Carbodiimides peptide synthesis with

Carbodiimide peptide synthesis. 1 -Hydroxybenzotriazole (HOBt) is frequently used to suppress racemization in peptide syntheses using carbodiimides. Copper(II) chloride decreases racemization, but also depresses the yield markedly. By judicious use of both additives, racemization can be prevented with only slight effect on the yield. 

H Hagenmeier, H Frank. Increased coupling yields in solid phase peptide synthesis with a modified carbodiimide coupling procedure. Hoppe-Seyler s Z Physiol Chem 353, 1973, 1972. 

Sheehan and another Nobel laureate Khorana showed independently that the mild carbodiimide mediated synthesis is very useful in the condensation reaction of N-blocked amino acids with a different amino acid to form a peptide bond. The O-blocking of carboxylic acid groups in peptide synthesis with amino nucleophiles, such as glycine esters or amides, " or novel silicon containing protective groups is also mediated by carbodiimides. Likewise, alcohols are protected with 4-benzyloxybutyric acid, using EDC/DMPA. ... 

Solid-Phase Peptide Synthesis with Carbodiimides... 

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 ... 

In solution-phase peptide synthesis, acylation of amino acids or peptides with N-protected azetidine-2-carboxylic acid is performed via the active esters, e.g. A-hydroxysuccin-imide 100 111-112 or pentachlorophenyl ester, m 117 as well as by the mixed anhydride 101114 or carbodiimide 118 methods. An attempt to prepare the A-carbonic acid anhydride by cycli-zation of A-(chloroformyl)azetidine-2-carboxylic acid with silver oxide in acetone or by addition of triethylamine in situ failed, presumably due to steric hindrance. 111 In SPPS, activation of the Fmoc-protected imino acid by HBTU 119,120 is reported. In solution-phase peptide synthesis, coupling of N-protected amino acids or peptides to C-protected azetidine-2-carboxylic acid or related peptides may be performed by active esters, 100 118 121 mixed anhydrides, 95 or similar methods. It may be worth mentioning that the probability of pip-erazine-2,5-dione formation from azetidine-2-carboxylic acid dipeptides is significantly reduced compared to proline dipeptides. 111 ... 

Polymeric carbodiimides are also used in peptide synthesis. For example, a polymeric carbodiimide with a N-cyclohexyl-N -phenyl endgroup is used in the synthesis of a metalloproteinase inhibitor. ... 

Polymeric substrates with carbodiimide end groups are often used in solid phase amidation reactions in peptide synthesis. 

Active esters are chirally stable under the usual conditions of coupling in peptide synthesis, but with the single exception of piperidino esters they may undergo isomerization if left in the presence of tertiary amines. In addition to their role as shelf-stable reagents, active esters are postulated as intermediates in carbodiimide-mediated reactions where a substituted hydroxylanoine is added in order to suppress the side reaction of epimerization in the... 

The advent of numerous new coupling reagents and the drawbacks discussed below have substantially decreased the popularity of mixed anhydrides for peptide synthesis in the laboratory. The mixed anhydride method is, therefore, a frequently overlooked technique which is an effective tool for peptide synthesis. This procedure has been comprehensively reviewed and, as stated by Meienhofer, the mixed anhydride method, when carried out under well-defined reaction conditions, is considerably less afflicted with undesired side reactions than, for example, the azide or carbodiimide procedures. 

The urea formed during the reaction is water soluble and can be extracted with water if the peptide synthesis is performed in organic solvents. It can be removed by dialysis or by gel filtration when used to couple haptens to high molecular weight carriers. In addition to the use of carbodiimides for the direct formation of peptide bonds, they can also be applied for the preparation of active esters, such as hydroxysuccinimide... 

Cyanamide, NsC—NH, in the tautomeric form HN==C=NH, is a carbodiimide and can be used in peptide synthesis. Diethylcyanamide, although incapable of tau-tomerism, functions as a dehydrating agent for the coupling of an N-protected amino acid with an amino acid ester. Best yields are obtained by heating the three components without solvent at 100° for 2 hrs. 

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