Synthesis and Conformational Analysis of a Cyclic Peptide Obtained via i to i+4 Intramolecular Side-Chain to Side-Chain Azide−Alkyne 1,3-Dipolar Cycloaddition

Abstract

Intramolecular side-chain to side-chain cyclization is an established approach to achieve stabilization of specific conformations and a recognized strategy to improve resistance toward proteolytic degradation. To this end, cyclizations, which are bioisosteric to the lactam-type side-chain to side-chain modification and do not require orthogonal protection schemes, are of great interest. Herein, we report the employment of Cu(I)-catalyzed 1,3-dipolar cycloaddition of side chains modified with azido and alkynyl functions and explore alternative synthetic routes to efficiently generate 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptides. The solid-phase assembly of the linear precursor including epsilon-azido norleucine and the propargylglycine (Pra) in positions i and i+4, respectively, was accomplished by either subjecting the resin-bound peptide to selective on-resin diazo transformation of a Lys into the Nle(epsilon-N3) or the incorporation of Fmoc-Nle(epsilon-N3)-OH during the stepwise build-up of the resin-bound peptide 1b. Solution-phase Cu(I)-catalyzed 1,3-dipolar cycloaddition converts the linear precursor Ac-Lys-Gly-Nle(epsilon-N3)-Ser-Ile-Gln-Pra-Leu-Arg-NH2 (2) into the 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptide [Ac-Lys-Gly-Xaa(&(1))-Ser-Ile-Gln-Yaa(&(2))-Leu-Arg-NH2][(&(1)(CH2)4-1,4-[1,2,3]triazolyl-CH2&(2))] (3). The conformational preferences of the model cyclopeptide 3 (III), which is derived from the sequence of a highly helical and potent i to i+4 side-chain to side-chain lactam-containing antagonist of parathyroid hormone-related peptide (PTHrP), are compared to the corresponding lactam analogue Ac[Lys(13)(&(1)),Asp(17)(&(2))]hPTHrP(11-19)NH2 (II). CD and NMR studies of 3 and II in water/hexafluoroacetone (HFA) (50:50, v/v) revealed a high prevalence of turn-helical structures involving in particular the cyclic regions of the molecule. Despite a slight difference of the backbone arrangement, the side-chains of Ser, Gln, and Ile located at the i+1 to i+3 of the ring-forming sequences share the same spatial orientation. Both cyclopeptides differ regarding the location of the turn-helical segment, which in II involves noncyclized residues while in 3 it overlaps with residues involved in the cyclic structure. Therefore, the synthetic accessibility and conformational similarity of i to i+4 side-chain to side-chain cyclopeptide containing the 1,4-disubstituted [1,2,3]triazolyl moiety to the lactam-type one may result in similar bioactivities.

CuI‐Catalyzed Azide–Alkyne Intramolecular i‐to‐(i+4) Side‐Chain‐to‐Side‐Chain Cyclization Promotes the Formation of Helix‐Like Secondary Structures

“…To this end, the recently introduced Cu I -catalyzed azide-alkyne 1,3-dipolar cycloaddition, a prototypic "click chemistry" reaction, [6][7][8] presents an interesting opportunity to develop a new paradigm for generating heterodetic cyclopeptides through pseudopeptidic and bioorthogonal intramolecular side-chain-to-sideScheme 1. Linear precursors IЈ-VIIIЈ (excluding IIIЈ that was reported previously [1] ): linear peptides in which Lys 13 and Asp 17 were replaced with ω-azido-and ω-yl-α-amino acid residues. i-to-(i+4) side-chain-to-side-chain 1,4-disubstituted 1,2,3-triazolyl-bridged cyclononapeptides (I-VIII, excluding III that was reported previously [1] ) are derived from the model peptide N α -Ac-PTHrP(11-19)NH 2 .…”

“…Linear precursors IЈ-VIIIЈ (excluding IIIЈ that was reported previously [1] ): linear peptides in which Lys 13 and Asp 17 were replaced with ω-azido-and ω-yl-α-amino acid residues. i-to-(i+4) side-chain-to-side-chain 1,4-disubstituted 1,2,3-triazolyl-bridged cyclononapeptides (I-VIII, excluding III that was reported previously [1] ) are derived from the model peptide N α -Ac-PTHrP(11-19)NH 2 .…”

Helical Content of a β3‐Octapeptide in Methanol: Molecular Dynamics Simulations Explain a Seeming Discrepancy between Conclusions Derived from CD and NMR Data

“…Recently, Baldwin [16] classified these factors as major ones such as hydrophobicity, van der Waals interactions, peptide hydrogen bonding, solvation, backbone conformational entropy, and auxiliary factors such as the linking of side chains. For a-peptides, the most common strategy for helix stabilization involves the covalent linking of adjacent turns of the helix with lactam, [17] triazole, [18] alkyl, or hydrazone bridges. [19,20] It was found that cyclization of a b-peptide [21] or a strategically placed salt bridge or disulfide linkage between residues i and i + 3 along the chain of residues of a 3 14 -helix [22,23] could lead to greater stability of the helix.…”

Cyclisierende Abspaltungen über dipolare Cycloadditionen: Polymergebundene Azidopeptidylphosphorane liefern konformativ fixierte cis‐Triazolylcyclopeptide als privilegierte Proteinbinder

“…[9] [10] sank die Effizienz der Cu I -katalysierten 1,4-selektiven Reaktionen beträchtlich bei Cyclisierungen, [11] insbesondere wenn diese an der Festphase durchgeführt wurden. [12] Deshalb wurde das 1,4-disubstituierte Triazol in den meisten dieser Fälle durch die Reaktion von Peptiden mit einem terminalen Azid-und Alkinrest in Lösung zu Cyclopeptiden eingeführt. [11][12][13] Alternativ wurde das 1,4-disubstituierte Triazol zunächst in die lineare Peptidsequenz eingebaut und das Cyclopeptid durch eine finale Lactamisierung erhalten.…”

“…[12] Deshalb wurde das 1,4-disubstituierte Triazol in den meisten dieser Fälle durch die Reaktion von Peptiden mit einem terminalen Azid-und Alkinrest in Lösung zu Cyclopeptiden eingeführt. [11][12][13] Alternativ wurde das 1,4-disubstituierte Triazol zunächst in die lineare Peptidsequenz eingebaut und das Cyclopeptid durch eine finale Lactamisierung erhalten. [14] Leider erwies sich die Synthese der biologisch privilegierten 1,5-disubstituierten Triazolylcyclopeptide als besonders schwierig.…”