A new strategy for synthesis of branched cyclic peptide by Asn side-chain hydrazide ligation

Aspartic Acid Side-Chain Benzyl Ester as a Multifunctionalization Precursor for Synthesis of Branched and Cyclic Arginylglycylaspartic Acid Peptides

“…Therefore, we employed the thio ester 6 to perform side-chain NCL with the N-terminal Cys-peptide CRGDRGDC, which contains two RGD units and a potential disulfide motif for cyclization. In our previous work, 15 a byproduct from the intramolecular cyclization of the side-chain thio ester with the neighboring amide nitrogen was observed. Here, we optimized the ligation condition in a pH 6.0 solution at a lower temperature of 4 °C to minimize the competitive intramolecular cyclization.…”

“…More examples of side-chain conversions of benzyl ester groups into hydrazide groups are available in the Supplementary Information. By following the approach of Liu and co-workers 16 and our previous method, 15 the side-chain hydrazide group was converted into the carbonyl azide 5 (Figure 1e) by treatment with NaNO 2 in acidic solution. Azide 5 was subsequently converted into the thio ester 6 (Figure 1f) by treatment with MPAA.…”

“…obtained in an improved yield (70%), with much less cyclization byproduct compared with the previous reported results. 15 With the purified branched peptide 7a in hand, we conducted the disulfide-bond-formation reaction at a concentration of 0.25 mM in a sodium phosphate buffer (pH 7.5, 0.2 M) containing 20% DMSO and bubbled with air. After four hours, the branched cyclic peptide 7 was obtained in 95% yield.…”

“…Sidechain ligation of 13 with the Cys-peptide CRGDRGDC provided the branched peptide 14a in 60% yield (Figures 3d and 3e). The side-reaction between the side-chain thio ester and the nitrogen of the neighboring amino acid 15 gave a cyclic byproduct with a more significant rate of formation than that of thio ester 6. This result is probably due to the lower steric hindrance of the amide in peptide 13 Asp-(side-chain)Lys compared with that of the amide of the Asp-Arg moiety in 6, when these amide nitrogen atoms were involved in the cyclization side-reaction.…”

“…12 The development of novel RGD peptides as tumor-targeting reagents or anticancer therapeutics has potential applications in cancer treatment. 13,14 In our previous work, 15 we employed peptide side-chain hydrazide ligation for the synthesis of branched and cyclic peptides. This strategy permitted the assembly of a hydra-zide functional group on the asparagine (Asn) side chain, and successive azidation with NaNO 2 , thio ester formation with (4-sulfanylphenyl)acetic acid (MPAA), and peptide side-chain ligation with an N-terminal cysteine (Cys) peptide then gave the target branched peptides, following Liu and co-workers' approach of linear peptide hydrazide ligation.…”