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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28. Carbohydrates
Monosaccharides - Forming Cyclic Hemiacetals
1:29 minutesProblem 47a
Textbook Question
Without referring to the chapter, draw the chair conformations of
(a) β-D-mannopyranose (the C2 epimer of glucose).
Verified step by step guidance1
Understand the structure of β-D-mannopyranose: It is the C2 epimer of β-D-glucopyranose, meaning the hydroxyl group (-OH) on carbon 2 (C2) is oriented differently compared to β-D-glucopyranose. In β-D-mannopyranose, the -OH group on C2 is axial (pointing down in the chair conformation), while in β-D-glucopyranose, it is equatorial (pointing outwards).
Draw the basic pyranose ring in the chair conformation: Start by sketching a six-membered ring in the chair form, which is the most stable conformation for pyranose sugars. Label the carbons from C1 to C6 in a clockwise direction, starting with the anomeric carbon (C1).
Place the substituents on the ring: For β-D-mannopyranose, the anomeric hydroxyl group (-OH) on C1 is equatorial (pointing outwards), as it is in the β-anomer. The hydroxyl group on C2 is axial (pointing down), and the remaining hydroxyl groups on C3, C4, and C6 are equatorial. The -CH2OH group on C5 is also equatorial.
Verify the stereochemistry: Double-check the orientation of each substituent to ensure it matches the β-D-mannopyranose structure. Remember that the axial and equatorial positions alternate around the ring, and the stereochemistry of each carbon is determined by the sugar's configuration.
Draw the final chair conformation: Clearly depict the chair conformation of β-D-mannopyranose with all substituents in their correct positions. Highlight the axial -OH group on C2 to emphasize the difference from β-D-glucopyranose.
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Video duration:
1mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chair Conformation
Chair conformation is a three-dimensional representation of cyclohexane and its derivatives, which minimizes steric strain and torsional strain. In this conformation, the carbon atoms are arranged in a staggered manner, allowing for more stable interactions between substituents. Understanding chair conformations is crucial for analyzing the spatial arrangement of atoms in cyclic sugars like mannopyranose.
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Pyranose Structure
Pyranose refers to a six-membered ring structure formed by the cyclization of monosaccharides, specifically aldoses and ketoses. In the case of D-mannopyranose, the ring consists of five carbon atoms and one oxygen atom. Recognizing the pyranose structure is essential for understanding the chemical properties and reactivity of sugars, as well as their conformational variations.
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Epimerization
Epimerization is the process by which two sugars differ in configuration at only one specific carbon atom. In the context of D-mannopyranose, it is the C2 epimer of glucose, meaning they differ in the orientation of the hydroxyl group at the second carbon. This concept is important for distinguishing between different sugars and understanding their biological roles and interactions.
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