Peptides dicyclohexylcarbodiimide

With the dicyclohexylcarbodiimide (DCQ reagent racemization is more pronounced in polar solvents such as DMF than in CHjCl2, for example. An efficient method for reduction of racemization in coupling with DCC is to use additives such as N-hydroxysuccinimide or l-hydroxybenzotriazole. A possible explanation for this effect of nucleophilic additives is that they compete with the amino component for the acyl group to form active esters, which in turn reaa without racemization. There are some other condensation agents (e.g. 2-ethyl-7-hydroxybenz[d]isoxazolium and l-ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline) that have been found not to lead to significant racemization. They have, however, not been widely tested in peptide synthesis. 

In one method treatment of a solution containing the N protected and the C protected ammo acids with N N dicyclohexylcarbodiimide (DCCI) leads directly to peptide bond formation... 

Section 27 17 Peptide bond formation between a protected ammo acid having a free carboxyl group and a protected ammo acid having a free ammo group can be accomplished with the aid of N N dicyclohexylcarbodiimide (DCCI)... 

Schemes are available, however, that start from the free carboxylic acid, plus an activator . Dicyclohexylcarbodiimide, DCC, has been extensively employed as a promoter in esterification reactions, and in protein chemistry for peptide bond formation [187]. Although the reagent is toxic, and a stoichiometric concentration or more is necessary, this procedure is very useful, especially when a new derivative is targeted. The reaction usually proceeds at room temperature, is not subject to steric hindrance, and the conditions are mild, so that several types of functional groups can be employed, including acid-sensitive unsaturated acyl groups. In combination with 4-pyrrolidinonepyridine, this reagent has been employed for the preparation of long-chain fatty esters of cellulose from carboxylic acids, as depicted in Fig. 5 [166,185,188] ...

For a review of peptide synthesis with dicyclohexylcarbodiimide and other coupling agents, see Klausner, Y.S. Bodansky, M. Synthesis, 1972, 453. 

The first version of SPPS to be developed used the t-Boc group as the amino-protecting group. f-Boc can be cleaved with relatively mild acidic treatment and TFA is usually used. The original coupling reagents utilized for SPPS were carbodiimides. In addition to dicyclohexylcarbodiimide (DCCI), N, (V -diisopropylcarbodiimide (DIPCDI) is often used. The mechanism of peptide coupling by carbodiimides was... 

M Slebioda, Z Wodecki, AM Kolodziejczyk. Formation of optically pure V-acyl-N,hT-dicyclohexylurea in W -dicyclohexylcarbodiimide-mediated peptide synthesis. Ini J Pept Prot Res 35, 539, 1990. 

JE Zimmerman, GW Callahan. The effect of active ester components on racemization in the synthesis of peptides by the dicyclohexylcarbodiimide method. J Am Chem Soc 89, 7151, 1967. 

W Konig, R Geiger. A new method for the synthesis of peptides activation of the carboxyl group with dicyclohexylcarbodiimide and 1-hydroxybenzotriazoles. Chem Ber 103, 788, 1970. 

The next residues were attached successively by dicyclohexylcarbodiimide-mediated coupling of Boc-amino acids with the free amino groups. The use of excess Boc-amino acid eliminated the need for capping after coupling. The last Boc-group and the benzyl-based side chain and carboxy-terminal protectors were removed at the end of the synthesis by acidolysis with hydrogen bromide in trifluoroacetic acid the latter was used instead of acetic acid to avoid acetylation of hydroxymethyl side chains (see Section 6.6). Catalytic hydrogenolysis of the peptide removed the nitro... 

The symmetrical anhydride is prepared using dicyclohexylcarbodiimide in dichloromethane, the urea and solvent are removed, and the anhydride is dissolved in dimethylformamide and added to the peptide-resin (see Section 2.5). The anhydride is a more selective acylating agent than the 0-acylisourea and, thus, gives cleaner reactions than do carbodiimides, but twice as much amino-acid derivative is required, so the method is wasteful. It avoids the acid-catalyzed cyclization of terminal glutaminyl to the pyroglutamate (see Section 6.16) and is particularly effective for acylating secondary amines (see Section 8.15). 

FIGURE 8.20 Peptides activated at an IV-methylamino-acid residue are postulated to epimer-ize because of the formation of the oxazolonium ion. Evidence for the latter resides in spectroscopic studies,96 and the isolation of a substituted pyrrole that was formed when methyl propiolate was added to a solution of Z-Ala-MeLeu-OH in tetrahydrofuran 10 minutes after dicyclohexylcarbodiimide had been added.95 The acetylenic compound effected a 1,3-dipolar cycloaddition reaction (B), with release of carbon dioxide, with the zwitter-ion that was generated (A) by loss of a proton by the oxazolonium ion. 

Coupling the substituents to the polyacid core is a key step. The reaction must have a high yield to limit purification problems and show high selectivity between the amines and alcohols present to limit side reactions. The amidifica-tion reaction chosen is a coupling reaction used in peptide chemistry. The reaction is carried out at room temperature in the presence of a coupling reagent such as NjAT -dicyclohexylcarbodiimide, l-(3-dimethylaminopropyl)-3-ethylcar-bodiimide or l-ethoxycarbonyl-2-ethoxyl-l,2-dihydroquinoline, possibly in the presence of an activator such as hydroxybenzotriazole or N-hydroxysuccimide (Fig. 8). 

The first studies on the sulfation of organic compounds, amino acids, and proteins have shown that pyridine/sulfur trioxide complex (pyridine/S03 or pyridine/Cl S03H),168-721 concentrated sulfuric acid,173,74 sulfuric acid//V,A -dicyclohexylcarbodiimide,175,761 and chloro-sulfonic acid177 are the most efficient reagents for the sulfation of tyrosine. More recently, alternative methods based on dimethylformamide/sulfur trioxide complex (DMF/S03),152,781 trimethylamine/sulfur trioxide (Me3N/S03),1152,1531 pyridinium acetylsulfate,137,791 and pyr-idinium trifluoroacetylsulfate1801 have been proposed to minimize side reactions which are difficult to control for the chemical sulfation of tyrosine peptides. 

This can then be coupled to a second amino acid with a blocked amino group using dicyclohexylcarbodiimide (Eq. 3-10). The removal of the blocking group and addition of a new amino acid residue can then be repeated as often as desired. The completed peptide is removed from the polystyrene by action of a strong acid such as HF. 

Another in situ procedure for activating carboxylic acids utilizes earbodiimides, such as dicyclohexylcarbodiimide (DCC). DCC (19) plays an important role in peptide synthesis. Addition of a carboxylic acid to the C-N double bond leads to the activated species, an acyl isourea 20, which upon attack by a nucleophile (and alcohol or an amine) releases the corresponding ester or amide along with 21 (for the mechanism, see Chapter 5). However, in the conversion of 5 to 7 the DCC procedure gives poor results. 

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