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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

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.

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?

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of 伪-D-galactopyranoseis +150.70 , and that of the 尾 anomer is +52.80 . When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.20. Determine the percentages of the two anomers present at equilibrium.

Draw the structures (using chair conformations of pyranoses) of the following disaccharides.

(a) 4-O-(伪 -D-glucopyranosyl)-D-galactopyranose

(b) 伪 -D-fructofuranosyl-尾 -D-mannopyranoside

(c) 6-O-(尾 -D-galactopyranosyl)-D-glucopyranose

Cytosine, uracil and guanine have tautomeric forms with aromatic hydroxyl groups. Draw these tautomeric forms.
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