Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 6: Alkoxy Radicals
Chapter 6 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 6 button below. The drawing underneath the button pictures the formation and reaction of an oxygen-centered, carbohydrate radical. To the right of the drawing is a description of the reaction. Below the drawing and its description is a summary of Chapter 6.

Chapter 6: Alkoxy Radicals

this drawing describes internal hydrogen abstraction by an oxygen-centered radical.

Drawing Description

In the reaction shown in the drawing on the left (Scheme 6 in Chapter 6) the alkoxy radical 13 internally transfers a hydrogen atom via a seven-membered-ring transition state even though a six-membered one is possible. The more electron-rich hydrogen atom (Ha) is abstracted by the electrophilic oxygen atom even though a normally less stable, seven-membered-ring transition state is involved.

Summary of Chapter 6

Alcohols are converted indirectly into alkoxy radicals through intermediate hypoiodites, ni­trates, and phthalimides. A common reaction of alkoxy radicals is hydrogen-atom abstraction. This reaction becomes synthetically useful when the abstraction is internal because regioselective formation of a carbon-centered radical takes place. This selectivity depends on a combination of factors that include transition-state ring size, stability of the developing radical, and polarity matching between reacting atoms.

Alkoxy radicals also can undergo carbon–carbon bond fragmentation that produces a carbonyl group and a carbon-centered radical. No matter which pathway is taken by the alkoxy radical (hydrogen-atom abstraction or β fragmentation), the resulting carbon-centered radical can undergo new ring formation, epimerization at a chiral center, ring opening, and other reactions characteristic of this radical intermediate.