Question

An important technique for establishing relative configurations among isomeric aldoses and ketoses is to convert both terminal carbon atoms to the same functional group. This can be done either by selective oxidation or reduction. As a specific example, nitric acid oxidation of Derythrose gives meso-tartaric acid (Section 3.4B). Similar oxidation of \(\mathrm{D}\)-threose gives \((2 S, 35)\)-tartaric acid. Gaven this information and the fact that D-erythrose and D-threose are diastereomers, draw Fischer projections for D-erythrose and D-threose. Check your answers aggainst Table 25.1.

Short answer

Answer: The Fischer projections for D-erythrose and D-threose are as follows: D-erythrose: \[\begin{array}{c} HOCH=O \\ \begin{array}{@{}c@{\;}|@{\;}c} \uparrow & \uparrow \\ S \text { H } O \text { C } S & S H O C S \\ \uparrow & \uparrow \\ H & H \end{array} \end{array} \] D-threose: \[\begin{array}{c} HOCH=O \\ \begin{array}{@{}c@{\;}|@{\;}c} \downarrow & \uparrow \\ S H O C S & S H O C S \\ \downarrow & \uparrow \\ H & H \end{array} \end{array} \]

Step-by-step solution

Understand D-erythrose and D-threose oxidation products

When D-erythrose is oxidized by nitric acid, it produces meso-tartaric acid. Meso-tartaric acid is a compound that has two chiral centers, but the molecule is achiral because it has a plane of symmetry. This means that both chiral centers must have the same configuration. When D-threose is oxidized, it produces (2S,3S)-tartaric acid. The compound has two chiral centers, and both have "S" configuration.

Draw the basic structures of aldoses

D-erythrose and D-threose are both aldoses with four carbon atoms, which means they are tetrose sugars. Their basic structures have an aldehyde group at the top and a primary alcohol group at the bottom. D-erythrose and D-threose are diastereomers, so they differ only in the configuration of their chiral centers.

Add chiral centers to the structures

We have two asymmetric carbon (chiral) centers in both D-erythrose and D-threose: C-2 and C-3. We will use the oxidation product information to deduce the configurations of these chiral centers.

Deduce the configurations for D-erythrose and D-threose

Since the oxidation of D-erythrose results in meso-tartaric acid, both chiral centers (C-2 and C-3) in D-erythrose must have the same configuration. As D-erythrose and D-threose are diastereomers, the configuration of at least one of the chiral centers should be different. D-threose oxidation results in (2S,3S) tartaric acid. We can deduce that the configurations of D-threose chiral centers are both “S".

Draw Fischer projections for D-erythrose and D-threose

Finally, we can draw the Fischer projections for D-erythrose and D-threose, taking into account the information we have about their chiral centers: D-erythrose: OH substituents at C-2 and C-3 must have the same configuration. The most stable configuration for D-erythrose is 'S' at carbons (C-2 and C-3). \[\mathrm{D}-\text { erythrose }:\] \[ \begin{array}{c} HOCH=O \\ \begin{array}{@{}c@{\;}|@{\;}c} \uparrow & \uparrow \\ S \text { H } O \text { C } S & S H O C S \\ \uparrow & \uparrow \\ H & H \end{array} \end{array} \] D-threose: Based on the oxidation information, both OH substituents should have the 'S' configuration for D-threose: \[\mathrm{D}-\text { threose }:\] \[ \begin{array}{c} HOCH=O \\ \begin{array}{@{}c@{\;}|@{\;}c} \downarrow & \uparrow \\ S H O C S & S H O C S \\ \downarrow & \uparrow \\ H & H \end{array} \end{array} \]

Key concepts

Aldose and Ketose Configurations

Understanding the difference between aldose and ketose sugars is fundamental in the study of carbohydrates. Aldoses are sugars that contain an aldehyde functional group, typically at the top of the Fischer projection, which is how they are most commonly represented. Ketoses, on the other hand, have a ketone functional group, normally positioned at the second carbon atom.

When looking at structures like D-erythrose and D-threose, which are aldoses with an aldehyde at the top, you can see how they serve as the building blocks for more complex sugars. Their Fischer projections allow us to see the spatial arrangement of groups around the carbon atoms, which is crucial for understanding their reactivity and behavior in biological systems.

For sugars like D-erythrose and D-threose, they can be represented in the Fischer projection with the aldehyde at the top, and the chiral centers laid out beneath it. By knowing that these two sugars are diastereomers, we also understand that they must differ in the configuration at least one chiral center, providing a unique three-dimensional structure to each sugar.

Oxidation of Sugars

Oxidation reactions in sugars occur when an oxidizing agent, such as nitric acid, reacts with the sugar molecules. For aldose sugars, the aldehyde group can be oxidized to form a carboxylic acid. This transformation is not only a way to derive new compounds but also a method to deduce the configuration of the original sugar molecule.

D-erythrose and D-threose undergo oxidation to yield tartaric acid isomers. By examining the specific isomers produced—we obtain meso-tartaric acid from D-erythrose and (2S,3S)-tartaric acid from D-threose—we can deduce the absolute configuration of the original sugar's chiral centers. This kind of oxidative cleavage is valuable for structural elucidation and determining stereochemistry in complex carbohydrates.

Diastereomers in Sugars

Diastereomers are stereoisomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other, which distinguishes them from enantiomers. In the context of sugars like D-erythrose and D-threose, understanding diastereomers is key because sugars often exist as multiple stereoisomers due to the presence of several chiral centers.

The Fischer projections provided by the exercise solution reveal that while both sugars are tetrose and aldoses, they have different configurations at their chiral centers, making them diastereomers. This difference in spatial arrangement affects properties like melting point, solubility, and how they interact with other molecules, including enzymes and receptors. It's essential to grasp this concept for applications in biochemistry and pharmaceuticals, where the specific orientation of atoms can determine the efficacy of a drug or the function of a biological molecule.

Chiral Centers in Carbohydrates

Chiral centers are carbon atoms that are attached to four different groups, leading to non-superimposable mirror images, or enantiomers. In carbohydrates, chiral centers play a critical role in determining the structure and function of the sugar molecules.

In our example, D-erythrose and D-threose, each contain two chiral centers. The relative configuration of these centers defines the sugar's specific form and reactivity. This is because different enzymes and receptors are often stereospecific, interacting only with the correct three-dimensional arrangement. Thus, recognizing and being able to determine these chiral centers using tools like Fischer projections is essential for students as it underpins much of the understanding needed for carbohydrate chemistry, ranging from energy metabolism to the assembly of complex biomolecules.

The knowledge about how to read and interpret these chiral centers' configurations from a Fischer projection will enable students to approach more complex problems in organic chemistry with confidence.