Peptide synthesis, using carbodiimides

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. 

Cyanamide is a relevant molecule in prebiotic chemistry, and it was recently shown that water-ice catalyzes the rearrangement of cyanamide to carbodiimide. Carbodiimide could act as a condensation agent in the assembly of amino acids into peptides. In the peptide synthesis, using substitued carbodiimides as condensation agents, formation of L—L bonds is favored over D—D bonds by a ratio of 6 1. ... 

T Miyazawa, T Otomatsu, Y Fukui, T Yamada, S Kuwata. Racemization-free and efficient peptide synthesis by the carbodiimide method using 1-hydroxybenzotriazole and copper(II) chloride simultaneously as additives. J Chem Soc Chem Commun 419, 1988. 

The utility of carbodiimide reagents was too important to consider abandoning them. Instead, it rapidly became obvious that carbodiimide activation could be used to prepare in situ active esters. Numerous nucleophilic additives were discovered and prepared for use in these reactions. The most important of these additives was 1,2,3-benzotriazol-l-ol (HOBt, 1), first reported for use in peptide synthesis by Konig and Geiger in 1970.[2(l As an example of a typical result, 1.2 equivalents of HOBt was added to the DCC coupling of Boc-Leu-Phe-OH to H-Val-OtBu in DMF as a solvent. Less than 1% of the l-d-l epimerized tripeptide was formed. When the reaction was carried out in the absence of HOBt, the amount of l-d-l product formed was 14.3%. Addition of HOBt to DCC reactions converts the intermediate G-acylisourea (2) (and any symmetrical anhydride) into the HOBt active ester 3 (Scheme 8). 

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 ... 

Dicyclohexyl carbodiimide (DCC) is one of the most frequently used coupling agents, especially in organic synthesis applications. It has been used for peptide synthesis since... 

In addition to DCC and EDC many other aliphatic carbodiimides are used in peptide synthesis. Examples include N-t-butyl-N -methylcarbodiimide (BMC), N-t-butyl-N -ethylcarbodiimide (BEC), N,N -dicyclopentylcarbodiimide andN-cyclohexyl-N -isopropylcarbodiimide (CIC). DCC, diisopropylcarbodiimide, BMC and BEC perform similarly in peptide synthesis. " N-cyclohexyl-N -isopropylcarbodiimide (CIC) is comparable or even better than DCC for mediation of peptide bond formation in sohd phase synthesis. Also, the solubility of the derived urea in dichloromethane is useful in the standard procedure. " ... 

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. 

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. ... 

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