Peptide bonds using reactive esters

Thiol esters, which are more reactive to nucleophiles than are the corresponding oxygen esters, have been prepared to activate carboxyl groups for both lactoniza-tion and peptide bond formation. For lactonization S-f-butyl and S-2-pyridyP esters are widely used. Some methods used to prepare thiol esters are shown below. The S-r-butyl ester is included in Reactivity Chart 6. 

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. 

Benzotriazolyl esters of A-(alkoxycarbonyl)-A-methyl amino acids have greatly diminished reactivity relative to those of unmethylated residues,however the succinimido esters react sufficiently to be of practical use.l A variant of esters of substituted hydroxylamines that can be used to generate amide and peptide bonds is the acyloxime resin.P A new type of active ester has been developed based on the reaction of acids with substituted pyrazolinones, the products being 1-phenylpyrazolinone enol esters 83 (Scheme 24).t More than a dozen of these esters of Fmoc-valine have been characterized. They are about as reactive as esters 63, are self indicating, and show promise for the future. 

The active ester methodology has been much applied mainly to form peptide bonds under mild conditions in both liquid- and solid-phase synthesis. In liquid-phase synthesis, most frequently the activation of a carboxyl group has employed HOSu, followed by esters of electron-withdrawing-containing phenols (nitrophe-nols of pentafluorophenols), the most reactive HOBt esters usually being unstable and used in solid-phase synthesis. The preparation of these active esters requires the presence of a coupling reagent such as DCC or DIC. 

More recently, the use of phosphonium (e.g., benzotriazole-1-yl-oxy-tris-pyrrolidino phosphonium hexafluorophosphate (PyBOP)) and uronium (e.g., 2-(1H- benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU)) coupling reagents have gained wide use in effecting peptide bond formation, particularly in SPPS (Fig. 7.6). The structures of PyBOP and HBTU both incorporate an equivalent of HOBt, and the final reactive Boc or Fmoc amino acid species is the corresponding OBt active ester (13). 

The efforts of D.A. Buckingham, in Duneden, to exploit the enhanced reactivity of peptides in Co-III complexes for rate enhancement in the hydrolysis of their esters and in the formation of a new peptide bonds brought interesting and potentially useful results. At Massey University the research of W.S. Hancock and his associates has led in the last decade to the design of new, selectively removable blocking groups. 

In some cases, up to 3% racemization has been observed when using the HOSu-DCC method, which renders it unsuitable for some types of peptide synthesis." Esters derived from HOSu are relatively reactive, as shown by a kinetic study of their rates of hydrolysis in aqueous buffers although, in some examples, endo-lV-hydroxy-5-norbornene-2,3-dicarboximide has proven superior to HOSu in peptide bond formation. Of course, the HOSu-DCC method is not limited to peptide synthesis and is well suited to the preparation of amides in general, both from ammonia and primary amines (e.g. eq 2). ... 

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