Each of the following transformations involves a Grignard reaction and requires the use of a protecting group. Identify reagents that can be used to achieve each transformation.

This question addresses the use of protecting groups in Grignard reactions, where specific functional groups are masked to direct the reaction to the desired sites. Protecting groups such as TMS ethers guard reactive sites until their deprotection regenerates the original functionality after the Grignard reaction has taken its course.

Explanation:

The question concerns the identification and use of protecting groups in Grignard reactions to synthesize particular organic compounds. In such transformations, a functional group that could potentially react with the Grignard reagent is masked to prevent reaction at that site. Once the desired transformation has been effected, the protecting group can be removed to reveal the original functional group.

For instance, when reacting a Grignard reagent with a compound containing both an ester and a ketone group, it may be necessary to protect the ketone as an acetal to ensure the Grignard reagent only reacts with the ester site. After the reaction with the ester is completed, the protecting group can be removed from the ketone through acidic hydrolysis, regenerating the original ketone functionality.

One common protecting group for alcohols in Grignard reactions is the trimethylsilyl (TMS) ether, which makes the alcoholic hydroxyl group unreactive towards Grignard reagents. After the Grignard reaction, the TMS group can be removed using a fluoride source such as TBAF, thus regenerating the alcohol.