One of the D-2 -ketohexoses is called sorbose. On treatment with NaBH4, sorbose yields a mixture

step 1 Problem 45 says if we reduce these sorbos, we're going to end up with these two molecules, the e

One of the D-2 -ketohexoses is called sorbose. On treatment...

Key Concepts

2-Ketohexoses

2-Ketohexoses are six?carbon sugars that contain a keto group (a carbonyl) at the C2 position. Their structure is characterized by having five hydroxyl groups and a carbonyl functionality that defines them as ketoses, distinguishing them from aldoses which contain an aldehyde group. Understanding the general backbone and functional groups of these sugars is fundamental when analyzing their chemical behavior and reactions.

Sodium Borohydride (NaBH4) Reduction

NaBH4 is a common reducing agent that selectively reduces carbonyl groups to alcohols without affecting most other functional groups. In the context of ketoses, the reduction of the keto group results in the formation of an additional hydroxyl group, converting the sugar into its corresponding sugar alcohol. This transformation plays a crucial role in structure determination because the reduction’s stereochemical outcomes can reveal information about the original stereochemistry at the carbonyl carbon.

Stereochemistry of Hydride Addition

The reduction of a planar carbonyl group by NaBH4 occurs via nucleophilic hydride attack from either face of the molecule, leading to the formation of two epimeric alcohols. This stereochemical outcome is significant when a reaction yields a mixture of products, as it reflects the non-stereoselective addition to the carbonyl carbon and helps in deducing the configuration of the original sugar. The observation of two distinct sugar alcohols indicates that the starting material’s carbonyl was achiral until the point of reduction.

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