Amino acid ester accelerators

Neutral amino acid ester accelerators should yield monomer formulations having better shelf-life than the corresponding amino acids. This has been established experimentally (39). 

It is now almost 50 years since Kroll (41) first reported that hydrolysis rates of amino acid esters were accelerated by the addition of certain divalent transition metal ions. His observations prompted numerous studies in this area, but even today exact descriptions of these... 

The Co(III) center is generally regarded as lacking the polarizing power of a proton but its attachment to the N-terminus of an amino acid ester, as in [Co(NH3)5(GlyOEt)]3, accelerates OH -catalyzed hydrolysis by ca. 100-fold. This is similar to the effect observed for N-protonation. 

Reaction 3, the intramolecular counterpart of reaction 2, has been observed for amino acid esters,156 amides157 and nitriles,158 where five- and six-membered chelate rings can be formed. In the case of the aminoacetonitrile complex (37) a rate enhancement of ca. 1011 occurs at pH 7, and this may be compared with an acceleration of ca. 106 for the reaction 1 analogue.159 For reaction 3, AH values become of considerable significance. 

The metal-accelerated hydrolysis of amino acid esters or amides comprises one of the best investigated types of metal-mediated reaction (Fig. 3-7). One of the reasons for this is the involvement of chelating ligands, which allows chemical characterisation of intermediates and products in favourable cases, and allows detailed mechanistic studies to be made. The reactions have obvious biological relevance and may provide good working models for the role of metals in metalloproteins. 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

We saw in Chapter 3 that the hydrolysis of chelated amino acid esters and amides was dramatically accelerated by the nucleophilic attack of external hydroxide ion or water and that cobalt(m) complexes provided an ideal framework for the mechanistic study of these reactions. Some of the earlier studies were concerned with the reactions of the cations [Co(en)2Cl(H2NCH2C02R)]2+, which contained a monodentate amino acid ester. In many respects these proved to be an unfortunate choice in that a number of mechanisms for their hydrolysis may be envisaged. The first involved attack by external hydroxide upon the monodentate A-bonded ester (Fig. 5-62). This process is little accelerated by co-ordination in a monodentate manner. 

How may we distinguish between pathways that involve external attack by hydroxide and those that involve co-ordinated hydroxide There is a considerable accumulation of data that suggest the two latter pathways are the most important (i.e., attack of external hydroxide upon monodentate amino acid ester is not greatly accelerated). The attack by external hydroxide may be studied independently and accurate rate constants may be determined for insertion in the composite rate equation with the two competing processes. In some cases it is possible to detect the five-co-ordinate and the other intermediates. Finally, some elegant labelling studies have provided very strong evidence for the exi-... 

One of the main questions in the cobalt(III)-promoted hydrolysis of activated amino acid esters is whether the ratedetermining step is addition of hydroxide to the carbonyl carbon, or loss of the alkoxide from the intermediate. Work with /3-alanine ester showed that below pH 8.5 the ratedetermining step was the elimination of alkoxide. At pH 10 and above, the rate-determining step changes and the addition of hydroxide to the activated ester becomes the rate-controlling step. This is due to the fact that above pH 10 the hydroxyl group of the intermediate becomes deprotonated (equation 7). The deprotonation of the hydroxyl group accelerates the loss of alkoxide by 10 times. ... 

Metal ion catalysis by the direct polarization mechanism can accelerate hydrolysis rates by factors of Kf or greater (Buckingham, 1977). For example, the Cu " -catalyzed hydrolysis of a-amino acid esters occurs at a rate that is six orders of magnitude faster than the uncatalyzed process (Bender and Brubacher, 1973 Hay and Morris, 1976). The metal ion chelate is thought to have the following structure ... 

The hydrolysis of chelated amino acid esters, H2NCHRCO2R, is known to be accelerated by metal ions, most notably cobalt(III). Dramatic enhancements are also observed with copper(II). Mechanistic studies of the hydrolysis of amino acid esters with copper(II) complexes of glycyl-DL-valine and dien (H2HCH2CH2NHCH2CH2NH2) have been reported/ The hydrolysis of benzyl-penicillin (30) by copper(II) salts to give (31) has been further investigated, and it is proposed that the key step involves intramolecular attack by metal-coordinated hydroxide in an intermediate of type (32). 

It was shown that microwave irradiation accelerated the 1,4 Michael addition of primary and cyclic secondary amines to acrylic esters, leading to several /j-amino acid derivatives in good yields within short reaction times [78] (Eq. 25). 

Microwave irradiation has been reported to accelerate the Michael addition of primary and cyclic secondary amines to esters of a ,/i-unsaturated Q -unsubstituted carboxylic acids to produce /3-amino acids. ... 

The search for RNAs with new catalytic functions has been aided by the development of a method that rapidly searches pools of random polymers of RNA and extracts those with particular activities SELEX is nothing less than accelerated evolution in a test tube (Box 26-3). It has been used to generate RNA molecules that bind to amino acids, organic dyes, nucleotides, cyano-cobalamin, and other molecules. Researchers have isolated ribozymes that catalyze ester and amide bond formation, Sn2 reactions, metallation of (addition of metal ions to) porphyrins, and carbon-carbon bond formation. The evolution of enzymatic cofactors with nucleotide handles that facilitate their binding to ribozymes might have further expanded the repertoire of chemical processes available to primitive metabolic systems. 

In addition to proofreading after formation of the aminoacyl-AMP intermediate, most aminoacyl-tRNA synthetases can also hydrolyze the ester linkage between amino acids and tRNAs in the aminoacyl-tRNAs. This hydrolysis is greatly accelerated for incorrectly charged tRNAs, providing yet a third filter to enhance the fidelity of the overall process. The few aminoacyl-tRNA synthetases that activate amino acids with no close structural relatives (Cys-tRNA synthetase, for example) demonstrate little or no proofreading activity in these cases, the active site for aminoacylation can sufficiently discriminate between the proper substrate and any incorrect amino acid. 

The active ester methodology, which is widely used in peptide chemistry, has found only limited application in depsipeptide synthesis. A more vigorous activation of the carboxy component is apparently required to form an ester bond compared to the peptide analogue. Nevertheless, active esters have been utilized for this purpose in combination with some catalyst additives. The first successful attempt in this direction was described by Mazur.1103 The modification of the 4-nitrophenyl ester procedure included addition of 1-10 equivalents of imidazole to the reaction mixture. This accelerated technique presumably involves formation of the highly reactive intermediate imidazolide. The reaction resulted in the preparation of model benzyloxycarbonyl didepsipeptide esters in good yields within several hours at room temperature from 4-nitrophenyl esters of Z-amino acids and the pentamethylbenzyl ester of glycolic acid, while in the absence of imidazole this reaction failed to give any product. 

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