Question

The so-called Ruff degradation is a chain-shortening reaction in which an aldose chain is shortened by one C atom, hexoses, for example, being converted into pentoses. In the Ruff degradation, the calcium salt of an aldonic acid (the corresponding carboxylic acid of an aldose) is oxidized with hydrogen peroxide. Ferric ion catalyzes the reaction. The calcium salt of the aldonic acid necessary for the Ruff degradation is obtained by oxidizing an aldose with an aqueous solution of bromine and then adding calcium hydroxide. The reaction scheme is as follows where \(R\) represents the rest of the chain of the aldose. (a) Show that D-glucose can be degraded into D-arabinose. Which other aldose can be degraded into D-arabinose? (b) Which two monosaccharides can be degraded into D-glyceraldehyde by employing the Ruff degradation only once?

Short answer

a) D-glucose and D-mannose can be degraded into D-arabinose. b) D-erythrose and D-threose can be degraded into D-glyceraldehyde.

Step-by-step solution

Convert D-glucose to D-arabinose

In order to show how D-glucose can be degraded into D-arabinose, let's apply the Ruff degradation process to D-glucose. D-glucose, being a hexose, loses one carbon atom producing an aldose pentose, which is D-arabinose.

Identify Other Aldose Convertible to D-arabinose

After successfully converting D-glucose to D-arabinose, let's look for other aldose that can be converted similarly. The other aldose that can be degraded to D-arabinose is D-mannose. D-mannose and D-glucose are epimers that differ only in the stereochemistry at carbon 2, hence D-mannose will also degrade to D-arabinose via the Ruff degradation.

Determine Monosaccharides Convertible to D-glyceraldehyde

The two monosaccharides that can be degraded into D-glyceraldehyde using Ruff degradation only once are D-erythrose and D-threose. Both are tetrose sugars having one carbon atom more than D-glyceraldehyde. Applying Ruff degradation to either of these would result in the triose sugar, D-glyceraldehyde.

Key concepts

Aldose

An aldose is a type of monosaccharide that includes an aldehyde group at its top chain carbon atom. Aldoses are simple sugars that form the backbone of more complex carbohydrates. Examples include D-glucose and D-mannose. The presence of the aldehyde group allows for specific reactions, like oxidation, to occur. Because of this vulnerability, aldoses are integral to biochemical pathways such as glycolysis and the Ruff degradation method.
In the Ruff degradation, aldoses undergo an oxidation process to transform into other simpler sugars. This chain-shortening reaction is crucial for understanding how complex carbohydrates can be broken down into more basic units.

Calcium Salt

Calcium salts play a pivotal role in the Ruff degradation process. In the context of this degradation, a calcium salt forms when an aldonic acid (an oxidized aldose) reacts with calcium hydroxide. This calcium salt is crucial because it stabilizes the reaction ingredients and is a necessary precursor in the chain-shortening Ruff degradation.
Through this process, the calcium salt ensures that the aldonic acid is delivered in the right form for the subsequent steps that eventually lead to the conversion of a hexose into a pentose, like D-glucose into D-arabinose.

Oxidation Process

The oxidation process in Ruff degradation is vital as it involves the conversion of an aldose into its corresponding aldonic acid. This transformation is achieved using an aqueous solution of bromine, which acts as the oxidizing agent.
The resulting aldonic acid then forms a calcium salt, poised for further reaction with hydrogen peroxide to achieve the desired chain-shortened product. This two-step oxidation method not only reveals the chemical versatility of aldoses but also underpins their potential transformation into simpler sugars.

• First, aldose is oxidized to aldonic acid.
• The aldonic acid is converted into its calcium salt form.
• The calcium salt undergoes further oxidation to effectuate chain-shortening.

Monosaccharides

Monosaccharides are the most basic units of carbohydrates. With simple structures, they serve as building blocks for more complex carbohydrates like disaccharides and polysaccharides. In the context of the Ruff degradation, monosaccharides such as hexoses or pentoses are specifically targeted for transformation.
In the presented exercise, hexoses like D-glucose can be reduced to pentoses such as D-arabinose. This reaction showcases the versatile nature of monosaccharides and their readiness to undergo chemical reactions that alter their carbon framework.

Carboxylic Acid

In the Ruff degradation, a key intermediate is the formation of an aldonic acid, which is a kind of carboxylic acid. Carboxylic acids are organic compounds that contain a carboxyl group (-COOH). In the oxidation of aldoses, the terminal aldehyde group is transformed into a carboxylic acid.
This conversion is crucial because it paves the way for the production of calcium salts, which are significant in the subsequent oxidation steps that lead to Ruff degradation. The presence of the carboxyl group changes the chemical properties and reactivity of the original aldose, enabling further transformation in the reaction chain.