Nitrophenyl ester, acylation cleavage

If an amine P-NH2 is used in the aqueous solution, one obtains RCONHP instead of RCOOH. Rates of cleavage of three acyl nitrophenyl esters were followed by the appearance of p-nitrophenolate ion as reflected by increased absorbances at 400 nm. The reaction was carried out at pH 9.0, in 0.02 M tris(hydroxymethyl)aminomethane buffer, at 25°C. Rate constants were determined from measurements under pseudo-first-order conditions, with the residue molarity of primary amine present in approximately tenfold excess. First-order rate graphs were linear for at least 80% of the reaction. With nitrophenyl acetate and nitrophenyl caproate, the initial ester concentration was 6.66xlO 5M. With nitrophenyl laur-ate at this concentration, aminolysis by polymer was too fast to follow and, therefore, both substrate and amine were diluted tenfold for rate measurements. 

To compare the catalytic effectiveness of our polymer with that reported for other substances that accentuate nitrophenyl ester cleavage, we26 have carried out a series of experiments (at pH 7.3) in which the residue molar concentration of polymer imidazole groups was substantially in excess of the concentration of substrate, p-nitrophenol acylate. Pseudo-first-order rate constants k[ were determined at each of a number of polymer concentrations. Under these conditions k[ was found to be linear with [P-Im]0, the initial residue concentration of methylene-imidazole groups ... 

Fig. 6 Dependence of transition state stabilization (p/fTS) on acyl chain length for the cleavage of m- and p-nitrophenyl alkanoates by ar-CD (filled points) and /3-CD (open points). Data from Table A.5.9, with points for the CK and C12 esters from...

The polyethylenimines are also effective in the cleavage of nitrophenyl-sulfate esters and nitrophenylphosphate esters. These have not yet been studied as extensively as the acyl esters, but interesting kinetic accelerations are already apparent. Nitrocatechol sulfate, for example, is very stable in aqueous solution at ambient temperature. In fact, even in the presence of 2 M imidazole no hydrolysis can be detected at room temperature. At 95°C in the presence of 2 M imidazole cleavage is barely perceptible. In contrast, a modified polyethylenimine with attached imidazole groups cleaves the sulfate ester at 20°C.34 Some kinetic parameters are compared in Table VI. It is obvious that accelerations of many orders of magnitude are effected by the polymer. 

Silyl esters are stable to nonaqueous reaction conditions, but are often too labile to mild acid or base or even neutral aqueous media to survive many simple manipulations. Thus, they have not found wide application in peptide synthesis. Due to easy formation and cleavage they may play an important role as intermediates in the synthesis of amino acid derivatives and for temporary carboxy protection in the preparation of small peptide fragments. The TMS group has been used for the solubilization of H-Arg-OH for the synthesis of Z-Arg(Z2)-OHP l and in the synthesis of Al -Nps- and Al -Tfa-protected amino acids.P Amino acid trimethylsilyl esters as well as the related A1 -TMS derivatives react rapidly with acylating agents and are used for the preparation of peptides with amino acid active esters, e.g. A-hydroxysuccinimide-, 4-nitrophenyl-, or 2,4,5-trichlorophenyl esters, or mixed anhydrides. 

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