Ch. 20 - The Organic Chemistry of Carbohydrates

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 20 - The Organic Chemistry of Carbohydrates

Problem 71

Chapter 21, Problem 71

Propose a mechanism for the rearrangement that converts an ⍺-hydroxyimine to an ⍺-aminoketone in the presence of a trace amount of acid.

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Step 1: Protonation of the hydroxyl group (-OH) on the α-hydroxyimine occurs in the presence of trace acid (H₃O⁺). This increases the electrophilicity of the carbon atom attached to the hydroxyl group, making it more susceptible to nucleophilic attack.

Step 2: The protonated hydroxyl group (-OH₂⁺) leaves as water (H₂O), generating a carbocation at the α-carbon. This carbocation is stabilized by resonance with the imine group.

Step 3: The lone pair of electrons on the nitrogen atom of the imine group migrates to form a double bond with the α-carbon, resulting in the formation of a new bond and shifting the positive charge onto the nitrogen atom.

Step 4: A molecule of water (H₂O) attacks the positively charged nitrogen atom, leading to the formation of an intermediate where the nitrogen is bonded to both the α-carbon and a hydroxyl group.

Step 5: Deprotonation of the hydroxyl group on the nitrogen occurs, resulting in the final product: an α-aminoketone, where the α-carbon is bonded to a ketone group and the nitrogen is bonded to a hydrogen atom.

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Key Concepts

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

Hydroxyimine Structure

A-hydroxyimines are compounds containing a hydroxyl group (-OH) and an imine group (C=N) on adjacent carbon atoms. This structure is crucial for understanding the rearrangement mechanism, as the hydroxyl group can participate in protonation and subsequent reactions under acidic conditions, facilitating the conversion to an a-aminoketone.

Protonation and Acid Catalysis

In the presence of a trace amount of acid, such as H3O+, protonation of the hydroxyl group occurs, enhancing its leaving ability. This step is essential in the mechanism, as it leads to the formation of a more reactive intermediate that can rearrange to form the a-aminoketone, demonstrating the role of acid catalysis in organic reactions.

Rearrangement Mechanism

The rearrangement mechanism involves the transformation of the a-hydroxyimine to an a-aminoketone through a series of steps, including protonation, loss of water, and reorganization of the double bond. Understanding this mechanism is vital for predicting the product and the reaction pathway, highlighting the importance of structural changes in organic synthesis.

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Definition of Claisen Rearrangement

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Textbook Question

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in axial positions, the two groups can react to form an acetal. This is called the anhydro form of the sugar (it has 'lost water'). The anhydro form of d-idose is shown here. Explain why about 80% of d-idose exists in the anhydro form in an aqueous solution at 100 °C, but only about 0.1% of d-glucose exists in the anhydro form under the same conditions.

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Textbook Question

An unknown disaccharide gives a positive Tollens' test. A glycosidase hydrolyzes it to D-galactose and D-mannose. When the disaccharide is treated with methyl iodide and Ag2O and then hydrolyzed with dilute HCl, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-O-methylmannose. Propose a structure for the disaccharide.

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Textbook Question

Trehalose, C₁₂H₂₂O₁₁, is a nonreducing sugar that is only 45% as sweet as sugar. When hydrolyzed by aqueous acid or the enzyme maltase, it forms only D-glucose. When it is treated with excess methyl iodide in the presence of Ag₂O and then hydrolyzed with water under acidic conditions, only 2,3,4,6-tetra-O-methyl-d-glucose is formed. Draw the structure of trehalose.

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Textbook Question

Predict whether D-altrose exists preferentially as a pyranose or a furanose. (Hint: In the most stable arrangement for a five-membered ring, all the adjacent substituents are trans.)

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