Peptides pentafluorophenyl ester

To a rapidly stirred mixture of abs dioxane and abs EtOH (40 1) at 95 °C, containing 4-pyrrolidi-nopyridine (0.1-1 equiv of the OPfp ester) and Pd/C, a soln of the peptide pentafluorophenyl ester in abs dioxane (one-tenth of the total volume used above, to yield a 1-2 mM soln) was added dropwise at 95 °C over 5 h, while simultaneously passing H2 through the soln. After completion of the reaction (monitored by HPLC), the mixture was cooled to rt, filtered, and the solvent was removed under reduced pressure. The crude cyclization product was purified by standard procedures. 

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. 

Attempts to prepare the compound l,4-(Bu3Sn 0)2C(F from tetralluoro-hydroquinone and tri-n-butyltin chloride have yielded only tri>n butyltin fluoride, whereas the phenate CfFs O-SnBuj is readily obtained from the chloride and pentafluorophenol. The conversion of pentafiuorophenol into JV-carbobenzoxyamino-acid and peptide pentafluorophenyl esters useful as intermediates in peptide synthesis has been described. ... 

A key step in the synthesis of 13-membered meta ansa and 14-membered para ansa peptide alkaloids involves catalytic hydrogenolysis of carbobenzyl-oxypeptide pentafluorophenyl esters. The most suitable solvent is dioxane with addition of a catalytic amount of pyrrolidinopyridine and 2% ethanol. Temperature should not exceed 90°C. The authors believe that after deblocking, the amino function remains on the surface until ring formation with the activated carboxylic function is accomplished (/5/). 

OS 25] [R 4] [P 17] For dipeptide formation from the pentafluorophenyl ester of (J )-2-phenylbutyric acid and (S)-a-methylbenzylamine an extent of racemization of 4.2% was found [86]. At higher concentration (0.5 instead of 0.1 M), a higher degree of racemization was found (7.8%). This experiment also served to demonstrate monitoring of the racemization of a simple carboxylic acid used in peptide synthesis. 

L Kisfaludy, M Low, O Nyeki, T Szirtes, I Schon. Utilization of pentafluorophenyl esters in peptide synthesis (DCC-XPhOH 1 3 complex). Liebigs Ann Chem 1421, 1973. 

The first successful synthesis of a biologically active cyclic peptide, gramicidin S, was accomplished by Schwyzer and Sieber[6,7l via the 4-nitrophenyl ester. The fact that -protected peptide esters can be deprotected to give the peptide active ester salts has made this approach popular not only in the synthesis of sequential polypeptides but also of cyclic peptides. Among the various active esters examined for this purpose, the pentafluorophenyl esters have emerged as the most reactive ones, although a high risk of epimerization is encountered when C-terminal chiral amino acids are involved. 

Unfortunately, A-(9-fluorenylmethoxycarbonyl)aziridine-2-carboxylic acid cannot be used in peptide synthesis, since N-deprotection of the respective peptides with secondary amines leads to oxazoline or dehydroamino acid side products. Similarly, N-(tert-butoxy-carbonyl)aziridine-2-carboxylic acid is inappropriate due to the instability of the aziridine moiety to TFA treatment. Attempts to convert A-tritylaziridine-2-carboxylic acid into homogenous and stable active esters as useful intermediates in peptide synthesis leads to positive results only in the case of the pentafluorophenyl ester. 47 Consequently, this active ester seems to be the method of choice for acylating peptides. The related Abhydroxysuc-cinimide and A-3-hydroxy-4-oxo-3,4-dihydro-l,2,3-benzotriazine ester could not be isolated in pure form and have therefore been used as crude products. 47 Access to 2-carbonylazir-idine peptides is also possible by carbodiimide-mediated coupling. Additionally, alkylamides of A-tritylaziridine-2-carboxylic acid are prepared by the azide method,1 5 yet this method fails in peptide coupling steps. 85 ... 

A better, higher yielding approach for the preparation of templates attached to different peptides employs a combination of solid-phase and solution-phase couplings (Scheme ll).121 A peptide chain is synthesized on a low-loading benzhydrylamine resin and reacted with a bifunctional electrophile such as the bis(pentafluorophenyl) ester 4 (R1 = Pfp) (low resin... 

The following peptide chains were constructed by solid-phase methodology al, which contained either 3 or 5 Gly-Pro-Hyp repeats on the N-terminus of H-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln-Arg-Gly-Val-Val-Gly-Cys(Acm)-Gly-Gly-OH a2, which contained 3 or 5 Gly-Pro-Hyp repeats on the N-terminus of H-Gly-Pro-Gln-Gly-Leu-Leu-Gly-Ala-Hyp-Gly-Ile-Leu-Gly-Cys(Acm)-Cys(StBu)-Gly-Gly-OH and al, which contained either 3 or 5 Gly-Pro-Hyp repeats on the N-terminus of H-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln-Arg-Gly-Val-Val-Gly-Leu-Cys(StBu)-Gly-Gly-OH. All three peptides were synthesized using Fmoc chemistry on HMP resin. Couplings were performed with HBTU/HOBt except for Cys residues, which were coupled as pentafluorophenyl esters to minimize racemization. Peptides were cleaved from the resin with H20/TFA (1 19) containing 1% Et3SiH. 

One method involves the use of preactivated Fmoc-protected amino acids (e.g., pentafluorophenyl ester). This method has the advantage that only one reagent is necessary since preparation of the amino acid solutions is very simple and the likelihood of mistakes is low. One disadvantage is the higher price of the amino acid derivatives, but due to the small amount of activated amino acids used the absolute difference falls in the range of a few dollars for synthesis of an entire peptide membrane array. Another disadvantage lies in the fact that activated esters are only commercially available for the standard amino acids. 

Acid cannot therefore be used for protection for the N-ter minus of the chain as it grows, so the solution is to use Fmoc. Each amino acid is introduced as its Fmoc-protected pentafluorophenyl ester (yet another electrophilicaUy activated electron-poor phenyl ester), and then the Fmoc group is cleaved with piperidine ready for the next residue to be added. The green blob in the diagram represents a polystyrene or polyamide bead, each of which carries many linkers and many growing peptide chains. 

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]. 

Scheme 8) are employed for the introduction of nitrogen alkylation into a peptide chain in order to suppress aggregation that interferes with chain assembly. Pentafluorophenyl esters of Fmoc-cysteine derivatives are employed in solid-phase synthesis to circumvent the enantiomerization that occurs during certain onium salt mediated couplings of the corresponding acids which have to be carried out in the presence of tertiary amines. The same phenomenon may apply to serine derivatives. Water-soluble tetrafluoro-4-sulfophenyl esters 22 are available. ... 

---