Compound A is an L-aldopentose. Oxidation of A with HNO3 yields an optically active aldaric acid B. Wohl degradation of A yields an L-aldotetrose C. Reduction of C with NaBH4 produces an optically active alditol D. Using this information, help draw structures A, B, C, and D.
The Correct Answer and Explanation is:
Correct Structures:
- Compound A (L-aldopentose): L-arabinose
- Compound B (aldaric acid): L-arabinaric acid
- Compound C (aldotetrose from Wohl degradation): L-erythrose
- Compound D (alditol from reduction): L-erythritol
Explanation
The problem begins with Compound A, which is described as an L-aldopentose. Aldopentoses are five-carbon sugars with an aldehyde group at C1 and hydroxyl groups on the remaining carbons. The term “L” refers to the stereochemistry relative to L-glyceraldehyde.
Among L-aldopentoses, we consider L-arabinose and L-xylose. However, further information helps narrow the identity.
Next, oxidation of A with nitric acid (HNO₃) forms Compound B, an aldaric acid. Nitric acid oxidizes both the aldehyde (C1) and the terminal alcohol (C5) to carboxylic acids. Importantly, Compound B is optically active. This rules out L-xylose, which would yield meso-xarabaric acid, a non-optically active (achiral) compound upon full oxidation. Therefore, Compound A must be L-arabinose, which forms optically active L-arabinaric acid.
Then, Wohl degradation removes one carbon from the aldehyde end (C1) of Compound A to give a four-carbon aldotetrose (Compound C). The product of this step from L-arabinose is L-erythrose, because it preserves the stereochemistry of carbons 2 through 4 of the original molecule.
Reduction of Compound C (L-erythrose) with NaBH₄ reduces the aldehyde group at C1 to a primary alcohol, yielding an alditol (Compound D). This reduction product is L-erythritol, which is optically active and retains chirality at both central carbon atoms (C2 and C3).
Each step in the sequence preserves or transforms the stereochemistry in a predictable way:
- Oxidation retains chirality at all centers.
- Wohl degradation removes the top carbon but retains the configuration of the remaining carbons.
- Reduction with NaBH₄ introduces no new stereocenters, only converts an aldehyde to an alcohol.
Therefore, the logical structures are:
- A: L-arabinose
- B: L-arabinaric acid
- C: L-erythrose
- D: L-erythritol
