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Chapter 23: Q6P. (page 1214)

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.

Short Answer

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Answer

(a)

(b)

(c)

(d)

Step by step solution

01

Epimers

Epimers are the compounds which are stereoisomers and differ at only one stereogenic center like the placing the OH group is different from the other. For example, D-glucose and D-galactose are epimers having only one chiral center with different configuration.

02

The structure of D-allose, D-talose and 2-idose and their comparison

(a) Epimers are stereoisomers which differs in configuration at any one stereogenic or chiral center. C3 epimer of glucose means configuration differs at carbon-3 of glucose which gives rise to different stereoisomer that is, D-allose. Position of hydroxyl group in glucose at carbon-3 is at left whereas position of hydroxyl group at carbon-3 of D-allose is at right.

The structure of D-allose


(b) Similarly, D-talose is C2 epimer of D-galactose and differs in configuration at carbon-2. Position of hydroxyl group in D-galactose is at right at carbon-2 whereas in D-talose, it is at left. D-idose is C3 epimer of D-talose and differs in configuration at carbon-3.

The structure of D-talose


(c) Position of hydroxyl group is at left at carbon-3 in D-talose whereas it is at right in D-idose.

The structure of D-idose

03

The C4 epimer of D-xylose

(d) C4 epimer of D-xylose is L-arabinose. Change in configuration at carbon-4 of D-xylose with respect to hydroxyl group changes the sugar series from 鈥淒鈥� to 鈥淟鈥� as the change occurred at bottom chiral center which as per convention, leads to formation of enantiomer. The D/L system names molecules by relating them to glyceraldehyde molecule.

The C4 epimer of D-xylose

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Most popular questions from this chapter

Raffinose is a trisaccharide (C18H32O16) isolated from cottonseed meal. Raffinose does not reduce Tollens reagent, and it does not mutarotate. Complete hydrolysis of raffinose gives D-glucose, D-fructose, and D-galactose. When raffinose is treated with invertase, the products are D-fructose and a reducing disaccharide called melibiose. Raffinose is unaffected by treatment with a-galactosidase, but an 伪 -galactosidase hydrolyzes it to D-galactose and sucrose. When raffinose is treated with dimethyl sulfate and base followed by hydrolysis, the products are 2,3,4-tri-O-methylglucose, 1,3,4,6-tetra-O-methylfructose, and 2,3,4,6-tetra-O-methylgalactose. Determine the complete structures of raffinose and melibiose and give a systematic name for melibiose.

The relative configurations of the stereoisomers of tartaric acid were established by the following synthesis:

(1) D-(+)-glyceraldehydediastereomers A and B (separated)

(2) Hydrolysis of A and B using aqueous Ba(OH)2 gave C and D, respectively.

(3) HNO3 oxidation of C and D gave (-)-tartaric acid and meso-tartaric acid, respectively.

(a) You know the absolute configuration of D-(+)-glyceraldehyde, Use Fischer projections to show the absolute configurations of products A, B, C, and D.

(b) Show the absolute configurations of the three stereoisomers of tartaric acid: (+)-tartaric acid, (-)-tartaric acid, and meso-tartaric acid.

Fructose is found in many fruits. From memory, draw fructose in

  1. the Fischer projection of the open chain.
  2. The most stable chair conformation of the most stable pyranose anomer.
  3. The Haworth projection of the most stable pyranose anomer

Show the products that result from hydrolysis of amygdalin in dilute acid. Can you suggest why amygdalin might be toxic to tumor (and possibly other) cells?

In 1891, Emil Fischer determined the structures of glucose and seven other D-aldohexoses using only simple chemical reactions and clever reasoning about stereochemistry and symmetry. He received the Nobel Prize for this work in 1902. Fischer has determined that D-glucose is an aldohexose, and he used Ruff degradation to degrade it to (+)-glyceraldehyde. Therefore, the eight D-aldohexose structures shown in Figure 23-3 are the possible structures for glucose.

Pretend that no names are shown in Figure 23-3 except for glyceraldehyde, and sue the following results to prove which of these structures represent glucose, mannose, arabinose, and erythrose.

(a)Upon Ruff degradation, glucose and mannose gives the same aldopentose: arabinose.Nitric acid oxidation of arabinose gives an optically active aldaric acid. What are the two possible structures of arabinose?

(b) Upon Ruff degradation, arabinose gives the aldotetrose erythrose. Nitric acid oxidation of erythrose gives an optically inactive aldaric acid, meso-tartaric acid. What is the structure of erythrose?

(c) Which of the two possible structures of arabinose is correct? What are the possible structures of glucose and mannose?

(d) Fischer鈥檚 genius was needed to distinguish between glucose and mannose. He developed a series of reactions to convert the aldehyde group of an aldose to an alcohol while converting the terminal alcohol to an aldehyde. In effect, he swapped the functional groups on the ends. When he interchanged the functional groups on D-mannose, he was astonished to find that the product was still D-mannose. Show how this information completes the proof of the mannose structure, and show how it implies the correct glucose structure.

(e) When Fischer interchanged the functional groups on D-glucose, the product was an unnatural L sugar. Show which unnatural sugar he must have formed, and show how it completes the proof of the glucose structure.
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