R

Peptide synthesis, Acid anhydrides, Oxidation o-

Polymeric ylides,

Mitsunobu,

Halogenation

Peptide synthesis

Fig. B.14.2. The first polymer reagents, either in the form of ion-exchange resins or covalently attached, were employed for a variety of simple transformations in the 1960s and 70s.

solution of the respective salt is added to the resin in excess, and removed after equilibration by washing with non-ionic solvents. Leaching of the reactive ions is, however, a general problem of this type of support. In principle, the immobilized ions can be exchanged by other competing ions in the solution. Today several important polymer reagents are still based on ionexchange resins; examples include the borohydride resins [6, 7] for reductive amination and the perruthenate resin [8] for oxidation.

The next generation of polymer-supported reagents was based on co-valently linked reactants. The concept was initiated in the 1960s and 1970s with the introduction of peptide coupling reagents such as supported car-bodiimides [3, 9], and active esters [10], of supported phosphines [11], and of polymeric bases (Figure B.14.2). A major advantage of covalent linking is to avoid leaching of the immobilized reagents from the resin. At that time polymer-supported chemistry was, however, still limited to few fundamental conversions and was not accepted as a useful synthetic method beyond the areas of peptide and oligonucleotide chemistry. Only when solid-phase chemistry became a powerful tool of combinatorial chemistry in the 1990s did polymer-supported reagents also gain acceptance and were employed for generation of compound libraries and for multi-step syntheses of natural products.

For these reasons research on polymer reagents has soared during recent years. Today, increasingly demanding reactions can be performed with the help of polymer reagents based on the successful isolation of reactive intermediates [2]. Further progress must be made in the development of support materials tailored as carriers of polymer reagents [12].

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