Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 1: Advantages and Disadvantages
              of Radical Reactions

Chapter 1 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 1 button below. The drawing underneath the button shows a reaction that produces a five-membered ring containing a highly hindered carbon atom. Beside the drawing is a more detailed description of the reaction. Below the drawing and its description is a summary of Chapter 1.

Chapter 1: Advantages and Disadvantages of Radical Reactions

This drawing shows a radical cyclization reaction.

Drawing Description

One advantage of radical reactions is that they can produce ring systems containing highly hindered carbon atoms. When reaction can afford either a five-membered or a six-membered ring, the thermody-namically less stable, five-membered ring typically forms. (Scheme 2 in Chapter 1, shown on the left, describes an example of a radical cyclization reaction that regioselectively gives the less stable of two possible ring systems.)

Summary of Chapter 1

Whenever a structural change is needed in a molecule, an early (if not the first) question is “What is the best way to accomplish this change?” For many years the answer to this question rarely involved a radical reaction (unless polymerization was taking place) because the synthetic potential of radical reactions was viewed in a negative light. As understanding of radical reactions blossomed during the latter part of the 20th Century, this situation changed, and radical-based processes were seen increasingly not only as synthetically viable possibilities but sometimes as the best choice. Nowhere was the option of conducting a radical reaction more attractive than in carbohydrate chemistry because the combined chemoselectivity, regioselectivity, and stereoselectivity of these reactions was particularly well suited for structural change in polyfunctional molecules.

 When deciding whether or not to conduct a radical reaction, certain information is crucial. It is important to know as completely as possible how intermediate radicals will form and, once formed, how these radicals will react with various reagents and solvents present in the reaction mixture. This information not only points to the expected product but also answers questions such as: What side reactions could take place? How might these reactions be avoided or minimized? What is the outcome of reactions that have been reported for similar compounds? A fitting way to begin framing the answers to these questions is by discussing the advantages and disadvantages of radical reactions. This discussion takes place in Chapter 1.