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Chapter 23: Q28P (page 1229)

Predict the products formed when the following sugars react with excess acetic anhydride and pyridine.

  1. α-D-glucopyranose
  2. β-D-ribofuranose

Short Answer

Expert verified
  1. penta-O-acetyl-α-D-glucopyranose
  2. tetra-O-acetyl-β-D-ribofuranose


Step by step solution

01

Formation of acetate esters

The hydroxy (-OH) groups of a sugar can be converted to acetate esters by the treatment with acetic anhydride and pyridine. The stereochemistry is usually retained at the anomeric carbon.

02

Pyranose and furanose

Pyranose is a six-membered cyclic hemiacetal while a furanose is a five-membered cyclic hemiacetal.

03

Formation of product

  1. α-D-glucopyranose on treatment with excess acetic anhydride and pyridine gives penta-O-acetyl-α-D-glucopyranoside as the product.

    Reaction
  2. β-D-ribofuranose on treatment with excess acetic anhydride and pyridine gives tetra-O-acetyl-β-D-ribofuranoside as the product.

    Reaction

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Most popular questions from this chapter

Draw the expected product of the reaction of the following sugars with excess methyl iodide and silver oxide.

  1. α-D-fructofuranose
  2. β-D-galactopyranose

The carbonyl group in D-galactose may be isomerized from C1 to C2 by brief treatment with dilute base (by enediol rearrangement, Section 23-7). The product is the C4 epimer of fructose. Draw the furanose structure of the product.

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

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

(b) α -D-fructofuranosyl-β -D-mannopyranoside

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

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 β -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′ and 3′ 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.

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 “epimer” of D-xylose. Notice that this “epimer” is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.
See all solutions

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