Question

A D-aldohexose A is formed from an aldopentose B by the Kiliani-Fischer synthesis. Reduction of A with forms an optically inactive alditol. Oxidation of B forms an optically active aldaric acid. What are the structures of A and B?

Short answer

Answer

Step-by-step solution

Kiliani Fischer synthesis

Kiliani Fischer synthesis constitutes the procedure to synthesize several monosaccharides. It is used to generate sugars that cannot be acquired from natural sources.

Oxidation and reduction of aldoses

The carbonyl group of a specific aldose is converted to primary alcohol using a reagent like NaBH₄ . The alcohol produced in this process is termed an alditol. Aldonic acid or aldaric acid is generated during the oxidation of aldoses.

Structures of A and B formed from Kiliani Fischer synthesis

The optically inactive aldols formed from the reduction of a D-aldohexose is given as:

Optically inactive aldols formed from the NaBH₄ reduction of a D-aldohexose

Two D-aldohexoses (A’ and A’’) produce optically inactive alditols on reduction. A’’ is formed from B’’ by Kiliani-Fischer synthesis. B’’ gives an optically active aldaric acid on oxidation. B’’ is B, and A’’ is A.

The other possibility (A’) is produced from an aldopentose B’ that provides an optically inactive aldaric acid on oxidation. The structures of A’, A’’, B’ and B’’ are given as:

The optically inactive compound from the oxidation of B’

Structures of A and B