldose. The gray, black, and red spheres represent hydrogen, carbon, and oxygen atoms, respectively.

    Problem 25-28

    The following structure is that of an L aldohexose in its pyranose form. Identify it, and tell whether it is an α or β anomer.

    The ball-and-stick model of a beta pyranose with three equatorial O H groups. The gray, black, and red spheres represent hydrogen, carbon, and oxygen atoms, respectively.
    The following model is that of an aldohexose: the= (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 β anomer of the sugar in its furanose form.

    Mechanism Problems

    Galactose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from UDP-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.

    u=

    Mannose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its 6-phosphate derivative from fructose 6-phosphate. No enzyme cofactor is required. Propose a mechanism.

    mannose,=

    Glucosamine, one of the eight essential monosaccharides (Section 25.7), is biosynthesized as its 6-phosphate derivative from fructose 6-phosphate by reaction with ammonia. Propose a mechanism.

    glucosamine,=

    D-Glucose reacts with acetone in the presence of acid to yield the nonreducing 1,2 : 5,6-diisopropylidene-D-glucofuranose. Propose a mechanism.

    The reaction of the Haworth projection of D-glucose with acetone in the presence of hydrochloric acid to yield the non-reducing 1, 2 : 5, 6 diisopropylidene D-glucofuranose.

    One of the steps in the biological pathway for carbohydrate metabolism is the conversion of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Propose a mechanism for the transformation.

    the=
    L-Fucose, one of the eight essential monosaccharides (Section 25.7), is biosynthesized from GDP-D-mannose by the following three-step reaction sequence, where GDP = guanosine diphosphate (a ribonucleoside diphosphate; Section 28.1):

    G D P- L-Fucose, one of the essential monosaccharides, is biosynthesized from G D P - D-mannose by a three-step reaction sequence. (a)
    Step 1 involves an oxidation to a ketone, a dehydration to an enone, and a conjugate reduction. The step requires 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

    structure= (b)

    Structure of a five-carbon atom ketopentose sugar with four -O H groups. (c)

    Structural representation of aldoheptose which is a heptose with an aldehyde group and five chiral centers.

    A ketotetrose
    (b)
    A ketopentose
    (c) A deoxyaldohexose (d) A five-carbon amino sugar
    D-ribose to L-xylose? What generalizations can you make about the following properties of the two sugars? (a)
    Melting point
    (b) Solubility in water (c) Specific rotation (d) Density

    Does ascorbic acid (vitamin C) have a D or L configuration?

    The structure of L-ascorbic acid. In the Fisher projection, the farthest chiral carbon has a hydroxyl group on the left hand side.

    Draw the three-dimensional furanose form of ascorbic acid (Problem 25-39), and assign R or S stereochemistry to each chirality center.
    R or S configuration to each chirality center in the following molecules: (a)

    three-carbon= (b)

    Two-carbon Fisher projection, phenyl top with OH on the right and methyl on the left at C 1, at C 2 methyl on the left, and OH at the bottom (c)

    Three-carbon Fisher projection, amine top and carboxylic acid on the right at C 1, at C 2 OH on the right, and at C 3 phenyl at the bottom

    The S enantiomer of 2-bromobutane
    (b)
    The R enantiomer of alanine, CH3CH(NH2)CO2H
    (c) The R enantiomer of 2-hydroxypropanoic acid (d) The S enantiomer of 3-methylhexane
    Draw Fischer projections for the two D aldoheptoses whose stereochemistry at C3, C4, C5, and C6 is the same as that of D-glucose at C2, C3, C4, and C5.

    The following cyclic structure is that of allose. Is this a furanose or pyranose form? Is it an α or a β anomer? Is it a D or an L sugar?

    The Haworth projection of beta anomer of pyranose sugar named allose.

    What is the complete name of the following sugar?

    the=
    the= (b)

    The structure of alpha-D-fructofuranose. (c)

    The structure of alpha-L-mannopyranose.

    Draw D-ribulose in its five-membered cyclic β-hemiacetal form.

    The Fisher projection of D-ribulose, ketone at C 2, two hydroxyl groups on the right at C 3, and C 4, and a hydroxyl at the bottom of C 4

    Look up the structure of D-talose in Figure 25.4, and draw the β anomer in its pyranose form. Identify the ring substituents as axial or equatorial.

    Carbohydrate Reactions

    β-D-talopyranose with each of the following reagents: (a)
    NaBH4 in H2O
    (b) Warm dilute HNO3 (c) Br2, H2O (d) CH3CH2OH, HCl (e) CH3I, Ag2O (f) (CH3CO)2O, pyridine
    How many D-2-ketohexoses are possible? Draw them.
    One of the D-2-ketohexoses is called sorbose. On treatment with NaBH4, sorbose yields a mixture of gulitol and iditol. What is the structure of sorbose?
    Another D-2-ketohexose, psicose, yields a mixture of allitol and altritol when reduced with NaBH4. What is the structure of psicose?
    L-Gulose can be prepared from D-glucose by a route that begins with oxidation to D-glucaric acid, which cyclizes to form two six-membered-ring lactones. Separating the lactones and reducing them with sodium amalgam gives D-glucose and L-gulose. What are the structures of the two lactones, and which one is reduced to L-gulose?
    Gentiobiose, a rare disaccharide found in saffron and gentian, is a reducing sugar and forms only D-glucose on hydrolysis with aqueous acid. Reaction of gentiobiose with iodomethane and Ag2O yields an octamethyl derivative, which can be hydrolyzed with aqueous acid to give 1 equivalent of 2,3,4,6-tetra-O-methyl-D-glucopyranose and 1 equivalent of 2,3,4-tri-O-methyl-D-glucopyranose. If gentiobiose contains a β-glycoside link, what is its structure?

