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Chapter 23: Q10P. (page 1218)

Question. (a) Figure 23-2 shows that the degradation of D-glucose gives D-arabinose, an aldopentose. Arabinose is most stable in its furanose form. Draw D-arabinofuranose.

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

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01

Haworth projections

Haworth projections are used to depict cyclic sugars. It indicates the stereochemical arrangement of groups on the ring system via up and down orientation of groups or atoms. The H and OH attachment orientation can be drawn in sugar molecules.

02

Furanose form of arabinose

(a) D-arabinofuranose is the furanose form of arabinose sugar. It is formed via hemiacetal formation, and arabinose is most stable in its furanose form, which is a five-membered cyclic form.

03

Furanose form of ribose

(b) Ribose, an aldopentose, commonly exists in furanose structure, a five-membered cyclic form. The cyclisation of ribose occurs via hemiacetal formation, which is due to an attack on the aldehyde by the carbon-4 hydroxyl to produce furanose form.

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Most popular questions from this chapter

Draw and name the enantiomers of the sugars shown in Figure 23-2. Give the relative configuration (D or L) and the sign of the rotation in each case.

Some protecting groups can block two OH groups of a carbohydrate at the same time. One such group is shown here, protecting the 4-OH and 6-OH groups of 尾 -D-glucose.

(a) What type of functional group is involved in this blocking group?

(b) What did glucose react with to form this protected compound?

(c) When this blocking group is added to glucose, a new chiral center is formed. Where is it? Draw the stereoisomer that has the other configuration at this chiral center. What is the relationship between these two stereoisomers of the protected compound?

(d) Which of the two stereoisomers in part (c) do you expect to be the major product? Why?

(e) A similar protecting group, called an acetonide, can block reaction at the 2鈥� and 3鈥� oxygens of a ribonucleoside. This protected derivative is formed by the reaction of the nucleoside with acetone under acid catalysis. From this information, draw the protected product formed by the reaction.

The Wohl degradation, an alternative to the Ruff degradation, is nearly the reverse of the Kiliani-Fischer synthesis. The aldose carbonyl group is converted to the oxime, which is dehydrated by acetic anhydride to the nitrile (a cyanohydrin). Cyanohydrin formation is reversible, and a basic hydrolysis allows the cyanohydrin to lose HCN. Using the following sequence of reagents, give equations for the individual reaction in the Wohl degradation of D-arabinose to D-erythrose. Mechanisms are not required.

(1) hydroxylamine hydrochloride

(2) acetic anhydride

(3) OH-,H2O

Erwin Chargaff鈥檚 discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff鈥檚 rule:

G = C and A = T

(a) Does Chargaff鈥檚 rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff鈥檚 rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff鈥檚 rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?

After a series of Kiliani鈥揊ischer syntheses on (+)-glyceraldehyde, an unknown sugar is isolated from the reaction mixture. The following experimental information is obtained:

(1) Molecular formula C6H12O6

(2) Undergoes mutarotation.

(3) Reacts with bromine water to give an aldonic acid.

(4) Reacts with HNO3 to give an optically active aldaric acid.

(5) Ruff degradation followed by HNO3 oxidation gives an optically inactive aldaric acid. (6) Two Ruff degradations followed by HNO3 oxidation give meso-tartaric acid.

(7) When the original sugar is treated with CH3I and Ag2O, a pentamethyl derivative is formed. Hydrolysis gives a tetramethyl derivative with a free hydroxy group on C5.

(a) Draw a Fischer projection for the open-chain form of this unknown sugar. Use Figure 23-3 to name the sugar.

(b) Draw the most stable conform

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