Incorporation of Acid-Labile Masking Groups for the Traceless Synthesis of C-Terminal Peptide α-Ketoacids

Abstract

An optimized protocol for the masking of α-ketoacids with acid-labile cyclic acetal protecting groups is reported. Unlike prior approaches, these new conditions allow the synthesis of protected α-ketoacids bearing aromatic, hindered alkyl, and protected polar side chains. Attachment to a Wang-type linker and solid support provides a resin that delivers fully unprotected C-terminal peptide α-ketoacids upon resin cleavage. These peptides are the key starting materials for chemical protein synthesis using the α-ketoacid-hydroxylamine ligation.

Photoprotected Peptide α‐Ketoacids and Hydroxylamines for Iterative and One‐Pot KAHA Ligations: Synthesis of NEDD8

“…These conditions allowed us to expand the scope of the protected amino acids to most of the canonical residues. [25] Based on this work, we selected diol 2 as a potential protecting group. The requisite, racemic starting material could be easily prepared in one step from the corresponding aldehydes.…”

“…Protected a-ketoacids with linker were prepared by our recently published method. [25] See the Supporting Information for details. All new intermediates and final products are reported herein.…”

Facile Synthesis of Internal and C‐Terminal Peptide α‐Ketoamides with Fmoc‐Solid Phase Peptide Synthesis

“…The basis for our approach is the synthesis of enantiopure protected α ‐ketoacids, which can be prepared on a multi‐gram scale in four steps from commercially available Fmoc α ‐amino acids . More than ten different protected α ‐ketoacid starting materials have been prepared by this convenient, unified route .…”

“…Based on our recent development of direct approaches to the formation of C‐terminal peptide α ‐ketoacids, we now report an approach to the fully automated solid phase synthesis of both internal and C‐terminal α ‐ketoamide peptides directly upon resin cleavage and without the need for additional deprotection or oxidation steps. This approach relies on acid‐labile, protected α ‐ketoacid building blocks and standard Fmoc‐amino acids with acid‐labile side chain protecting groups including tert ‐butyl, trityl and Boc . Importantly – and in contrast to other methods – our approach allows the formation of enantiopure peptide α ‐ketoamides without detectable epimerization during their synthesis or purification by reverse phase HPLC .…”

Chemical synthesis of a homoserine-mutant of the antibacterial, head-to-tail cyclized protein AS-48 by α-ketoacid–hydroxylamine (KAHA) ligation

“…28–30 The requisite peptide α-ketoacids and hydroxylamines can be conveniently prepared by Fmoc–SPPS and we have applied this reaction to the synthesis of numerous linear proteins as well as small cyclic peptides. 31,32 The homoserine-forming variant, which uses ( S )-5-oxaproline as the ligation partner, has the unique property of forming depsipeptides as the primary ligation products, a feature that can greatly aid in the preparation of hydrophobic sequences.…”