Amine acylation, nitrophenyle esters

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. 

With nonacylated polyethylenimines (Table II) the rate constant is increased by a factor of about 4 over that of propylamine. This small enhancement may be due merely to the fact that a greater fraction of primary amine groups in the polymer are in the basic, NH2 state. With these polyethylenimines, as with propylamine, k drops with increasing length of the hydrocarbon chain of the acyl nitrophenyl ester. 

In the second major method of peptide synthesis the carboxyl group is activated by converting it to an active ester, usually a p-nitrophenyl ester. Recall from Section 20.12 that esters react with ammonia and amines to give fflnides. p-Nitrophenyl esters are much more reactive than methyl and ethyl esters in these reactions because p-nitrophenoxide is a better (less basic) leaving group than methoxide and ethoxide. Simply allowing the active ester and a C-protected amino acid to stand in a suitable solvent is sufficient to bring about peptide bond formation by nucleophilic acyl substitution. 

First-Order Rate Constants for Amine Acylation by p-Nitrophenyl Esters"- ... 

Diphenylthieno[3,4-d][l,3]dioxol-2-one 5,5-dioxide (304) can serve as an activating agent for peptide synthesis (76AG(E)444). The esters (305) are formed readily on admixture of a carboxylic acid with (304) in an aprotic solvent in the presence of pyridine. The activated esters (305) are stable, crystallizable compounds which react with amines readily to furnish the corresponding amides (Scheme 65). Competition experiments reveal that the esters (305) are more effective acyl transfer agents than the p- and o-nitrophenyl esters often used in peptide synthesis. 

Table 3.1 Fi rst-order rate constants (k x 102 min)a for amine acylations by p-nitrophenyl esters.b...

Reaction between amines and the reactive HPMA copolymers forms the basis for attachment of oligopeptide sequences, targeting residues or therapeutic agents to the polymer precursor (Fig. 3). Reaction of aliphatic amines (tertiary butylamine and diisopropylamine) in dimethylsulphoxide at 25 C with reactive copolymers containing different side-chains has been used to characterize copolymer reactivity.It has been shown that copolymers of HPMA with p-nitrophenyl esters of a-(acylamino) acids react faster with amines than copolymers with p-nitrophenyl esters of g-(acyl-amino) acids or, e-(acylamino) acids.21... 

A great deal of work has been done on the in situ activation of carboxylic acids toward nucleophilic substitution by amines. This type of reaction forms the backbone of the methods for synthesis of peptides and proteins. Among the methods for carboxylate activation which have achieved importance in peptide synthesis that were mentioned in Section 3.4.1 are the carbodiimide and isoxazolium methods. Since amines are better nucleophiles than alcohols, the leaving group in a potential acylation reagent need not be as reactive as is necessary for alcohols. The p-nitrophenyl and 2,4,5-trichlorophenyl esters of amino acids are sufficiently reactive toward amines to be useful in peptide synthesis. Acyl derivatives of hydroxysuccinimide are also useful for synthesis of peptides and other types of amides.Like the p-nitrophenyl esters, the acylated A-hydroxysuccinimides can be isolated and purified, but rapidly react with free amino groups. The A-hydroxy-succinimide that is liberated is easily removed because of its solubility in dilute... 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

Table II lists first-order rate constants, corrected for hydrolysis of ester in buffer alone. Propylamine served as a reference amine in its presence k (in min-1) for aminolysis decreased progressively from 0.98 x 10-2 to 0.51 x 10-2 to 0.05 x 10-2 as the length of the acyl group increased from 2 to 12 carbons (see Table II). The sharp drop for nitrophenyl laurate may be the result of micelle formation23 even at concentrations of 6x 10-6 M...

The answer concerns the different kinetic behavior of chymotrypsin toward amide and ester substrates. Substrate A is N-acetyf-L-phenyfalanine p-nitrophenyl amide, rather than N-acetyl-L-phenylalanine p-nitrophenyf ester for which the initial burst activity was described in the text. The burst is observed if the first step of a reaction (in this case, acyl-chymotrypsin formation, together with release of p-nitrophenyl amine) is much faster than the second step (release of N-acetyl-phenylalanine and free chymotrypsin). With the amide substrate, however, the relative rates of the two steps are more nearly equal therefore no burst is observed. 

To avoid hydrolysis of the no-carrier-added F-labeled active esters in aqueous buffered solution, the acylation route can be inverted, i.e., radiolabeled amines are prepared and coupled with activated carboxyls at the peptide (O Table 42.9, amidation). Oxytocin was successfully coupled with 2-[ F]fluoroethylamine in yields >90% hy both preactivation (NHS, 4-nitrophenyl and pentafluorophenyl esters) and in situ activation of the C-terminus (TBTU) (Jelinski et al. 2002). 

By acylation of amines with active derivatives of a-keto acids pyruvic acid chloride (767), p-nitrophenyl pyruvate 167, 169), carbodimide method 86. 166, 178), mixed anhydrides with phosphorous acid dichloride 436) and by activation with the cyclic esters of oxalic acid with 4,6-diphenyl-thieno-[3,4-d]-[l,3]-dioxolone-dioxide (H. Schmidt and W. Steglich, 346A). 

---