Four-dimensional orthogonal solid-phase synthesis of new scaffolds based on cyclic tetra-β-peptides

Optimized Stepwise Synthesis of the API Liraglutide Using BAL Resin and Pseudoprolines

The number of peptide-based active pharmaceutical ingredients (APIs) has increased enormously in recent years. Furthermore, the emerging new peptide drug candidates are more complex and larger. For the industrial solid-phase synthesis of C-carboxylic acid peptides, the two main resins available, Wang and chlorotrityl chloride (CTC), have a number of drawbacks. In this context, resins that form an amide bond with the first amino acid are more robust than Wang and CTC resins. Here, we address the use of the backbone (BAL) resin for the synthesis of the peptide liraglutide. The BAL resin, in conjunction with the use of pseudoprolines to avoid aggregation, allows the stepwise solid-phase synthesis of this API in excellent purity and yield.

p-Nitromandelic Acid as a Highly Acid-Stable Safety-Catch Linker for Solid-Phase Synthesis of Peptide and Depsipeptide Acids

[reaction: see text] p-Nitromandelic acid as a safety-catch linker for Boc/Bzl-SPPS of base-labile compounds like peptides and depsipeptides is described. This linker permits acidic removal of side-chain protection groups from the resin. For cleavage from the solid support, the p-nitro group was reduced with tin(II) chloride. After washing off the reducing agents, the (depsi)peptide acids with or without the side-chain protection schemes were obtained by microwave irradiation at 50 degrees C with 5% TFA in dioxane.

Cyclic peptoids

Foldamers are an intriguing family of biomimetic oligomers that exhibit a propensity to adopt stable secondary structures. N-Substituted glycine oligomers, or "peptoids", are a prototypical example of these foldamer systems and are known to form a helix resembling that of polyproline type I. Ongoing studies seek to improve the stability of peptoid folding and to discover new secondary structure motifs. Here, we report that peptoids undergo highly efficient head-to-tail macrocyclization reactions. A diverse array of peptoid sequences from pentamers to 20mers were converted to macrocyclic products within 5 min at room temperature. The introduction of the covalent constraint enhances conformational ordering, allowing for the crystallization of a cyclic peptoid hexamer and octamer. We present the first X-ray crystallographic structures of peptoid hetero-oligomers, revealing that peptoid macrocycles can form a reverse-turn conformation.

Stable Helicalβ3-Peptides in Water via Covalent Bridging of Side Chains

An on-bead cyclization protocol of beta 3-peptides was developed, providing easy access to cyclic beta 3-peptides. With this methodology, a small library of helical cyclic beta 3-peptides was synthesized and investigated with CD spectroscopy. Covalent bridging of two side chains in beta 3-peptides significantly stabilized their helical conformation in aqueous solutions and turned out to be superior to the previously described electrostatic interactions.

Orthogonally Protected Cyclo-β-tetrapeptides as Solid-Supported Scaffolds for the Synthesis of Glycoclusters

Two novel peptide scaffolds, viz. cyclo[(N(alpha)-Alloc)Dpr-beta-Ala-(N(alpha)-Fmoc)Dpr-beta-Ala] (1) and cyclo[(N(alpha)-Alloc)Dpr-alpha-azido-beta-aminopropanoyl-(N(alpha)-Fmoc)Dpr-beta-Ala] (2), composed of orthogonally protected 2,3-diaminopropanoyl (Dpr) and beta-alanyl residues, have been described. Fmoc chemistry on a backbone amide linker derivatized resin has been used for the chain assembly. Selective removal of the 4-methyltrityl (Mtt) and 1-methyl-1-phenylethyl protections (PhiPr) exposes the beta-amino and carboxyl terminus, respectively, and on-resin cyclization then gives the desired orthogonally protected cyclo-beta-tetrapeptides (1 and 2). The alpha-amino groups, bearing the Fmoc and Alloc protections and the azide mask, allow stepwise orthogonal derivatization of these solid-supported cyclo-beta-tetrapeptide cores (1 and 2). This has been demonstrated by attachments of various sugar units [viz., acetyl- or toluoyl-protected carboxymethyl alpha-d-glycopyranosides (13-15) and methyl 6-O-(4-nitrophenoxycarbonyl)-alpha-d-glycopyranosides (22-24)] to obtain diverse di- and trivalent glycoclusters (33-42). Acidolytic release (TFA) from the support, followed by conventional NaOMe-catalyzed transesterification (33-40) or hydrazine-induced acyl substitution in DMF (41 and 42), gives the fully deprotected clusters (43-52) as final products.

Cyclic β-Tetra- and Pentapeptides: Synthesis through On-Resin Cyclization and Conformational Studies by X-Ray, NMR and CD Spectroscopy and Theoretical Calculations

The solution-phase synthesis of the simplest cyclic beta-tetrapeptide, cyclo(beta-Ala)4 (4), as well as the solid-phase syntheses through side chain anchoring and on-resin cyclization of the cyclic beta3-tetrapeptide cyclo(-beta3hPhe-beta3hLeu-beta3hLys-beta3hGln-) (14) and the first cyclic beta3-pentapeptide cyclo(-beta3hVal-beta3hPhe-beta3hLeu-beta3hLys-beta3hLys-) (19) are reported. Extensive computational as well as spectroscopic studies, including X-ray and NMR spectroscopy, were undertaken to determine the preferred conformations of these unnatural oligomers in solution and in the solid state. cyclo(beta-Ala)4 (4) with no chiral side chains is shown to exist as a mixture of rapidly interchanging conformers in solution, whereas inclusion of chiral side chains in the cyclo-beta3-tetrapeptide causes stabilization of one dominating conformer. The cyclic beta3-pentapeptide on the other hand shows larger conformational freedom. The X-ray structure of achiral cyclo(beta-Ala)4 (4) displays a Ci-symmetrical 16-membered ring with adjacent C=O and N-H atoms pointing pair wise up and down with respect to the ring plane. CD spectroscopic examinations of all cyclic beta-peptides were undertaken and revealed results valuable as starting point for further structural investigations of these entities.

Solid-Phase Synthesis of Lidocaine and Procainamide Analogues Using Backbone Amide Linker (BAL) Anchoring

New solid-phase strategies have been developed for the synthesis of lidocaine (1) and procainamide (2) analogues, using backbone amide linker (BAL) anchoring. Both sets were prepared starting from a common resin-bound intermediate, followed by four general steps: (i) attachment of a primary aliphatic or aromatic amine to the solid support via reductive amination (as monitored by a novel test involving reaction of 2,4-dinitrophenylhydrazine with residual aldehyde groups); (ii) acylation of the resultant secondary amine; (iii) displacement of halide with an amine; and (iv) trifluoroacetic acid-mediated release from the support. A manual parallel strategy was followed to provide 60 novel compounds, of which two dozen have not been previously described. In most cases, initial crude purities were >80%, and overall isolated yields were in the 40-88% range.