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Chapter 23: Q33P (page 1231)

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

Short Answer

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Wohl degradation of D-arabinose to D-erythrose

Step by step solution

01

Ruff degradation (shortening of the carbon chain)

Synthesis of new sugars and its structure can be known by a process known as Ruff degradation. The sugar chains are shortened in Ruff degradation. It is a two- step process which is given as below:

(1) oxidation of aldose to aldonoic acid by bromine water (Br2/H2O).

(2) aldonic acid formed is treated with hydrogen peroxide (H2O2) and ferric sulfate, Fe2(SO4)3 that oxidises the carboxyl group to carbon dioxide (CO2) and the resulting aldose is formed with one fewer carbon atom.

02

Kiliani- Fischer synthesis (lengthening of the carbon chain)

Adding one carbon atom to the aldehyde end of the aldose lengthens an aldose chain. The sugar chain is lengthened as a result of this process with a new carbon atom at C1 position and the aldehyde group (C1 position earlier) is now moved to C2 position. For determining the structure of existing sugars and also for synthesizing new sugars, Kiliani-Fischer synthesis is used.

03

The equation

Wohl degradation of D-arabinose to D-erythrose

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Most popular questions from this chapter

Draw the structure of the individual mutarotating 伪 and 尾 anomers of maltose.

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.

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.

Which of the sugars mentioned in Problem 23-53 and 23-54 are reducing sugars? Which ones undergo mutarotation?

Aldohexoses A and B both undergo Ruff degradation to give aldopentose C. On treatment with warm nitric acid, aldopentose C gives an optically active aldaric acid. B also reacts with warm nitric acid to give an optically active aldaric acid, but A reacts to give an optically inactive aldaric acid. Aldopentose C is degraded to aldotetrose D, which gives optically active tartaric acid when it is treated with nitric acid. Aldotetrose D is degraded to (+)-glyceraldehyde. Deduce the structures of sugars A,B,C and D, and use Figure 23-3 to determine the correct names of these sugars.

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