Peptides ester aminolysis

There are two basic strategies for enzyme-catalyzed peptide synthesis equiUbrium- and kineticaHy controlled synthesis. The former is the direct reversal of proteolysis and involves the condensation of an amino component with unactivated carboxyl component. The latter proceeds by the aminolysis of an activated peptide ester. 

Thus, the family of azolides represents a versatile system of reagents with graduated reactivity, as will be shown in the following section by a comparison of kinetic data. Subsequent chapters will then demonstrate that this reactivity gradation is found as well for alcoholysis to esters, aminolysis to amides and peptides, hydrazinolysis, and a great variety of other azolide reactions. The preparative value of azolides is not limited to these acyl-transfer reactions, however. For example, azolides offer new synthetic routes to aldehydes and ketones via carboxylic acid azolides. In all these reactions it is of special value that the transformation of carboxylic acids to their azolides is achieved very easily in most cases the azolides need not even be isolated (Chapter 2). 

For the mechanism of azolide hydrolysis under specific conditions like, for example, in micelles,[24] in the presence of cycloamyloses,[25] or transition metals,[26] see the references noted and the literature cited therein. Thorough investigation of the hydrolysis of azolides is certainly important for studying the reactivity of those compounds in chemical and biochemical systems.[27] On the other hand, from the point of view of synthetic chemistry, interest is centred instead on die potential for chemical transformations e.g., alcoholysis to esters, aminolysis to amides or peptides, acylation of carboxylic acids to anhydrides and of peroxides to peroxycarboxylic acids, as well as certain C-acylations and a variety of other preparative applications. 

M Bodanszky. Synthesis of peptides by aminolysis of nitrophenyl esters. Nature (London) 75, 685, 1955. 

LM Siemens, FW Rottnek, LS Trzupek. Selective catalysis of ester aminolysis. An approach to peptide active esters, (phenylthiomethyl esters). J Org Chem 55, 3507,... 

Preparation of the thioacids and their esters via SPPS is mainly restricted to the Boc methodology. The popular Fmoc approach is limited by aminolysis of the thioester bond during removal of the Fmoc group by piperidine. However, a modified Fmoc-deprotecting mixture (1-methyl pyrrolidine/hexamethyleneimine/HOBt/DMSO/NMP 25 2 2 35.5 35.5) gave the final desired peptide ester with 24% yield J24 ... 

Aminolysis by an Amino Acid or Peptide Ester or Amide... 

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). 

Protease Ester, amide hydrolysis, ester aminolysis, peptide synthesis Many stable proteases No D-proteases... 

Phenol-type resins 13-15 have also been used to prepare LH-RH, enkephalin and bradykinin peptide analogs. Peptides were successfully cleaved by ammonoly-sis (NH3 in MeOH-DMF or DMF, 5-18h) or aminolysis with protected amino acid or peptide esters bearing the free a-amino group [24, 25, 35]. Marshall s mercaptophenol resin (16) has been used to prepare libraries of piperazine-2-carboxamides (consisting of 22 different amines) and 6-carboxybenzopyran-4-ones. Cleavage was performed using an excess of appropriate amines in pyridine for 24 to 48 h [36, 37]. Tetrahydro-P-carboline-3-carboxamides have also been similarly synthesized and released [38]. 

The first reported solid-phase synthesis of head-to-tail cyclic peptides was based on the intramolecular aminolysis of resin-bound o-nitrophenyl esters. The cyclization proceeds concurrently to cleave the peptide from the resin, after deprotection and neutralization of the AT-terminal residue (Scheme 2A). Accordingly, Fridkin et al. [3] reported the preparation of several simple, unhindered cyclopeptides, such as cyc/o(Ala-Gly-Ala-Ala). Similarly, Flanigan and Marshall [4] obtained activation of the resin-bound peptide ester, after elongation of the peptide chain, by oxidation of the 4-(methyl-thio)phenyl (MTP) linker to a sulfonyl ester. Subsquent deblocking of the A-terminal residue and intramolecular condensation yielded the desired cyclic peptide. However, this method was found not to be suitable for the synthesis of longer and more hindered cyclic peptides [5]. 

By using active peptide esters with high molecular ester components, inter-molecular aminolysis can be suppressed. — E Glycyl glycine cross-linked poly-(4-hydroxy-3-nitrostyrene) ester hydrobromide suspended in dimethylformamide, neutralized with triethylamine, allowed to stand 12 hrs. at room temp., and the resulting poly-4-hydroxy-3-nitrostyrene removed by filtration diketo-piperazine. Y 75%. F. e. and ester components s. M. Fridkin, A. Patchornik, and E. Katchalski, Am. Soc. 87, 4646 (1965) peptides s. T. Wieland and G. Birr, Ang. Gh. 78, 303 (1966) with active a-aminocarboxylic acid polyphenolesters s. M. Fridkin, A. Patchornik, and E. Katchalski, Am. Soc, 88, 3164 (1966). 

Peptide synthesis via ester aminolysis (kinetic control)... 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

Peptides can be formed by aminolysis of iV-aminoacyl-3,5-dimethylpyrazole with free amino acid esters 303 as shown in Table 5-5. 

JH Jones, GT Young. Anchimeric acceleration of aminolysis of esters and its application to peptide synthesis. Chem Commun 35, 1967. 

FIGURE 2.17 Peptide-bond formation from benzotriazol-l-yl-oxy-m s(dimethy-lamino)phosphonium hexafluorophosphate-mediated reactions of Af-alkoxycarbonylamino acids.48 The peptide can originate by aminolysis of either of two precursors the acyloxyphos-phonium cation and the benzotriazolyl ester. 

FIGURE 2.21 Experimental evidence indicates that products from BOP-mediated reactions do not originate from the benzotriazolyl ester. The use of BOP allows successful coupling of A-alkoxycarbonyl-A-methylamino acids, whereas the benzotriazolyl esters of these acids undergo aminolysis only with great difficulty. The higher ratio of products obtained from the BOP-mediated reaction in the competing reactions described implies a compound other than the benzotriazolyl ester as the precursor of the peptides. 

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