Peptide bond formation with carbodiimide

Carbodiimide method, a procedure for peptide bond formation using carbodi-imides, R-N=C=N-R, such as dicyclohexyl carbodiimide (DCC), diisopropyl carbodiimide (DIC) and water-soluble carbodi-imides. The carbodiimide reacts in a one-pot procedure with the carboxylate anion of the carboxy component to form a highly reactive O-acylisourea intermediate. The former reacts immediately with the amino function of the amino component to yield the desired peptide derivative and the urea byproduct. Indeed, a more complex mechanism must be taken into consideration. Unwanted side reactions are racemization via the oxazolone mechanism and formation of the unreactive N-acylurea by base-catalyzed acyl migration from the isourea oxygen to nitrogen. The side reactions can be diminished by preparing the O-acylisourea at 0 °C... 

On reviewing the couphng methods proposed for application in solid phase peptide synthesis one finds that most known methods of peptide bond formation have been tried but no new approach was designed specially for the acylation of the polymer bound amine component. Therefore we will limit the discussion in this section to two well established procedures, coupling with the help of carbodiimides and acylation via active esters. 

This reagent is a dehydrating agent which has been used much less for peptide bond formation than the carbodiimides. Reaction first occurs with a carboxylate function after base catalyzed decomposition of the isoxazolium salt ... 

The chemical synthesis of adenosine and guanosine triphosphate may be undertaken in a fashion analogous to their biosynthesis phosphorylation of the low energy monophosphate precursors. Phosphorylation is accomplished with phosphoric acid and a coupling (dehydrating) agent, DCC. Use of carbodiimides has already been discussed in connection with peptide bond formation. It will be recalled that reaction proceeds by way of an anhydride... 

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... 

The second reagent, dicyclohexylcarbodiimide (DCC, DCCI), introduced by J.C. Sheehan (Plate 41) and G.P. Hess for the formation of the peptide bond [40] brought about a revolution in synthesis. Carbodiimides (R—N=C=N—R) had been known for a long time [41] and their reactivity had been exploited before for the preparation of esters and anhydrides. According to H.G. Khorana [42] the condensation reaction starts with the addition of the carboxyl component to one of the double bonds in the carbodiimide to yield the reactive... 

The introduction of carbodiimides (Sheehan and Hess 1955), particularly dicy-clohexylcarbodiimide (DCC, DCCI) as reagents for the formation of the peptide bond was a major event in the history of peptide synthesis. The novel feature of coupling reagents was that they could be added to the mixture of the carboxyl component and the amine component. Thus, activation and coupling proceed concurrently. While amines do react with carbodiimides (yielding guanidine derivatives) the rate of this reaction is negligible when compared with the rapid rate observed in the addition of carboxylic acids to one of the double bonds of a carbodiimide ... 

Diphenylphosphinic mixed anhydrides have been utilized to form peptide bonds. Peptides are easier to isolate by this method than by employing 1,3-Dicyclohexylcarbodiimide. These anhydrides are the method of choice for the formation of amides of 2-alkenoic acids (eq 1 ). Carbodiimide and acyl carbonate methods proved to be inferior. Primary amines result in better yields than secondary amines. This activation protocol can be employed to form thiol esters (eq 2) p-Amino acids are readily converted to p-lactams with chlorodiphenylphosphine oxide (eq 3). Secondary amines work best. This activation protocol has been utilized to convert acids to amines via a Curtius rearrangement. Phenols have been generated from diene acids, presumably via base-induced elimination of diphenylphosphinic acid from the mixed anhydrides to form ketenes which spontaneously cyclize. Acids have been converted to ketones via activation followed by reaction with organometallic reagents (eq 4)."... 

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