Peptides, acyl transfer

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). 

Much more important than these reactions, however, are the reactions of CDI and its analogues with carboxylic acids, leading to AAacylazoles, from which (by acyl transfer) esters, amides, peptides, hydrazides, hydroxamic acids, as well as anhydrides and various C-acylation products may be obtained. The potential of these and other reactions will be shown in the following chapters. In most of these reactions it is not necessary to isolate the intermediate AAacylazoles. Instead, in the normal procedure the appropriate nucleophile reactant (an alcohol in the ester synthesis, or an amino acid in the peptide synthesis) is added to a solution of an AAacylimidazole, formed by reaction of a carboxylic acid with CDI. Thus, CDI and its analogues offer an especially convenient vehicle for activation of... 

FIGURE 7.34 Decomposition of the symmetrical anhydride of A-methoxycarbonyl-valine (R1 = CH3) in basic media.2 (A) The anhydride is in equilibrium with the acid anion and the 2-alkoxy-5(4//)-oxazolone. (B) The anhydride undergoes intramolecular acyl transfer to the urethane nitrogen, producing thelV.AT-fcwmethoxycarbonyldipeptide. (A) and (B) are initiated by proton abstraction. Double insertion of glycine can be explained by aminolysis of the AA -diprotected peptide that is activated by conversion to anhydride Moc-Gly-(Moc)Gly-0-Gly-Moc by reaction with the oxazolone. (C) The A,A -diacylated peptide eventually cyclizes to the IV.AT-disubstituted hydantoin as it ejects methoxy anion or (D) releases methoxycarbonyl from the peptide bond leading to formation of the -substituted dipeptide ester. 

DM Coltart. Peptide segment coupling by prior ligation and proximity-induced intramolecular acyl transfer. Tetrahedron 56, 3449-3491, 2000. 

In the 1990s, short peptides,and other nucleophiles °° ° were used as organocatalysts for a number of enantioselective acyl transfer processes transformations that set the stage for the more recent research in the area of nucleophilic catalysis.One of the most appealing approaches to enantioselective acyl transfer was outlined by Fu using an azaferrocene catalyst (6) [Eq. (11.6)]. While these pyridyl systems are not organic catalysts in the strictest sense, these azaferrocene compounds function as chiral dimethylaminopyridine equivalents for a broad range of acyl transfer processes ... 

Miller and co-workers have taken a totally different approach to design an efficient catalyst for enantioselective acylation. Their strategy relied on the use of a pep-tide-based backbone incorporating a 3-(l-imidazolyl)-(5)-alanine unit as the catalytic core. Upon treatment with an achiral acyl source these biomimetic enantioselective acyl transfer catalysts allow the formation of an acyl imidazolium ion in proximity to the chiral environment generated by the folding of the peptide [3, 159-174]. 

Halotriazoles can act as halogenating agents and A-acyltriazoles can act as acyl transfer reagents. Triazole can be used for the synthesis of peptide bonds and is superior to imidazole in that less racemization is observed. It can also be used to transfer the t-butyloxycarbonyl (t-Boc) protecting group to the nitrogen of amino acids. For details see Polya <84CHEC-I(5)733, p. 786). 

Diphenylthieno[3,4-d][l,3]dioxol-2-one 5,5-dioxide (304) can serve as an activating agent for peptide synthesis (76AG(E)444). The esters (305) are formed readily on admixture of a carboxylic acid with (304) in an aprotic solvent in the presence of pyridine. The activated esters (305) are stable, crystallizable compounds which react with amines readily to furnish the corresponding amides (Scheme 65). Competition experiments reveal that the esters (305) are more effective acyl transfer agents than the p- and o-nitrophenyl esters often used in peptide synthesis. 

It is clear that this mechanistic interpretation is not unique, because there is no direct evidence for the proposed intermediates. For example, alternative possible explanations are (1) The amino group in the aminoenzyme may not be covalently bound, but may merely be activated by the enzyme and (2) similarly, the acyl transfer reaction of equation 16.31 could occur by the direct attack of Leu-Tyr-Leu on the enzyme-bound Leu-Tyr-Leu. However, M. S. Silver and S, L. T. James169,170 have proposed a further interpretation, based on the observation that small peptides stimulate the pepsin-catalyzed hydrolysis of other peptides by being first synthesized into larger peptides in a condensation reaction that is the reverse of the hydrolytic step e.g., equations 16.33. The idea of the condensation of two small peptides to give a larger peptide at a rate that is relatively fast compared with hydrolysis of the small peptides is quite reasonable... 

We were also able to use FAB mass spectrometry to determine the amino acid sequence around the active site serine in the acyl transference domain of rabbit mammary fatty acid synthase.6 The synthase was labelled in the acyl transferase domain(s) by the formation of O-ester intermediates after incubation with [" " C]-acetyl- or malonyl-CoA (Fig. 2A). The modified protein was then digested with elastase (Fig. 2B), and radioactive material isolated via successive purification steps on Sephadex G-50 and reverse phase HPLC. The isolated peptides were then sequenced by FAB MS. The data summarized in Table II established the sequences of both the acetyl and malonyl hexapeptides to be N-acyl-Ser-leu-Gly-Glu-Val-Ala. 

Yamada, S. Sugaki, T. Matsuzaki, K. Twisted amides as selective acylating agents for hydroxyl groups under neutral conditions models for activated peptides during enzymatic acyl transfer reactions. J. Org. 

Scheme 6. The Hnb-protecting group serves as an activated O- N acyl transfer auxiliary for the efficient synthesis of difficult peptides.

Miller has developed various short peptides containing 3-(l-imidazolyl)-(S)-alanine as the catalytic core these biomimetic enantioselective acyl transfer catalysts allow the formation of an acyl imidazolium intermediate in a chiral environment formed by folding of the peptide [109-123]. The first catalyst of this type to be reported was tripeptide 27 in 1998 [109]. This peptide incorporates a C-terminal (R)-methylbenzylamide to encourage order-inducing stacking interactions in the acylimidazolium intermediate (Scheme 8.6). 

Another biologically compatible, site-specific method for poly(ethylene glycol) (PEG) conjugation to proteins was described by Sato (2002), who reported the use of the transglutaminase enzyme to catalyze the acyl transfer between the y-carboxyamide group of glutamine and alkylamines under mild conditions. This method could easily be adopted for peptide-PEG conjugation. 

Intermediate 23b can be obtained by a direct nucleophilic attack of the uncharged amino group (present at low equilibrium concentration at pH 7) but the mechanism of this step may be more complex as shown by the observation of a catalytic effect of CO2 in amidations using CDI [191]. Moreover, the reaction of the carboxylate group followed by an intramolecular acyl transfer is also a possibility. CDI-promoted peptide formation was shown to display... 

N-Acylsaccharins (13) possess a certain potential as acylating agents. They will acylate amines, but will react with water or alcohols only when acid or base is present.167 The method was used to acylate a-amino-penicillanic acid.170 Micheel162-165 has based a peptide (38) synthesis on the acyl transfer from 31 [Z = carbobenzoxy, obtained through reaction with DCC or with pseudosaccharin chloride (6) or with thionyl chloride and imidazole] to amino acids. Pseudosaccharin anhydride 323, lee js thg product of a condensation between 6 and 1, mostly from hydrolysis of 6. Formation of 32 tends to occur in nonprotic solvents with base catalysis, even when practical precautions are taken to exclude moisture. Water and protic solvents seem to shield the anion 19 and prevent attack on 6. 

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