Chapter 5 - Functional Group Exchange Reactions: Protecting Groups

Abstract

Many problems arise during a synthesis. Some synthetic targets contain more than one functional group. If one group reacts with a reagent competitively with another group, or if one modifies the reactivity of the other, a synthesis can be in serious trouble. Ketones and aldehydes react similarly with nucleophilic species (e.g., a Grignard reagent, Section 11.4) and if both units are in the same molecule at the same time it is an obvious point of concern. Aldehydes are generally more reactive, but the difference in rate may be insufficient to make the reaction chemoselective. The presence of two or more alcohol moieties in a molecule, or an alcohol and an amine in the same molecule at the same time will also cause problems since similar reactions are possible. One practical solution to such problems is to temporarily block one or more reactive positions by transformation into a new functional group that will not interfere with the desired transformation. That blocking group is called a protecting group. Using a protecting-group strategy in a synthesis requires at least two chemical reactions, however. The first reaction transforms the interfering functional group into a different one that will not compete with the desired reaction, and is termed protection. The second chemical step transforms the protecting group back into the original group at a later stage of the synthesis. This latter process is termed deprotection.