Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 10: Aldehydes and Ketones

Chapter 10 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 10 button below. The drawing underneath the button pictures the migration of the aldehydo group in the C-3 side chain to C-2. Beside the drawing is a description of the reaction. Below the drawing and its description is a summary of Chapter 10.

Chapter 10: Aldehydes and Ketones

This drawing shows the migration of an aldehydo group.

Drawing Description

Formation of a radical centered at C-2 is the first step in a migration reaction that transfers an aldehydo group from the side chain attached to C-3 to C-2. The drawing on the left, which is Scheme 8 in Chapter 10, contains a proposed mechanism for this migration process.

Summary of Chapter 10

Aldehydo and keto groups in carbohydrates react internally with carbon-centered radicals to form cyclic alkoxy radicals. Most of these reactions involve aldehydes; both five- and six-membered rings can be formed. When ring opening of the newly formed alkoxy radical takes place, it does so to produce the more stable of the two possible, carbon-centered radicals. Such ring opening can be part of a process that causes aldehydo group migration.

Tin-centered radicals add to aldehydes to generate tin ketyls, intermediates that can add to compounds with multiple bonds. Samarium ketyls, more common than their tin counterparts, undergo similar reaction. Reaction of samarium(II) iodide with carbohydrates containing two appropriately placed aldehydo groups converts one group to a ketyl that then adds to the second in route to formation of a pinacol.

Ketone photolysis forms carbon-centered radicals by breaking the bond between the car­bonyl carbon atom and one of the α carbon atoms (an α-cleavage). Typical radical reactions then take place; in addition, α-cleavage in some nucleotides and oligonucleotides is followed by radical cation formation. α-Cleavage and hydrogen-atom abstraction take place in photolysis of cyclic ketones to produce diradicals.