Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Protecting Groups

• Protecting groups are used in synthesis to temporarily mask the characteristic chemistry of a functional group because it interferes with another reaction.
• A good protecting group should be easy to put on, easy to remove and in high yielding reactions, and inert to the conditions of the reaction required.
• For example, consider the following scenario: How can you perform the following reaction ?

The overall transformation required is ester to primary alcohol.  This is a reduction of the ester, which requires LiAlH4, but that will reduce the ketone as well which we don't want. We can avoid this problem if we "change" the ketone to a different functional group first. Conceptually, this is like being able to put a cover (shown below) over the ketone while we do the reduction, then remove the cover.

In reality, the "molecular cover" is a protecting group. In this example, we protect the ketone as an acetal (which is an ether and doesn't react with LiAlH4)

Then we can reduce the ester to the primary alcohol.

Finally we can remove the protecting group:

Overall, this gives us the complete scheme:

© Dr. Ian Hunt, Department of Chemistry