Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 7: Unprotected Carbohydrates

Chapter 7 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 7 button below. The drawing underneath the button gives an orbital picture of the abstraction of H-1 from D-glucopyranose anomers. Beside the drawing is a description of the abstraction process. Below the drawing and its description is a summary of Chapter 7.

Chapter 7: Unprotected Carbohydrates

This drawing shows abstraction of H-1 by the sulfate radical anion.

Drawing Description

Abstraction of  H-1 occurs more rapidly from β-D-glucopyranose than from its α-anomer because orbital alignment in the developing radical during reaction of the β-anomer stabilizes the transition state more effectively than does orbital align-ment during reaction of the α-anomer. (The drawing shown on the left is Figure 3 in Chapter 7.)

Summary of Chapter 7

The hydroxyl radical and the sulfate radical anion both abstract hydrogen atoms from unprotected carbohydrates. In each case first-formed, carbon-centered radicals are produced. The hydroxyl radical is so reactive that it shows little regioselectivity when reacting with D-glucose; that is, spectroscopic evidence indicates that hydrogen-atom abstraction occurs from each of the six carbon atoms in this molecule. The hydroxyl radical remains unselective in reaction with other simple sugars that contain only pyranoid rings, but it does regioselectively abstract H-5' from the furanoid ring in sucrose. The sulfate radical anion is a more selective abstracting agent. Hydrogen-atom abstraction occurs primarily from C-2, C-5, and C-6 in α-D-glucopyranose and at C-1, C-5, and C-6 in β-D-glucopyranose.

First-formed radicals derived from D-glucose undergo acid-catalyzed rearrangement under strongly acidic conditions to produce carbonyl-conjugated radicals. Under basic conditions first-formed radicals produce radical anions that form semidiones. When oxygen is present in the reaction mixture, first-formed radicals react to give peroxy radicals.