Carboxylic acid function, alternatives

The structural versatility of pseudopoly (amino acids) can be increased further by considering dipeptides as monomeric starting materials as well. In this case polymerizations can be designed that involve one of the amino acid side chains and the C terminus, one of the amino acid side chains and the N terminus, or both of the amino acid side chains as reactive groups. The use of dipeptides as monomers in the manner described above results in the formation of copolymers in which amide bonds and nonamide linkages strictly alternate (Fig. 3). It is noteworthy that these polymers have both an amino function and a carboxylic acid function as pendant chains. This feature should facilitate the attachment of drug molecules or crosslinkers,... 

In a slightly different approach, the introduction of a metal complex is also possible as a metallo amino acid. For example, ferrocenyl-alanine (Fer) was prepared as early as 1957 [67]. Also, stereoselective preparations [68-71] and its biological activity were reported [72], and the compound has been inserted into peptide sequences just as a normal amino acid would (see Fig. 7 for an example) [35, 71]. In this case, the metallocene constitutes the side chain. Alternatively, the amino and carboxylic acid function can be on two different Cp rings, yielding, in the simplest possible case, l -aminoferrocene-l-carboxylic acid (Fca, see Fig. 7 for an example). This compound has also been reported in the literature, although it was difficult to obtain in pure form [73,74], It has since been incorporated into oligomers... 

A conceptual alternative way to the activation of the carboxylic acid function is the reaction of carboxylic acids with amino groups activated as isocyanates - and isothiocyanates (equation 16). Preparation of these derivatives is racemization free. The reaction proceeds via mixed acid anhydrides in aromatic hydrocarbon solvents at elevated temperatures, and decarboxylation leads to the V-substituted amide. Pyridine as solvent enhances the conversion rate but increases also the amount of the urea side product via disproportionation. Application to peptide chemistry is limited, because peptide ester fragments tend to form hydantoins. ... 

The first came from a continuing investigation of alternative tricyclic systems as carriers of the carboxylic acid function and the finding that compounds (5) in which X or Y equals sulphone, for example, had high activity. One of these, the thioxanthone dioxide BW 437C, had a higher ratio of oral to intravenous activity than we had observed before. 

Protection and Deprotection.—In a substantial paper, Barton and co-workers have discussed in detail the protection of carboxylic acid functions in penicillin derivatives by conversion into the corresponding acylhydrazines, acylhydrazones, and dihydro-heteroaromatic amides. An alternative method for the methoxymethyl-ation of free acids, which avoids the use of toxic chloromethyl methyl ether, is by prior conversion into the zinc bromide salt followed by reaction with acetyl chloride and methylaP (Scheme 14). 

Therefore, it is expected that films containing polymer 2 and a dicarboxylic acid should crosslink upon heating, as shown in Scheme 2. Alternatively, the thermal reaction of 2 with a suitably designed monofunctional carboxylic acid acting as a solubility modifier, should also lead to a product insoluble in water as a result of the modification of both the oxazoline ring and the carboxylic acid function. This type of thermal reaction, which may be done in the pre-bake step, is particularly attractive as it meets our first objective of insolubilization of the entire film prior to irradiation. 

In both chemical steps, a second active site carboxylate residue functions alternately as a general acid and general base catalyst to promote leaving group... 

There has been a slight increase in activity in this area compared with that in the previous two year period. For the polymeric esters of acrylic, methacrylic acids, and related polymers the simplest reaction, apart from thermal depolymerization, is hydrolysis, and one or two papers on this subject have appeared. One of these concerns a comparison of the kinetics of hydrolysis of a number of methacrylate esters and a further two deal with the formation of copolymers containing carboxylic acid functions. Methyl trifluoroacrylate forms alternating copolymers with cE-olefins (ethylene, propylene, isobutylene) and these are readily hydrolysed in boiling aqueous methanolic sodium hydroxide to yield hydrophilic fluoropolymers. Hydrolysis is reported to be nearly quantitative with no chain scission. An alternating copolymer is also formed by radical polymerization of maleic anhydride with A-vinyl succinimide. On hydrolysis this copolymer is... 

Acrylic acid-grafted polylactide (PLA-g-AA) and multihydroxyl-functionalized multiwalled carbon nanotubes have been melt blended to improve thermal stability and mechanical properties of the composite. The formation of a covalent bond (ester linkage) resulted in a significant improvement in compatibility [98]. Alternatively, carboxylic acid-functionalized multiwalled carbon nanotubes were grafted onto PLEA by a one-step in situ polycondensation reaction [99]. 

As an alternative to pendent sulfonate groups, polythiophene derivatives which possess pendent carboxylic acids have been investigated. The lower acidity of the carboxylic acid functionality (/7ATa 4.8) compared to... 

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