Subjecting D-(-) -threose to a Kiliani-Fischer synthesis...

Subjecting $\mathrm{D}-(-)$ -threose to a Kiliani-Fischer synthesis yields two other epimeric aldopentoses, $\mathrm{D}-(+)$ -xylose and $\mathrm{D}-(-)$ -lyxose. $\mathrm{D}-(+)$ - Xylose can be oxidized (with nitric acid) to an optically inactive aldaric acid, while similar oxidation of $\mathrm{D}-(-)$ -lyxose gives an optically active product. What are the structures of $\mathrm{D}-(+)$ -xylose and $\mathrm{D}-(-)$ -lyxose?

Key Concepts

Kiliani?Fischer Synthesis

A classic method in carbohydrate chemistry used to homologate an aldose sugar by one carbon unit. It involves the formation of a cyanohydrin intermediate followed by hydrolysis, leading to a mixture of epimeric sugars, which is central to the production of different aldopentose stereoisomers from a tetrose precursor.

Epimerization in Carbohydrate Chemistry

Epimerization refers to the process where two stereoisomers (epimers) differ in configuration at only one stereogenic center. In the context of carbohydrate synthesis, certain reactions can change the configuration at one chiral center, producing different sugars that have distinct physical and optical properties.

Stereochemistry of Aldopentoses

The detailed three?dimensional arrangement of atoms in aldopentoses, such as the positioning of hydroxyl groups relative to the aldehyde group, is crucial in defining their identities. The specific configurations (D or L designation and the positive or negative optical rotation) determine the sugar's reactivity and the outcome of further chemical transformations.

Oxidation Reactions and Optical Activity

Oxidation of sugars, for example using nitric acid, can convert an aldose into an aldaric acid. The product’s optical activity (or lack thereof) depends on its symmetry and chiral centers. In a reaction producing an optically inactive product, the molecule often gains a plane of symmetry, whereas retaining chirality results in an optically active compound.

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