Polymeric protecting groups

Synthesis of large heterocycles usually involves condensation reactions of two difunctional molecules. Such molecules tend to polymerize. So far two special techniques have been described above to avoid this important side-reaaion , namely high dilution and use of templates. The general procedure to avoid polymerizations in reactions between difunctional molecules is, of course, the application of protecting groups as described in sections 4.1.2 and 2.6. 

E. C. Blossey and D. C. Neckers, Eds., Solid Phase Synthesis, Halsted, New York, 1975 P. Hodge and D. C. Sherrington, Eds., Polymer-Supported Reactions in Organic Synthesis, Wiley-Interscience, New York, 1980. A comprehensive review of polymeric protective groups by J. M. J. Frechet is included in this book. 

There is a large range of resins available for SPOS. These resins are derivatised polymer supports with a range of linkers. The roles of linkers are (i) to provide point(s) of attachment for the tethered molecule, akin to a solid supported protecting group(s), (ii) to provide distance from the polymeric backbone in order to minimise interactions with the backbone, (iii) to enable cleavage of product molecules under conditions compatible with the stability of the molecules and the reaction conditions employed for chemical transformations. Hence in order to... 

Peptide synthesis requires the use of selective protecting groups. An N-protected amino acid with a free carboxyl group is coupled to an O-protected amino acid with a free amino group in the presence of dicydohexvlcarbodi-imide (DCC). Amide formation occurs, the protecting groups are removed, and the sequence is repeated. Amines are usually protected as their teit-butoxy-carbonyl (Boc) derivatives, and acids are protected as esters. This synthetic sequence is often carried out by the Merrifield solid-phase method, in which the peptide is esterified to an insoluble polymeric support. 

Some tailor-made homopolymers can serve as starting points for chemical modifications to yield new species. Poly(hydroxyethyl methacrylate) and poly(glyceryl methacrylate) 16), already mentioned, are obtained upon hydrolysis of the OH-protecting groups that allow the anionic polymerization to proceed. Another example is the acid hydrolysis of poly(t-butyl methacrylate), a reaction which proceeds easily to completion, yielding poly(methacrylic acid) of known degree of polymerization and narrow molecular weight distribution 44 45). 

Preparation of 3-vinylindole (84) via Cope elimination of N,N-diethyltrypt-amine-N-oxide has been reported [87], An alternate approach based on the Wittig reaction of the readily accessible N-phenylsulfonylindole-3-carbalde-hyde failed because cleavage of the sulfonyl protecting group easily produced an anion whose neutralization led to polymerization [86]. 

FIGURE 3 Schematic representation of a pseudopoly (amino acid) derived from the side chain polymerization of a dipeptide carrying protecting groups X and Y. The wavy line symbolizes a nonamide bond. In this polymer, the amino acid side chains are an integral part of the polymer backbone while the termini have become pendant chains. In the backbone, amide and nonamide bonds strictly alternate. 

An interesting observation was made in regard to the polymeriza-bility of N-acylhydroxyproline derivatives The monomers with the smallest protecting groups such as N-acetyl and N-pivaloylhydroxy-proline gave rise to the polymers with the lowest molecular weights (Table 1). When these monomers were polymerized, the initially... 

The MejSi protecting groups in 58 a and 59 a could be removed by treatment with sodium tetraborate (borax) in MeOH/THF, yielding 58c and 59 c, respectively, as very unstable compounds [84]. Any attempts to obtain characterizable two-dimensional all-carbon network structures [3,4] by oxidative polymerization of 59 c have failed. 

We tried to synthesize various polysilanes with a phenol group(25), however, the only one we were able to obtain was Polysilane(II). The synthetic route is shown in Figure 4. We chose a trimethylsily 1 group as the protecting group of the phenol moiety, because it is easy to remove after the polymerization without damaging the Si-Si main chain, however, it has been reported that in some reactions, the Si-O-C bond cleavage takes place with Na dispersion(27). 

The protecting group is removed by hydrogenation. The deprotected hydroxymethyl groups are used as initiators for polymerization of new e-CL. The process was repeated to the third generation to give a narrow molecular weight... 

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