Question

The Kiliani-Fischer chain extension is a method for lengthening an aldose chain by one carbon, giving two new aldoses that differ in stereochemistry at C2. D-Erythrose, for instance, yields a mixture of D-ribose and D-arabinose on Kiliani-Fischer chain extension. \(\begin{array}{lll}\text { D-Erythrose } & \text { D-Pibose } & \text { D-Arabinose }\end{array}\) (a) What product(s) would you expect from Kiliani-Fischer reaction of D-ribose? (b) What aldopentose would give a mixture of L-gulose and \(\mathrm{L}\) -idose on Kiliani-Fischer chain extension?

Short answer

(a) D-allose and D-altrose; (b) L-ribose.

Step-by-step solution

Understanding the Initial Aldose

D-Ribose is an aldopentose, which means it is a five-carbon sugar with an aldehyde group. When it undergoes Kiliani-Fischer chain extension, it will form an aldohexose.

Performing the Chain Extension

Kiliani-Fischer chain extension involves adding one carbon to the aldose, which is done by converting the aldose into a cyanohydrin, then hydrolyzing it to an aldonic acid and reducing it to an aldohexose. The new carbon forms a new chiral center leading to two possible stereoisomers.

Determining the New Aldohexoses from D-Ribose

For D-ribose, the extension will result in the formation of D-allose and D-altrose. These two aldohexoses differ in configuration at the newly formed carbon, which is now C2 in the new six-carbon chain.

Identifying the Aldopentose for L-Gulose and L-Idose

To form L-gulose and L-idose from an aldopentose, we need an aldopentose that adds a carbon creating these two configurations. This aldopentose is L-ribose. When L-ribose undergoes Kiliani-Fischer chain extension, it forms L-gulose and L-idose.

Key concepts

Aldose

An aldose is a type of monosaccharide, or simple sugar, that contains an aldehyde group in its structure. The general formula for an aldose is \(C_n(H_2O)_n\). Aldoses are categorized based on the number of carbon atoms they carry.
- **Triose:** 3 carbon atoms
- **Tetrose:** 4 carbon atoms
- **Pentose:** 5 carbon atoms
- **Hexose:** 6 carbon atoms

D-ribose is an example of an aldopentose, indicating it has five carbons including an aldehyde group. The Kiliani-Fischer chain extension is typically applied to aldoses to increase their carbon chain, transforming them from an aldopentose to an aldohexose. This extension process is crucial because it allows the exploration of new sugars with varied structures and potential chemical activities.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms within a molecule and how this affects its chemical behavior. When performing a reaction like the Kiliani-Fischer chain extension, stereochemistry becomes very essential.

During the chain extension, a new carbon is added, resulting in new chiral centers. A chiral center is a carbon atom bonded to four different groups. This newly generated chiral center brings about the formation of different stereoisomers — molecules that are identical in bonding but different in spatial orientation.

• The Kiliani-Fischer process results in the creation of two isomers due to the different possible configurations at the new chiral center.
• D-allose and D-altrose are examples that arise from D-ribose due to stereochemical variations at C2.

Understanding stereochemistry enables chemists to predict and discern which spatial configurations will form and how they might behave chemically.

Cyanohydrin

In chemistry, a cyanohydrin is a functional group consisting of a nitrile and a hydroxyl group bonded to the same carbon atom. Cyanohydrins play a pivotal role in the Kiliani-Fischer chain extension process.

Here’s how it fits in the process:

• The initial step involves converting the aldose to a cyanohydrin. This provides the carbon skeleton necessary to extend the sugar chain.
• In simple terms, a cyanohydrin is formed by the reaction of the aldose with hydrogen cyanide (HCN), thereby introducing an additional carbon atom with a CN (cyano) group.

The formation of the cyanohydrin is critical because it sets up the subsequent steps where the cyano group is eventually transformed into a carboxyl group, paving the way for further chemical changes in the sugar structure.

Aldonic Acid

An aldonic acid is formed from the oxidation of aldose sugars, where the aldehyde group of the sugar is converted into a carboxyl group. This conversion is an integral step in the Kiliani-Fischer chain extension. The process follows these steps:

• The cyanohydrin is hydrolyzed, transforming it into an aldonic acid. This reaction involves converting the cyanhydrin's cyano group to a carboxyl group.
• Subsequently, this aldonic acid undergoes further reaction to yield another sugar form — the desired aldohexose.

Aldonic acids are important because they serve as intermediates in the extension process, allowing for the generation of longer sugar chains with new configurations. Understanding aldonic acid transformation is crucial for mastering chemical processes that modify simple sugar structures and create diversity in carbohydrate chemistry.