Promising General Solution to the Problem of Ligating Peptides and Glycopeptides

Abstract

Our global goal is that of synthesizing complex polypeptides and glycopeptides in homogeneous form. Chemistry-derived access to homogeneous biologics could well have useful consequences in the discovery of drugs and vaccines. The key finding in this report is that thio acids can become highly competent acyl donors following even trace levels of oxidative activation, thereby undergoing amide bond formation upon reaction with N-terminal peptides. Though our data set does not establish the specific mechanism of this reaction, a framework to account for the fact that minute levels of oxidation actuate amide bond formation with high turnover is offered. An apparently general coupling of thio acids (including complex peptide thio acids with N-termini of complex peptides) has thus been realized. These ligations are conducted with minimal α-epimerization in the C-terminal group and allow for the coupling of N-terminal and C-terminal glycopeptides en route to homogeneous glycoproteins.

6.11 N-Acylation Reactions of Amines

“…358 Danishefsky and Wang have reported a useful protocol for the synthesis of peptide bonds from condensation of thioacids and amines that proceeds under extremely mild conditions in the presence of the additive HOBT (Figure 55(a)). 359 Treatment of thioacids and amines with CuSO 4 ·5H 2 O generates a catalytic copper sulfide species in methanol that mediates rapid and racemization-free peptide bond formation, 360 thus complementing earlier reports of silver(I)-mediated peptide bond-forming reactions in the presence of N-hydroxysuccinimide (vide supra). 361 Xian and coworkers have shown that in situ activation of thioacids with an orgononitrite reagent affords highly reactive S-nitrosothio acid intermediates that react with amines to afford amides in good yield (Figure 55(b)).…”

“…362 Danishefsky and coworkers have employed thioacids in a chemical ligation strategy that employs a sterically hindered isonitrile to generate a reactive intermediate that is intercepted by an amine to afford either secondary or tertiary amide bonds (Figure 55(c)). 363 This method was subsequently extended for the chemical ligation of peptides and glycopeptides, 359 and has been used for the macrolactamization of acyclic peptide precursors to afford synthetic seco-cyclosporins. 364 Liebeskind and coworkers reacted thioacids with bis(trimethylsilyl) acetamide (BSA) to afford O-silylthionoesters, which rearrange via a silatropic switch mechanism to afford sterically demanding secondary and tertiary amides, and peptides ( Figure 55(d)).…”

Solid-phase peptide synthesis and solid-phase fragment coupling mediated by isonitriles

“…The experiments summarized in Table 1 serve to clarify and integrate the findings herein, conducted under SPPS conditions, with the previously described (20,27) solutionbased amidation of thioacids. Previously, we had demonstrated a highly exploitable oxidatively activable pathway for enhancing the acyl-donating properties of thioacids (18). In solution phase, oxidative amidation is operative even in the absence of isonitrile mediation.…”

“…It was in the context of this problem that we could envision a major advantage of the chemistry developed above. Thus, we have shown that a suitably rendered isonitrile could selectively activate a thioacid functional group at room temperature in the presence of carboxylic acid, amide, guanidine, or phenol (18). The resulting thio-FCMA (or its corresponding HOBt ester) acylates primary amine more readily than the alcohol, imidazole, indole, or phenol-based putative acyl acceptors.…”

“…1). The efficiency of the amidation was further improved through the use of hydroxybenzotriazole (HOBt) (18), which could well give rise to HOBt ester 7, although this pathway has not been mechanistically proven.…”