Draw the Fischer projection of a D-aldopentose B) Provide the arrow-pushing mechanism showing how this sugar can isomerize under basic conditions to its epimer

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(8 pts )A) Draw the Fischer projection of a D-aldopentose B) Provide the arrow-pushing mechanism showing how this sugar can isomerize under basic conditions to its epimer

The Correct Answer and Explanation is:

A) Fischer Projection of a D-Aldopentose:

There are several D-aldopentoses, but one of the most common is D-ribose. Here’s the Fischer projection of D-ribose (an example of a D-aldopentose):

mathematicaCopyEdit     CHO
      |
H – C – OH
      |
H – C – OH
      |
H – C – OH
      |
     CH2OH

The key feature of a D-aldopentose is:

  • A five-carbon sugar (pentose),
  • An aldehyde group at C1 (making it an aldose),
  • The OH on the highest-numbered chiral carbon (C4 in this case) is on the right, making it D.

B) Mechanism: Base-Catalyzed Epimerization of a D-Aldopentose

Under basic conditions, an aldopentose such as D-ribose can epimerize at C2, converting into its epimer (e.g., D-arabinose). This happens via an enediol intermediate. Here’s the arrow-pushing mechanism:

  1. Deprotonation:
    • A base (like OH⁻) abstracts the proton from the α-hydrogen next to the aldehyde (at C2), forming an enolate ion.
  2. Enediol Formation:
    • The electrons from the enolate form a double bond between C1 and C2, and the π electrons of the carbonyl move to oxygen, forming an enediol intermediate (with OH groups on both C1 and C2 and a C=C bond between them).
  3. Reprotonation:
    • The enediol tautomerizes back to the aldose form, but now the proton can be added to either face of C2, giving either the original sugar (D-ribose) or its C2-epimer (D-arabinose).

Explanation

A D-aldopentose is a five-carbon sugar with an aldehyde group and D-configuration, meaning the OH group on the highest numbered chiral carbon is on the right in a Fischer projection. An example is D-ribose, with the configuration (from top to bottom): CHO, H–C–OH, H–C–OH, H–C–OH, CH₂OH. D-aldopentoses can undergo isomerization reactions under basic conditions, one common type being epimerization—a change in configuration at a single chiral center.

The mechanism proceeds via an enediol intermediate. In base, the α-hydrogen next to the aldehyde (on C2) is slightly acidic. Hydroxide abstracts this proton, forming a resonance-stabilized enolate ion. The enolate tautomerizes to form an enediol: a molecule with a double bond between C1 and C2 and hydroxyl groups on both.

This enediol is a symmetrical intermediate. When it tautomerizes back to an aldose, protonation can occur on either face of the planar C2. As a result, the hydroxyl group at C2 may end up on either the same side (reforming the original sugar) or the opposite side (forming the epimer at C2).

Thus, D-ribose can convert into D-arabinose through this reversible process. This mechanism is important in carbohydrate chemistry and underlies the Lobry de Bruyn–Alberda van Ekenstein transformation, a base-catalyzed isomerization involving aldoses, ketoses, and their epimers.

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