Multiple Choice
Which type of reaction is responsible for the cyclization of monosaccharides in the Calvin cycle?
28. Carbohydrates
Monosaccharides - Cyclization
Problem 9
Textbook Question
Talose is the C4 epimer of mannose. Draw the chair conformation of D-talopyranose.
Verified step by step guidance1
Understand the problem: Talose is the C4 epimer of mannose, meaning that the configuration of the hydroxyl group (-OH) on the fourth carbon (C4) is inverted compared to mannose. We are tasked with drawing the chair conformation of D-talopyranose, which is the cyclic form of D-talose in a six-membered ring structure.
Start by determining the Fischer projection of D-talose. Since it is the C4 epimer of mannose, copy the Fischer projection of D-mannose and invert the configuration of the hydroxyl group on C4. Ensure the other stereocenters (C2, C3, and C5) remain the same as in D-mannose.
Convert the Fischer projection of D-talose into a Haworth projection for D-talopyranose. To do this, cyclize the molecule by forming a bond between the hydroxyl group on C5 and the carbonyl carbon (C1), creating a hemiacetal. Assign the anomeric configuration (α or β) based on the orientation of the newly formed hydroxyl group on C1.
Translate the Haworth projection into a chair conformation. Place the six-membered ring in a chair form, ensuring that the substituents (hydroxyl groups and hydrogen atoms) on each carbon are positioned either axially (parallel to the ring's axis) or equatorially (perpendicular to the ring's axis). Use the stereochemistry from the Haworth projection to determine the correct positions of the substituents.
Double-check the stereochemistry of the chair conformation to ensure it matches the original Fischer projection of D-talose. Verify that the hydroxyl group on C4 is in the correct position to reflect the epimeric relationship with mannose, and confirm that the other substituents are consistent with the D-talose configuration.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Epimers
Epimers are a specific type of diastereomer that differ in configuration at only one stereogenic center. In the case of talose and mannose, they are both aldohexoses that differ at the C4 carbon. Understanding epimerism is crucial for distinguishing between similar sugars and predicting their chemical behavior.
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Chair Conformation
The chair conformation is a three-dimensional representation of cyclohexane and its derivatives, which minimizes steric strain and torsional strain. For sugars like D-talopyranose, drawing the chair conformation helps visualize the spatial arrangement of substituents around the ring, which is essential for understanding their reactivity and interactions.
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Pyranose Structure
Pyranose refers to a six-membered ring structure formed by the cyclization of monosaccharides, typically involving an aldehyde or ketone group. In D-talopyranose, the ring structure is stabilized by the formation of an acetal linkage, which is important for its stability and biological function. Recognizing the pyranose form is key to understanding carbohydrate chemistry.
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