Discussion
Find out the correct structure of L-Ribose if the structure of D-Ribose is :
- A.
- B.
- C.
- D.
Answer: B
L-Ribose is structure B.
L-Ribose is the enantiomer of D-ribose, so every stereocentre must be inverted. In the zigzag drawing of D-ribose the OH groups at C4, C3 and C2 are hashed, wedged and hashed. Inverting all three gives wedged, hashed, wedged, with the same carbon skeleton and the CHO and CH2OH ends unchanged, which is exactly structure B.
Structure A inverts only C4, structure C inverts C4 and C3, and structure D inverts C2. Inverting some but not all centres gives a diastereomer (another pentose), not the enantiomer.
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