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

(Chemical Equation Presented) 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 ∈-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(∈-N ₃) or the incorporation of Fmoc-Nle(∈-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(∈-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[Lys¹³(&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.