Peptide bonds formation with carboxy activation

Aminolysis of peptide esters occurs uneventfully but this approach to peptide bond formation is not used routinely because access to enantiomerically pure active esters of peptides is straightforward only when the activated residue is glycyl or prolyl (Section 3.2.2). Succinimido esters of small peptides with glycine or proline at the carboxy terminus have been used extensively for the preparation of larger segments in solution. Aminolysis by dipeptide ester 85 of dipeptide succinimido ester 84, obtained through the mixed anhydride (Scheme 19), gave enantiomerically pure (<0.5% l-d-l-l isomer) protected tetrapeptide 86 in 88% yield (Scheme 25). 

A simplified synthesis relies on the potential to protect difunctional compounds as cyclic derivatives. For example, 1,2-diols are masked as cyclic acetals (Section 24-8), hydroxy acids as lactones (Section 19-9), amino acids as lactams (Section 19-10), and dicarboxylic acids as anhydrides (Section 19-8). The last two possibilities merit consideration as applied to Asp. However, direct lactam formation can be quickly ruled out because of the complications of ring strain (although /3-lactams have been used in the preparation of aspartame). This problem is absent with respect to dehydration to the five-membered ring anhydride. Because anhydrides are activated carboxylic acid derivatives (Section 20-3), the Asp anhydride can be coupled directly with Phe-OCHa without the help of added DCC. Nucleophilic attack of the amino end of Phe-OCHs occurs preferentially at the desired position, albeit not completely so 19% of the product derives from peptide-bond formation at the /3-carboxy group of Asp. 

CDI 13 (33) allows one-pot amide bond formation and is also used for large-scale peptide chemistry (34). Initially, the mechanism may involve the formation of acyl carboxy imidazole and imidazole. Both intermediates react together to lead to the activated species as the acyl imidazole 14. Then the amine is added to undergo aminolysis (see Fig. 4). As imidazole is generated in situ, the reaction does not need an additional base and it is usually compatible with amine HCl salts (35). Incidentally, the acyl imidazole intermediate can also be isolated and stored. Some simple acyl imidazoles are even available commercially. 

The active ester methodology, which is widely used in peptide chemistry, has found only limited application in depsipeptide synthesis. A more vigorous activation of the carboxy component is apparently required to form an ester bond compared to the peptide analogue. Nevertheless, active esters have been utilized for this purpose in combination with some catalyst additives. The first successful attempt in this direction was described by Mazur.1103 The modification of the 4-nitrophenyl ester procedure included addition of 1-10 equivalents of imidazole to the reaction mixture. This accelerated technique presumably involves formation of the highly reactive intermediate imidazolide. The reaction resulted in the preparation of model benzyloxycarbonyl didepsipeptide esters in good yields within several hours at room temperature from 4-nitrophenyl esters of Z-amino acids and the pentamethylbenzyl ester of glycolic acid, while in the absence of imidazole this reaction failed to give any product. 

In addition to suitable protection of other coreactive functionalities, activation of the carboxy group prior to reaction with the amino group is required for a controlled formation of the amide bond between two a-amino acid or peptide components (Scheme 1). For this purpose activated species can be generated separately, isolated, and stored for subsequent use. Alternatively, the activation can be carried out in situ with specific coupling reagents. Independently of the mode of activation, an electron-withdrawing group X must be incorporated at the acyl carbon in a transient mode in the in situ activation or in the formation of reactive intermediates. 

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