Nitrophenyl ester method

Schmidt and coworkers [80] developed the pentafluorophenyl ester method for synthesizing cyclopeptides, particularly applicable for 13- and 14-membered ansa peptides. This procedure is superior to the p-nitrophenyl ester method in respect to short reaction time and easy separation of products. Evans and Ellman [81] applied this method to the synthesis of the cyclic tripeptide K-13 146). As shown in Scheme 48, the reaction of the linear precursor 143 with pentafluorophenol and DCC afforded pentafluorophenyl ester 144 in 87-93% yield. Then, under catalytic hydrogenation condition in the presence of a mild base and ethanol, 144 was cyclized to 145 in up to 70% yield. There are more applications of the pentafluorophenyl ester procedure in recent literature [82]. 

Phenolic esters are obtained similarly. The presence of a nitro group in the aromatic nucleus and the use of pyridine as solvent facilitates the reaction. This reaction is recommended for the characterization of phenols. 2,4,5-Trichlorophenyl-, pentachlorophenyl-, 4-nitrophenyl- and thiophenyl esters of N-acylamino acids are prepared in this manner. These aromatic esters are used in the stepwise lengthening of peptides, du Vigneaud and coworkers synthesized lysine vasopressin from a nonapeptide which they prepared stepwise using the nitrophenyl ester method. Room temperature esterification of dicarboxylic acids and diphenols are also carbodiimide mediated using the 1 1 complex derived from DMAP and p-toluenesulfonic acid as catalyst Methacrylic acid is also esterified with phenols using carbodiimides and DMPA to mediate the reaction. ... 

Hydroxyl (serine) and sulfhydryl groups (cysteine) are also silylated. These derivatives can be used in peptide synthesis by the phosphoryl chloride, imidazole, mixed anhydride, or p-nitrophenyl ester methods. An attractive feature is that the silyl groups are removed merely by treatment with water in the normal workup. Trimethylchlorosilane is not suitable because of the amphoteric nature of the amino acids. 

M. Bodanszky, V. du Vigneaud, Synthesis of oxytocin by the nitrophenyl ester method. Nature (Lond.) 183 1324-1325 (1959) also A method of synthesis of long peptide chains using a synthesis of ox3rtocin as an example. J. Amer. Chem. Soc. 81 5688-5691 (1959)... 

Efficient syntheses of biologically active peptides, such as oxytocin or secretin, entirely b the nitrophenyl ester method stimulated further efforts toward the development of still more reactive esters but from the numerous activated intermediates proposed only the p-nitrophenyl esters and the esters of N-hydroxysuccinimide (Anderson et al. 1963) are commercially available at this time. The N-hydroxysuccimide esters are highly reactive and yield the desired peptides with the expected ease... 

The di-/-butoxycarbonyl amino acids have been applied to solution peptide synthesis using the 1,3-DicycIohexyIcarbodumide or p-nitrophenyl ester method in DMF for coupling. ... 

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. 

A general step ahead in polycondensation was achieved by the application of the active ester method by DeTar et al.19) and Kovacs et al.291 Very soon, the nitrophenyl ester, the pentachlorophenyl ester, or the hydroxysucdnimido ester were used exclusively. The esters of the protected tripeptides could be purified by crystallization, then the N-protecting group was split off and the free peptide esters were purified again. Addition of base starts the polycondensation, resulting quickly in the formation of a viscous solution at low temperature. 

Activated esters of A-alkoxycarbonylamino acids are prepared by two approaches, activation of the acid followed by reaction with the hydroxy compound, and trans-esterification. Most of the products are stable enough to be purified by washing a solution of the ester in an organic solvent with aqueous solutions. A few that are not crystalline are purified by passage through a column of silica. The commonly used method for their preparation is addition of dicyclohexylcarbodiimide to a cold mixture of the reactants in dimethylformamide or ethyl acetate. The first Boc-amino acid nitrophenyl esters were obtained using pyridine as solvent. Pyridine generates the nitrophenoxide ion that is more reactive. For one type of ester, 2-hydroxypyridino... 

