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Propose an acid-catalyzed mechanism by which cytosine can form a β-1 N-linkage with 2-deoxy-β-Dribofuranose to produce a deoxynucleoside (DNA) called deoxycytidine.
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28. Carbohydrates
Monosaccharides - N-Glycosides
4:07 minutesProblem 44a
Textbook Question
An aliphatic aminoglycoside is relatively stable to base, but it is quickly hydrolyzed by dilute acid. Propose a mechanism for the acid-catalyzed hydrolysis.
Verified step by step guidance1
Step 1: Recognize that the reaction involves acid-catalyzed hydrolysis of an aliphatic aminoglycoside. The structure provided shows a glycosidic bond between a sugar moiety and an amine group (R2N). Acid catalysis typically involves protonation of the oxygen atom in the glycosidic bond, making it more susceptible to cleavage.
Step 2: Protonation occurs at the oxygen atom of the glycosidic bond by hydronium ion (H3O+). This increases the electrophilicity of the carbon atom attached to the oxygen, making it more prone to nucleophilic attack or bond cleavage.
Step 3: The protonated glycosidic bond undergoes cleavage. The bond between the sugar and the amine group breaks, resulting in the formation of a positively charged amine (R2NH2+) and a sugar molecule. This step is facilitated by the acidic environment.
Step 4: The sugar molecule formed may undergo further protonation or rearrangement depending on the reaction conditions, but the primary products are the free sugar and the protonated amine group.
Step 5: The mechanism highlights the role of acid in destabilizing the glycosidic bond and promoting hydrolysis. This explains why the aminoglycoside is stable in base but hydrolyzed quickly in acid.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Catalyzed Hydrolysis
Acid-catalyzed hydrolysis is a reaction where an acid donates protons (H+) to facilitate the breaking of chemical bonds in a molecule, leading to the formation of new products. In the case of aminoglycosides, the presence of hydronium ions (H3O+) enhances the nucleophilicity of water, allowing it to attack the glycosidic bond, resulting in the cleavage of the molecule into simpler components, such as sugar and an amine.
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Acid Catalyzed
Aminoglycosides Structure
Aminoglycosides are a class of antibiotics characterized by their amino sugar components and glycosidic linkages. Their structure typically includes one or more amino groups attached to sugar moieties, which are crucial for their biological activity. Understanding the structural features of aminoglycosides is essential for predicting their reactivity, particularly how they respond to acidic conditions during hydrolysis.
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Mechanism of Hydrolysis
The mechanism of hydrolysis involves a series of steps where the acid protonates the leaving group, making it a better leaving entity. This is followed by the nucleophilic attack of water on the electrophilic carbon, leading to the formation of a carbocation intermediate. The subsequent steps involve the breakdown of the carbocation and the release of the products, which in this case are a sugar and a protonated amine (R2NH2+).
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