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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 21a
Chapter 23, Problem 21a

Draw the structures of the compound methyl α-D-galactopyranoside.
Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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Step 1: Understand the problem. The task involves drawing the structure of methyl α-D-galactopyranoside and understanding the epimeric relationships between allose, glucose, ribose, and arabinose. An epimer is a stereoisomer that differs in configuration at only one chiral center.
Step 2: Start with methyl α-D-galactopyranoside. Galactose is a hexose sugar with the D-configuration. In the α-anomer, the anomeric hydroxyl group (-OH) is on the opposite side of the ring relative to the CH2OH group. Add a methyl group (-CH3) to the anomeric oxygen to form the glycosidic bond.
Step 3: Draw the structure of D-galactopyranose in its cyclic pyranose form. Use the Fischer projection of D-galactose to determine the stereochemistry of the hydroxyl groups. In D-galactose, the hydroxyl groups on carbons 2, 3, and 4 are oriented as follows: C2 (right), C3 (left), C4 (left). Convert this to the Haworth projection for the pyranose form.
Step 4: For allose and glucose, identify the C3 epimeric relationship. Glucose has the hydroxyl group on C3 pointing to the right in the Fischer projection, while in allose, the hydroxyl group on C3 points to the left. Draw the Fischer projection of glucose and modify the C3 configuration to obtain allose.
Step 5: For ribose and arabinose, identify the C2 epimeric relationship. Ribose has the hydroxyl group on C2 pointing to the right in the Fischer projection, while in arabinose, the hydroxyl group on C2 points to the left. Draw the Fischer projection of arabinose and modify the C2 configuration to obtain ribose.

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Key Concepts

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

Anomeric Carbon

The anomeric carbon is the carbon atom in a sugar that is derived from the carbonyl carbon of the open-chain form. In cyclic forms of sugars, it is the carbon that determines the alpha or beta configuration of the sugar. For methyl a-D-galactopyranoside, the anomeric carbon is crucial for understanding its structure and reactivity.
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Epimers

Epimers are a specific type of diastereomer that differ in configuration at only one specific carbon atom. In the context of the question, allose is an epimer of glucose at the C3 position, while ribose is an epimer of arabinose at the C2 position. Understanding epimerism is essential for drawing and distinguishing between these sugar structures.
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Pyranose and Furanose Forms

Pyranose and furanose refer to the cyclic forms of sugars, where pyranose is a six-membered ring and furanose is a five-membered ring. Methyl a-D-galactopyranoside specifically refers to the pyranose form of galactose. Recognizing these ring structures is vital for accurately representing the compounds in the question.
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Related Practice
Textbook Question

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has two –OH protons, and removing one or the other gives two different enolate ions.

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Textbook Question

The mechanism of glycoside formation is the same as the second part of the mechanism for acetal formation. Propose a mechanism for the formation of methyl β-D-glucopyranoside.

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Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(e)

(f)

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Textbook Question

Draw the structures of the compound ethyl β-D-ribofuranoside.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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Textbook Question

Draw the structures of the compound α-D-allopyranose.

Allose is the C3 epimer of glucose, and ribose is the C2 epimer of arabinose.

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Textbook Question

Which of the following are reducing sugars? Comment on the common name sucrose for table sugar.

(c) α-D-allopyranose

(d) ethyl β-D-ribofuranoside

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