Solid-phase synthesis of tailed cyclic peptides: The use of α-allyl-protected aspartic acid leads to aspartimide and tetramethylguanidinium formation

Abstract

This paper discusses the application of a method developed for cyclic peptide synthesis using allyl-based sidechain-protecting groups to obtain a so-called tailed cyclic peptide, a cyclic peptide bearing a side-chain anchoring tail. The method used for the synthesis ofcyclo[-D-Val-Arg-Gly-Asp-Asp(-eAhx-Cys-NH2)-] incorporates the c~-allylprotected aspartic acid Fmoc-L-Asp-OA1. A major side reaction, resulting in aspartimide formation, was observed when Fmoc-L-Asp-OA1 was incorporated into the sequence at the N-terminus of 6-aminohexanoic acid (eAhx). This side reaction leads to an aspartimidyl linear peptide with the same molecular weight as the expected cyclized peptide. Additionally, the undesired peptide contains a free amino terminus, which was responsible for further side reactions during the subsequent steps of the synthesis, mainly tetramethylguanidinium formation (M + 98) in TBTUinduced cyclization, and acetylation (M +42).

Automated Solid-Phase Synthesis of Cyclic Peptides Bearing a Side-Chain Tail Designed for Subsequent Chemical Grafting

“…The cause of this problem cysteine on a side chain of the cycle was developed and adapted for an automatic synthesizer. In this method, is an incomplete washing of the resin after allyl palladium cleavage and Fmoc removal which lead to side Fmoc-Cys(Trt)-OH and Fmoc-6-aminohexanoic acid (32). Mass spectrome-can be a major factor for the formation of impurities try analysis of the side products obtained here shows during solid-phase synthesis (36).…”

“…Since the introduction of solid-phase peptide synthepalladium(0) allyl removal (32). According to our exper-sis by Merrifield and its adaptation to continuous-flow iments, this side reaction can be due to the presence of systems, a constant effort has been devoted in deveacetic acid in the solubilization solution of palladium(0) loping solid-phase methods for the synthesis of molewhich is used for the allyl cleavage and which was cules of increasing complexity, like branched peptides, probably not completely removed during the subse-cyclic peptides, or multiple antigenic peptide systems quent steps of the synthesis.…”

The cyclization of peptides and depsipeptides

“…Yet, even now, the number of variables associated with the cyclization step still makes it a 'test it and see' approach, although percolating through the most recent reports is the deduction that HOAt -based couplings have advantages of yield and low racemization when compared with the others. In making choices, however, it would be best to steer away from the tetramethyluronium derivatives as there is evidence [36,39] that the tetramethylguanidinium group is involved in end-capping the amino group involved in macrocyclization. It is not a trivial matter to reach structure (1) in Scheme 1, i.e.…”

Design and synthesis of biologically active peptides: A ‘tail’ of amino acids can modulate activity of synthetic cyclic peptides

“…Further innovative modifications to peptide structure, and likely to their function, could be achieved by the addition of an amino acid ‘tail’ to cyclic structures [12, 13]. Noted biological peptides that have a ring-and-tail structure include arginine vasopressin and analogs.…”

“…Delforge et al . [12, 13] used a C-terminus to N-terminus peptide bond ( i.e ., main chain atoms) to generate synthetic cyclic peptides, then added a tail of amino acids for desired functionality (including serving as a linker to an affinity resin). Additional amino acids, outside of the cyclic portion of the peptide ( i.e ., attached to a side chain), may be expected to alter physical properties of the peptide in terms of hydrophobicity or hydrophilicity (perhaps to enhance solubility) and could potentially do so without disrupting the pharmacophore portion of a cyclized peptide.…”