What is Protecting group?

Example 48.8 (Making one dipeptide, not four)

Glycine, $\ce{H2N-CH2-COOH}$, and alanine, $\ce{H2N-CH(CH3)-COOH}$, each carry an amine group and an acid group. An amide bond between the acid of glycine and the amine of alanine gives the dipeptide Gly–Ala. Mixed directly, the two amino acids give four dipeptides, Gly–Ala, Ala–Gly, Gly–Gly and Ala–Ala, in a mixture hard to separate. The plan:

1. protect the amine of glycine with a group written $\ce{Z}$, and the acid of alanine as its methyl ester;
2. form the amide bond: only one acid group and one amine group are left free;
3. remove both protecting groups.

Example 23.8 (A ketone in the way)

To add a Grignard reagent to the aldehyde group of a molecule that also carries a ketone, no simple acetal choice exists (aldehydes form acetals more readily than ketones, so the wrong group would be protected); the order of the steps or the reagents must change. Protection is a strategy, not a reflex.