The alternative method for making activated esters is base-catalyzed transesterification. Fmoc-amino acids are esterified in excellent yields by reaction with pentafluorophenyl trifluoroacetate at 40°C in the presence of pyridine (Figure 7.13). A mixed anhydride is formed initially, and the anhydride is then attacked by the pentafluorophenoxy anion that is generated by the pyridine. Succinimido, chlorophe-nyl, and nitrophenyl esters were made by this method when it was introduced decades ago. A unique variant of this approach is the use of mixed carbonates that contain an isopropenyl group [Cf C CfyO-COjR]. These react with hydroxy compounds in the presence of triethylamine or 4-dimethylaminopyridine (see Section 4.19) to give the esters and acetone.30 35... 

Poly( L-lysine trifluoroacetate )(4) was prepared according to the method of Sela ( 5 ). The p-nitrophenyl esters ( 1, 2, 3 )... 

Selected entries from Methods in Enzymology [vol, page(s)] Active site titration [by 2-hydroxy-5-nitro-a-toluenesulfonic acid sultone, 19, 6-14 by p-nitrophenyl ester substrates, 19, 14-20 by rapidly reversible, covalently bound substrates, 19, 14-20 by slowly reversible, covalently bound inhibitors, 19, 6-14] assay,... 

Solid-phase techniques can be applied to the N-activated esters.16 In the activated ester method, 4-nitrophenyl, 12 21-27 2,4-dinitrophenyl, 614 28 2,4,5-trichlorophenyl, 29 and penta-fluorophenyl 8 esters can be used, the amide bonds being formed with aminolytic release of the corresponding phenol (Scheme 4). For example, the use of pentafluorophenyl esters in the synthesis of pure azapeptides (named azatides for the first time) proved to be advantageous. 8 ... 

Peptide Ac-AKEAAHAEAAEAA-NH2 was synthesized using Boc chemistry on MBHA resin (100-200 mesh, 1% DVB, substitution level 0.53 mmol-g 1). Purification was carried out by HPLC on a 2.2-cm Vydac C18 column using a gradient of MeCN/H,0 in 0.1% TFA (5 to 40% over 40 min) at a flow rate of SmL-min Mesoporphyrin IX was converted into the iron(III) complex by refluxing with excess Fe(OAc)2 in AcOH.11"1 The bis(4-nitrophenyl) ester of iron(III) mesoporphyrin IX was subsequently prepared using the method of Collman et al.12001... 

Cyclopeptides.1 Cyclization of polypeptides under conditions of high dilution usually proceeds in low yield. A new method involves cyclization of co-peptide-pentafluorophenyl esters in dioxane in the presence of 4-pyrrolidinopyridine and an alcohol (ethanol or f-butyl alcohol). p-Nitrophenyl esters can also be employed, but separation of p-nitrophenol from product is more difficult than that of pen-tafluorophenol. 

The literature data on the preparation of phosphono-dipeptides from 1-aminoalkanephosphonic acids showed 7-7 that the yields of condensation reactions are usually small or moderate. Moreover,the use of bulky N-blocked amino acids drastically decreased the reaction yield. Thus following Martell s method— we wre unsuccesful in the preparation of dipeptides containing N-terminal valine or leucine,while peptides of phenylalanine were obtained in 5—10% yield.Also the active ester method appeared to give small yields of the desired products. Our studies using p-nitrophenyl- and cyanomethyl esters of N-phtaloyl amino acids confirmed these observations. 

The reaction of 42 with 43 was attempted with dicyciohexylcarbodiimide (DCC) or water-soluble carbodiimide in solvent systems. All these reactions seemed to be unsuccessful because of the low solubility and low reactivity of 43. Among the methods of activation of a carboxy group, only the activated ester method of p-nitrophenyl ester was successful. In the acid chloride method a side reaction with the amino group of adenine was reported56. ... 

NMR analysis of the product indicate that Tyr and Lys were modified (53). A homobifunctionalp-nitrophenyl ester was proposed for the preparation of neoglycoproteins (26). In this method, one of the active ester groups is used to react with the co-aminoaUcyl glycoside and the product thereof then will be conjugated to a protein or other amino-bearing material. 

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