Polymers as Supports and Protecting Groups

Polymers as Supports and Protecting Groups,—Although almost all of the early work involving polymer supports was concentrated on repetitive sequential syntheses such as those of polypeptides, nucleosides, and polysaccharides, many other applications have now been devised and have led to a better understanding of both the advantages and limitations of polymer-supported species. There have been a number of reviews.  

A polymer-supported rhodium-phosphine hydrogenation catalyst, in which the diphenylphosphine residue is spaced from the polymer backbone, hydrogenates oct-l-ene at a rate which is similar to or faster than that of hydrogenation by homogeneous catalysts.  

A polymer-supported reducing agent, prepared by treating poly(2-vinyIpyridine) with BHj-MCjS, rapidly reduces aldehydes and ketones in high yield.  

The chlorination of organic compounds may be achieved, under mild conditions, using chlorine, as ICIj, incorporated into an anion exchange resin.  

There have been several reports on oxidation reactions by copper(ii)-poly(4-vinylpyridine) complexes such as those of L-ascorbic acid, oxyanions of sulphur, phenol derivatives, and the dimerization of acetylenes.  

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M. J. Frechet, Tetrahedron 37, 663-683 (1981), Synthesis and Applications of Organic Polymers as Supports and Protecting Groups . 

J. M. J. Frechet, Synthesis and applications of organic polymers as supports and protecting groups. Tetrahedron 1981, 37, 663-683. 

The present chapter will review the current state-of-the-art regarding the use of polymers as supports for catalysts and related systems. The variety of applications where polymer supports have proved invaluable has expanded enormously in the last decade or so, and to review comprehensively even the limited area of polymer-supported reagents, catalysts and protected groups [1] would be a major undertaking. This chapter will not therefore attempt to do so, but instead will refer to the many excellent books and reviews on this subject, and then describe some more recent works which exemplify the strategically important developments that are currently underway. 

Polymer-supported esters are widely used in solid-phase peptide synthesis, and extensive information on this specialized protection is reported annually. Some activated esters that have been used as macrolide precursors and some that have been used in peptide synthesis are also described in this chapter the many activated esters that are used in peptide synthesis are discussed elsewhere. A useful list, with references, of many protected amino acids (e.g., -NH2, COOH, and side-chain-protected compounds) has been compiled/ Some general methods for the preparation of esters are provided at the beginning of this chapter conditions that are unique to a protective group are described with that group/ Some esters that have been used as protective groups are included in Reactivity Chart 6. 

In principle, linker groups are polymer-enlarged versions of blocking functions used in regular solution-phase chemistry. Therefore, enzymatic transformations that may be employed for the removal of protecting groups in solution, in principle may also open up alternative opportunities for releasing compounds from polymeric supports. The linkers developed so far can be divided into exo- and endo-linkers (Fig. 10.1) cleavable by exo- respectively endo-enzymes, as proposed by Flitsch et al. [6]. 

Ester formation is the main and most efficient means of protecting carboxylic acids. The protection of a carboxylic acid as an amide is infrequent, as its removal normally requires drastic conditions. Most of the work concerned with the use of light-sensitive protecting-groups for carboxylic acids is in peptide synthesis. Carboxylic acids are protected as photosensitive esters, including ester linkages to polymer supports or as (difficult to prepare) photosensitive amides. Many of these techniques may be readily applied to sugar acids. 

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