Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 53

Chapter 23, Problem 53

Draw the following sugar derivatives.
(a) methyl β-D-glucopyranoside
(b) 2,3,4,6-tetra-O-methyl-D-mannopyranose
(c) 1,3,6-tri-O-methyl-D-fructofuranose
(d) methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside

Verified step by step guidance

Step 1: Understand the structure of the parent sugar for each derivative. For example, glucopyranoside is derived from glucose, mannopyranose from mannose, fructofuranose from fructose, and galactopyranoside from galactose. These sugars are cyclic forms of their respective monosaccharides.

Step 2: Identify the modifications specified in the problem. For example, 'methyl' indicates the addition of a methoxy group (-OCH₃) at specific positions, and 'tetra-O-methyl' or 'tri-O-methyl' indicates multiple hydroxyl groups (-OH) are replaced with methoxy groups.

Step 3: Draw the cyclic structure of the sugar in its correct stereochemical form. For example, for β-D-glucopyranoside, draw the six-membered pyranose ring with the hydroxyl groups and substituents in their correct stereochemical positions (axial or equatorial).

Step 4: Add the specified substituents. For example, for methyl β-D-glucopyranoside, replace the anomeric hydroxyl group (-OH) with a methoxy group (-OCH₃) while maintaining the β-configuration. For tetra-O-methyl derivatives, replace the hydroxyl groups at the specified positions (e.g., 2, 3, 4, and 6) with methoxy groups.

Step 5: Verify the stereochemistry and connectivity of the final structure. Ensure that the substituents are correctly placed and that the stereochemistry matches the D- or L-configuration and the α- or β-anomeric designation as specified in the problem.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Sugar Structure and Anomeric Carbon

Understanding the structure of sugars, particularly their cyclic forms, is essential. Sugars like glucose and galactose can exist in pyranose (six-membered) or furanose (five-membered) rings. The anomeric carbon, which is the carbon derived from the carbonyl group during cyclization, plays a crucial role in determining the sugar's reactivity and properties.

Methylation of Sugars

Methylation involves the addition of methyl groups (-CH3) to hydroxyl (-OH) groups on sugars, which alters their solubility and reactivity. In the context of sugar derivatives, understanding how to represent these modifications is key to drawing the correct structures, as each methylation affects the overall configuration and properties of the sugar.

Stereochemistry in Carbohydrates

Stereochemistry is vital in organic chemistry, especially for carbohydrates, as the spatial arrangement of atoms can significantly influence their biological function. Recognizing the D- and L- configurations, as well as the orientation of hydroxyl groups in sugar derivatives, is crucial for accurately depicting and understanding their structures.

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Related Practice

Textbook Question

Which of the following sugars are reducing sugars? Which ones would undergo mutarotation?

(a) methyl β-D-glucopyranoside

(b) 2,3,4,6-tetra-O-methyl-D-mannopyranose

(d) methyl 2,3,4,6-tetra-O-methyl-β-D-galactopyranoside

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Textbook Question

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

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Textbook Question

Predict the products obtained when D-galactose reacts with each reagent.

(k) excess HIO₄

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Textbook Question

Erwin Chargaff’s 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’s rule:

G = C and A = T

(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff’s 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’s 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?

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Textbook Question

Predict the products obtained when D-galactose reacts with each reagent.

(i) Br₂, H₂O, then H₂O₂ and Fe₂(SO₄)₃

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Textbook Question

Predict the products obtained when D-galactose reacts with each reagent.

(j) (1) KCN/HCN; (2) H₂, Pd/BaSO₄; (3) H₃O⁺

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