Fluka is a global supplier of the highest quality coupling reagents, available from research to large-scale quantities. We focus on a broad assortment of versatile peptide coupling reagents, carbodiimides and additives for achieving fast, efficient, selective and racemization-free amidation in a wide variety of solvent systems.

Carbodiimides

The most popular in-situ condensing agents are the carbodiimides.^[1-3] The reaction of a carboxylic acid with a carbodiimide is believed to involve a labile O-acylisourea (see figure).

N,N4-dicyclohexylcarbodiimide (DCC) is extensively used in Boc/Bzl-peptide synthesis, because the dicyclohexylurea (DCU) by-product is easily removed from the reaction vessel in presence of trifluoroacetic acid during the Boc-deprotection protocol. In the Fmoc/t-Bu chemistry, diisopropylcarbodiimide gives rise to a more DMF-soluble urea by-product and is therefore highly recommended.^4,5 N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (WSC, EDC) is widely used in solution phase, as it generates a urea by-product which can be easily removed from the reaction medium by extraction with water.^6,7

Another important water-soluble carbodiimide is N-cyclohexyl-N'-(2-morpholinoethyl)carbodiimide-methyl-p-toluenesulfonate (CMC).^8,9 A major drawback of the cabodiimide procedure is the dehydration of side-chain carboxamides of Asn- and Gln-residues to the corresponding nitriles. This problem is completely avoided when using carbodiimides in combination with additives like hydroxylamine derivatives, such as 1-hydroxybenzotriazole (HOBt)¹⁰ or 7-aza-1-hydroxybenzotriazole (HOAt).^11-13 These additives lead to an efficient suppression of racemization and total exclusion of dehydration of carboxamide residues while generating highly active ester species. Carbodiimide-mediated activation is achieved without base catalysis otherwise responsible for considerable rates of racemization in sensitive Fmoc-Cys(Trt)-OH residue.^14,15

REFERENCES:

1. Sheehan, J. C., Hess, G.P., J. Am. Chem. Soc., 1955, 77, 1067.
2. Rich, D. H., Singh, J., in The Peptides: Analysis, Synthesis, Biology, (E. Gross, J. Meienhofer, eds), Vol. 1, Academic Press, New York 1979, 242.
3. Beyermann, M. et al., Int. J. Pept. Protein Res., 1991, 37, 25.
4. Sarantakis, D. et al., Biochem. Biophys. Res. Commun., 1976, 73, 336.
5. Hudson, D. et al., in Peptide Chemistry, Proc. 23rd Japn. Pept. Symp., (Y. Kiso, ed.), Protein Research Foundation, Osaka, 1986, 4113.
6. Sheehan, J. C., Ledis, S. L., J. Am. Chem. Soc., 1973, 95, 875.
7. Sakakibara, S., Biopolymers (Pept. Sci.), 1995, 37, 17.
8. Sheehan, J. C., Hlavka, J. J., J. Org. Chem., 1956, 21, 4395.
9. Kunz, H., Angew, Chem., 1978, 90, 63.
10. Kvnig, W. Geiger, R., Chem. Ber. 1970, 103, 788.
11. Carpino, L. A., J. Am. Chem. Soc. 1993, 115, 4397.
12. Carpino, L. A.et al., J. Chem. Soc., Chem. Commun., 1994, 201.
13. Carpino, L. A. et al., Tetrahedron Lett.,1994, 35, 2279.
14. Kaiser, T. et al., Tetrahedron Lett., 1996, 37, 1187.
15. Meisenbach, M. et al., J. Chem. Soc., Chem. Commun., 1997, 849.