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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

Does lactose mutarotate? Is it a reducing sugar? Explain. Draw the two anomeric forms of lactose.

Question. Cellulose is converted to cellulose acetateby treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibres. Show the structure of cellulose acetate.

Question. Retroviruses like HIV, the pathogen responsible for AIDS, incorporate an RNA template that is copied into DNA during infection. The reverse transcriptaseenzyme that copies RNA into DNA is relatively nonselective and error-prone, leading to a high mutation rate. Its lack of selectivity is exploited by the anti-HIV drug AZT (3鈥�-azido-2鈥�,3鈥�-dideoxythymidine), which becomes phosphorylated and is incorporated by reverse transcriptase into DNA, where it acts as a chain terminator. Mammalian DNA polymerases are more selective, having a low affinity for AZT, so its toxicity is relatively low.

  1. Draw the structures of AZT and natural deoxythymidine.
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Question:

a) Draw D-allose, the C3 epimer of glucose.

b) Draw D-talose, the C2 epimer of D-galactose.

c) Draw D-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.

d) Draw the C4 鈥渆pimer鈥� of D-xylose. Notice that this 鈥渆pimer鈥� is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.

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.
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