Functionalization methods protection-deprotection approaches

In addition to the alkene-functionalized polymer approach of making side-chain functionalized polymer, other approaches have also been reported. For example, based on the protection-deprotection chemistry of ortho-nitrobenzyl moiety, Barner-Kowollik and coworkers investigated light-triggered release of thiols at ambient temperature and developed a novel method to modify functionalization... 

Another important application of the polymer reagent method is in the synthesis of cyclic peptides. In this approach, the amino-protected linear peptide (7(5) is converted into the corresponding polymeric active ester (17) by coupling it to a cross-linked hydroxyl function containing polymer (eg. 12) on deprotection of the amino group and subsequent neutralization, the cyclic peptide (18) is obtained in good yields 109 110> ... 

Since the required polymer is a functionalized polystyrene, the most sensible approach would be to co-polymerize styrene and some 3,4-dihydroxystyrene, perhaps protected as an acetal or a silyl derivative. A proportion of the benzene rings in the polystyrene would have the correct functionalization and the crown ether could be built on to them by passing a large excess of a suitable reagent, such as one of those we discussed in Problem 2 of this chapter or in the main text (p. 1456), deprotecting as required. A potassium salt would be used as a base in the final cyclization to take advantage of complexation by the crown ether. The various methods of polymerizing styrene (radical, anionic, etc.) are described in the chapter (pp. 1459-62). 

The classical chemical peptide synthesis is a synthesis in a homogeneous solution t41 46l. Even in the 1950s this approach had started to gain industrial importance followed by the solid-phase technique in the early 1960s, invented by the Nobel laureate Bruce Merrifieldl47-501. The most fundamental time-consuming operations in chemical peptide synthesis (sometimes not free from undesirable side reactions) are the selective protection, and after synthesis the deprotection of the a-amino function, the carboxyl group and the various side chain functionalities of trifunctional amino acids. Despite the development of numerous efficient protection methods based on chemical techniques, the whole process is rather slow as all intermediate products have to be purified and characterized after each reaction step. The formation of each peptide bond requires the activation of the carboxylic acid function of the carboxyl moiety. 

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