    General Problems

    All aldoses exhibit mutarotation. For example, α-D-galactopyranose has [α]D = +150.7, and β-D-galactopyranose has [α]D = +52.8. If either anomer is dissolved in water and allowed to reach equilibrium, the specific rotation of the solution is +80.2. What are the percentages of each anomer at equilibrium? Draw the pyranose forms of both anomers.
    What other D aldohexose gives the same alditol as D-talose?
    Which of the eight D aldohexoses give the same aldaric acids as their L enantiomers?
    Which of the other three D aldopentoses gives the same aldaric acid as D-lyxose?
    L-galactose, and then answer the following questions: (a)
    Which other aldohexose gives the same aldaric acid as L-galactose on oxidation with warm HNO3?
    (b) Is this other aldohexose a D sugar or an L sugar? (c) Draw this other aldohexose in its most stable pyranose conformation.
    Amygdalin, or laetrile, is a cyanogenic glycoside first isolated in 1830 from almond and apricot seeds. If acidic hydrolysis of amygdalin liberates HCN, along with benzaldehyde cyanohydrin with gentiobiose (Problem 25-54), what is its structure?
    Trehalose is a nonreducing disaccharide that is hydrolyzed by aqueous acid to yield 2 equivalents of D-glucose. Methylation followed by hydrolysis yields 2 equivalents of 2,3,4,6-tetra-O-methylglucose. How many structures are possible for trehalose?
    Trehalose (Problem 25-61) is cleaved by enzymes that hydrolyze α-glycosides but not by enzymes that hydrolyze β-glycosides. What is the structure and systematic name of trehalose?
    Isotrehalose and neotrehalose are chemically similar to trehalose (Problems 25-61 and 25-62) except that neotrehalose is hydrolyzed only by β-glycosidase enzymes, whereas isotrehalose is hydrolyzed by both α- and β-glycosidase enzymes. What are the structures of isotrehalose and neotrehalose?
    D-Mannose reacts with acetone to give a diisopropylidene derivative (Problem 25-33) that is still reducing toward Tollens’ reagent. Propose a likely structure for this derivative.
    Glucose and mannose can be interconverted (in low yield) by treatment with dilute aqueous NaOH. Propose a mechanism.
    Propose a mechanism to account for the fact that D-gluconic acid and D-mannonic acid are interconverted when either is heated in pyridine solvent.
    The cyclitols are a group of carbocyclic sugar derivatives having the general formulation 1,2,3,4,5,6-cyclohexanehexol. How many stereoisomeric cyclitols are possible? Draw them in their chair forms.
    Compound A is a D aldopentose that can be oxidized to an optically inactive aldaric acid B. On Kiliani–Fischer chain extension, A is converted into C and D; C can be oxidized to an optically active aldaric acid E, but D is oxidized to an optically inactive aldaric acid F. What are the structures of A–F?
    Simple sugars undergo reaction with phenylhydrazine, PhNH–NH2, to yield crystalline derivatives called osazones. The reaction is a bit complex, however, as shown by the fact that glucose and fructose yield the same osazone. d-glucose= (a)
    Draw the structure of a third sugar that yields the same osazone as glucose and fructose.
    (b) Using glucose as the example, the first step in osazone formation is reaction of the sugar with phenylhydrazine to yield an imine called a phenylhydrazone. Draw the structure of the product. (c) The second and third steps in osazone formation are tautomerization of the phenylhydrazone to give an enol, followed by elimination of aniline to give a keto imine. Draw the structures of both the enol tautomer and the keto imine. (d) The final step is reaction of the keto imine with 2 equivalents of phenylhydrazine to yield the osazone plus ammonia. Propose a mechanism for this step.
    When heated to 100 °C, D-idose undergoes a reversible loss of water and exists primarily as 1,6-anhydro-D-idopyranose.

    Fischer projection of D-idose, two hydroxyl groups on the left at C2, C3, two hydroxyl groups on the right at C3, C5 in equilibrium with idopyranose and water. (a)
    Draw D-idose in its pyranose form, showing the more stable chair conformation of the ring.
    (b) Which is more stable, α-D-idopyranose or β-D-idopyranose? Explain. (c) Draw 1,6-anhydro-D-idopyranose in its most stable conformation. (d) When heated to 100 °C under the same conditions as those used for D-idose, D-glucose does not lose water and does not exist in a 1,6-anhydro form. Explain.

    Acetyl coenzyme A (acetyl CoA) is the key intermediate in food metabolism. What sugar is present in acetyl CoA?

    the=

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