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Ch. 23 - Carbohydrates and Nucleic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 23 - Carbohydrates and Nucleic AcidsProblem 54
Chapter 23, Problem 54

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

Verified step by step guidance
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Step 1: Understand the problem. The task is to draw the chair conformations of the given disaccharides. Each disaccharide consists of two monosaccharides linked by a glycosidic bond. The configuration (α or β) and the specific carbons involved in the glycosidic bond are provided in the problem.
Step 2: For part (a), 4-O-(α-d-glucopyranosyl)-d-galactopyranose: Start by drawing the chair conformation of d-galactopyranose. Identify the hydroxyl group on carbon-4 (C4) of galactopyranose, as this is where the glycosidic bond will form. Then, draw the α-d-glucopyranosyl unit in its chair conformation and connect its anomeric carbon (C1) to the C4 of galactopyranose via an α-glycosidic bond.
Step 3: For part (b), α-d-fructofuranosyl-β-d-mannopyranoside: Begin by drawing the chair conformation of β-d-mannopyranose. Identify the anomeric carbon (C1) of mannopyranose, as this is where the glycosidic bond will form. Then, draw the furanose ring of α-d-fructofuranose and connect its anomeric carbon (C2) to the C1 of mannopyranose via an α-glycosidic bond.
Step 4: For part (c), 6-O-(β-d-galactopyranosyl)-d-glucopyranose: Start by drawing the chair conformation of d-glucopyranose. Identify the hydroxyl group on carbon-6 (C6) of glucopyranose, as this is where the glycosidic bond will form. Then, draw the β-d-galactopyranosyl unit in its chair conformation and connect its anomeric carbon (C1) to the C6 of glucopyranose via a β-glycosidic bond.
Step 5: Ensure stereochemistry is correct. Verify that the α or β configuration of the glycosidic bonds is consistent with the problem statement. For α, the anomeric hydroxyl group is axial (down) in the chair conformation, and for β, it is equatorial (up). Double-check the orientation of all substituents on the monosaccharides to ensure accuracy.

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Key Concepts

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

Pyranose and Furanose Structures

Pyranoses are six-membered cyclic forms of sugars, while furanoses are five-membered forms. Understanding these structures is crucial for drawing disaccharides, as they dictate the spatial arrangement of atoms and the overall conformation of the sugar. The chair conformation of pyranoses minimizes steric strain and allows for a more stable representation of the molecule.
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Glycosidic Bonds

Glycosidic bonds are covalent linkages formed between the anomeric carbon of one sugar and a hydroxyl group of another. The type of glycosidic bond (e.g., α or β) influences the properties and reactivity of the disaccharide. Recognizing the specific glycosidic linkages in the given disaccharides is essential for accurately drawing their structures.
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Chair Conformation

The chair conformation is a three-dimensional representation of cyclohexane derivatives, including pyranoses, that minimizes steric hindrance and torsional strain. This conformation allows for the most stable arrangement of substituents on the ring. When drawing disaccharides, using chair conformations helps visualize the spatial orientation of the sugar units and their substituents.
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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

(c) 1,3,6-tri-O-methyl-D-fructofuranose

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

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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

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

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Textbook Question

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

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

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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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