Ammonia peptide synthesis

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. 

However, sulfonamides are much more difficult to hydrolyze back to the amine than are carboxamides. In peptide synthesis (Section 25-7C) the commonly used sulfonyl protecting groups are 4-methylbenzenesulfonyl or 4-bromo-benzenesulfonyl groups. These groups can be removed as necessary from the sulfonamide by reduction with sodium metal in liquid ammonia ... 

The suitability of this technique for acid-labile (142) and photocleavable linkers (143) has been demonstrated. Other cleavage conditions that do not produce residues (e.g., cleavage with gaseous ammonia) could also be used in theory. TOF-secondary ion mass spectrometry (TOF-SIMS) (144) has also been validated to monitor SP peptide synthesis (145) and could in future increase the versatility of MS monitoring of SP reactions. 

The ethylaminocarbonyl (ethylcarbamyl, EC) derivatives of N-protected cysteine or cysteine are readily obtained by reaction with ethyl isocyanate under anhydrous conditions (Scheme 18) [231,233] pjjjg S-protection is stable toward acids such as HBr/AcOH or TFA, as well as to the standard conditions of peptide synthesis. Deprotection is achieved by treatment with 1M aqueous sodium hydroxide, hydrazine, or ammonia in methanol,or by exposure to sU-ver(I) or mercury(II) ions.t l... 

The peptide synthesis consists of stepwise-condensation and segment-condensation procedures. In both procedures, it is possible to purify the reaction product at each reaction step. Therefore, protected peptides with a high degree of purity can be obtained from a solution method. The choice of the reagents used in the final deprotection step determines the main strategy of peptide synthesis. The final deprotection procedures are mainly (1) catalytic hydrogenolysis in the presence of palladium,(2) sodium treatment in liquid ammonia for 10 to 15 seconds, repetitively over 30 minutes,(3) TFA treatment at room temperature for 1 hour,t l (4) HF treatment at 0°C for 1 hour,t l or (5) hard-soft acid-base procedure.0 ... 

Repetitive Ugi reactions are known. This product probably arises from a reaction between the carboxylic acid, the isocyanide, and the imine formed from the aldehyde or ketone and ammonia or the primary amine. The use of an A pro-tected amino acid or peptide as the carboxylic acid component and/or the use of an isocyanide containing a C-protected carboxyl group allows the reaction to be used for peptide synthesis. 

Peptide synthesis. The p-methoxybenzyl group is regarded as superior to the benzyl group for S-protection. It is readily cleaved from the final peptide either by sodium in liquid ammonia or by boiling with trifluoroacetic acid or with anhydrous hydrogen fluoride. ... 

Stable for convenient manipulation. For these reasons /-butyl azidoformate is used widely in peptide synthesis. The group is stable to hydrogenation and to sodium in liquid ammonia and is more resistant to alkali than the carbobenzoxy group. It is readily removed by hydrogen bromide in acetic acid. Selective removal of this group in the presence of a carbobenzoxy group can be accomplished with hydrogen chloride in acetic acid, ether, ethyl acetate, or nitromethane. 

Fischer peptide synthesis. Formation of polypeptides by treatment of an a-chloro or a-bro-mo acyl chloride with an amino acid ester, hydrolysis to the acid, and conversion to a new acid chloride that is again condensed with a second amino acid ester, and so on. The terminal chloride is finally converted to an amino group with ammonia. 

A succinyl anchor is commonly cleaved by concentrated aqueous ammonia at ambient temperature for 1-2 h. It is possible to largely preserve the integrity of the succinyl linker and retain most of the N,P-deprotected oligonucleotide on the support by using ethanolamine deprotection [210]. The main problem with the succinate ester attached to the primary amino group on polymer is its propensity for base-catalyzed cycfization into the corresponding succinimide [211], similarly to the aspartimide formation in the Fmoc peptide synthesis. This unwanted side-reaction can be prevented by joining the succinate to the secondary amine, for... 

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