3

The union of two amino acids to form a peptide requires suppression of the reactivities of the functional groups that are not incorporated into the peptide bond (see Section 1.5). This is achieved by combining each group with another compound in a manner that allows removal of the added moieties at will. These moieties are referred to as protecting groups. The starting materials for peptide synthesis are thus protected amino acids. Several characteristics are essential or desirable for a good protector. First and foremost, it must be removable, preferably with ease and by a mechanism that does not lead to side reactions. The suppression of reactivity should be complete and last throughout the synthesis, and the products generated by the protecting moiety must be separable from the target molecule. Both preparation of the amino acid derivative and removal of the protector must take place with preservation of the chiral integrity of the residue. Finally, the derivative should be crystalline and stable during storage and obtainable by a process that is not too laborious or expensive. In some cases, compromises are accepted by choice or necessity.