Synthesis of trifunctional cyclo-β-tripeptide templates

Abstract

The concept of template-assembled synthetic proteins (TASP) describes a central scaffold that predefines the three dimensional structure for diverse molecules linked to this platform. Cyclic β-tripeptides are interesting candidates for use as templates due to their conformationally defined structure, stability to enzymatic degradation, and ability to form intermolecular stacked tubular structures. To validate the applicability of cyclic β-tripeptides within the TASP concept, an efficient synthesis of the cyclopeptide with orthogonal functionalization of the side chains is desired. A solid-phase-supported route with on-resin cyclization is described, employing the aryl hydrazide linker cleavable by oxidation. An orthogonal protection-group strategy allows functionalization of the central cyclic β-tripeptide with up to three different peptide fragments or fluorescent labels.

Keywords: cyclic β-tripeptide scaffold; orthogonal protection groups; peptide synthesis; template-assembled synthetic proteins (TASP); β-amino acids.

Figure 1

Trifunctional cyclic β-tripeptide forming an intermolecular stack of rings by backbone hydrogen bonding.

Figure 2

β-Amino acids 1–3 with orthogonal side-chain protection obtained by Arndt–Eistert homologation followed by Wolff rearrangement; cyclo-β-tripeptide template 4 as obtained by coupling of amino acids 1–3 followed by cyclization.

Scheme 1

Synthesis of cyclic peptides employing the oxidation-labile aryl hydrazide linker [11,24].

Figure 3

Functional units provided as carboxylic acids for the attachment to the cyclo-β-peptide: 5(6)-tetramethylcarboxyrhodamine 5, thymine 6, Cbz protected cytosine with a carboxylic acid linker 7 [25], and fully protected peptide sequence of penetratin 8 [26].

Scheme 2

Functionalization of the cyclic β-tripeptide 4.

Figure 4

Cyclic β-tripeptides varying in two side-chain functionalities and containing an additional azide moiety, e.g., for ligation with click chemistry.