Chemoselective cyclization of unprotected linear peptides by α-ketoacid–hydroxylamine amide-ligation

Abstract

Cyclic peptides are important synthetic targets due to their constrained conformation, enhanced metabolic stability and improved bioavailability, which combine to make them promising lead compounds for drug candidates. They are typically synthesized by a multi-step sequence of carefully orchestrated protecting group manipulations and cyclization of side-chain protected linear precursors. In the present manuscript we disclose an alternative approach to the synthesis of peptide macrocycles by the α-ketoacid-hydroxylamine (KAHA) ligation. This reaction allows readily prepared linear peptides to be cyclized without reagents or side-chain protecting groups and delivers a native backbone amide bond at the ligation site. The precursors are prepared with Fmoc-based solid phase peptide synthesis using reagents that we have previously disclosed. No post-cyclization manipulations or deprotections other than purification are required. This protocol was applied to five different cyclic peptide natural products of varying ring sizes and side chain functionalities.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

“…The KAHA ligation employs N ‐terminal peptide hydroxylamines and C ‐terminal peptide α–ketoacids, which react chemoselectively to form native peptidic bonds. On this basis, a new and convenient strategy for peptide cyclization was also developed . As shown in Figure , the nitrone‐sulfur ylide peptide was prepared using a MBHA resin‐bond sulfonium salt through standard 9‐fluorenylmethoxycarbonyl‐based solid phase peptide synthesis (Fmoc‐SPPS).…”

Chemical Protein Synthesis with the KAHA Ligation

“…Upon reaction with the free N-terminus of a peptide, the intermediately formed imine is oxidized to the nitrone, which can be hydrolyzed on-resin by treatment with hydroxylamine hydrochloride and imidazole in NMP. The resulting N-hydroxy amine peptide can undergo on-resin ligations or can be cleaved from the resin [44].…”

“…3.2.2) [44]. The linear peptides were assembled by Fmoc SPPS and the terminal nitrone-protected N-hydroxy aminoacid was coupled with standard reagents (Scheme 33).…”

Synthesis of 3a‐hydroxyhexahydropyrrolo[2,3‐B]Indole‐2‐carboxamide, an oxidation product of tryptophan present in natural products

“…Residues (Xaa = Gly, Ala, Asn(Tr), Glu( t ‐Bu), His( N im ‐Tr), Ile, Leu, Lys(Boc), Phe, Pro, Ser( t ‐Bu), Thr( t ‐Bu), Tyr( t‐ Bu), Val, Sar, N α ‐Me‐Ala, N α ‐Me‐Leu) in the dipeptide sequences ( 57 ) were not significantly oxidized on side chains resulting in reasonably high yielding mixtures of syn‐cis and anti‐cis diastereomers (Scheme ). While in this case side chains were protected, unprotected residues such as Lys, Asn, Glu, Tyr, Ser and ornithine embedded in several peptide sequences were shown to be stable to DMDO exposure for 10 minutes at room temperature . Following the saponification of the N α ‐Tr‐HO‐HPIC‐Xaa‐OMe methyl ester, the resulting carboxylate was readily incorporated into a peptide sequence by standard peptide chemistry .…”