title: A backbone amide protecting group for overcoming difficult sequences and suppressing aspartimide formation.
abstract: A backbone amide bond protecting group, 2-hydroxy-4-methoxy-5-nitrobenzyl (Hmnb), improved the synthesis of aggregation and aspartimide-prone peptides. Introduction of Hmnb is automated and carried out during peptide assembly by addition of 4-methoxy-5-nitrosalicylaldehyde to the peptidyl-resin and on-resin reduction to the secondary amine. Acylation of the hindered secondary amine is aided by the formation of an internal nitrophenol ester that undergoes a favourable O,N intramolecular acyl transfer. This activated ester participates in the coupling and generally gives complete reaction with standard coupling conditions. Hmnb is easily available in a single preparative step from commercially available material. Different methods for removing the amide protecting group were explored. The protecting group is labile to acidolysis, following reduction of the nitro group to the aniline. The two main uses of backbone protection of preventing aspartimide formation and of overcoming difficult sequences are demonstrated, first with the synthesis of a challenging aspartimide-prone test sequence and then with the classic difficult sequence ACP (65-74) and a 23-mer homopolymer of polyalanine.
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.
authors.forename: Abu-Baker M
authors.initials: AB
authors.lastname: Abdel-Aal
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.
authors.forename: George
authors.initials: G
authors.lastname: Papageorgiou
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: Raz
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.
authors.forename: Martin
authors.initials: M
authors.lastname: Quibell
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.|Sorbonne Universités, UPMC Université Paris 06, LBM, 4 place Jussieu, 75005, Paris, France.|CNRS, UMR 7203, LBM, Paris, France.|Département de Chimie, LBM, ENS, PSL Research University, 24 Rue Lhomond, 75005, Paris, France.
authors.forename: Fabienne
authors.initials: F
authors.lastname: Burlina
authors.affiliation: The Francis Crick Institute, Mill Hill Laboratory, The Ridgeway, London, NW7 1AA, UK.
authors.forename: John
authors.initials: J
authors.lastname: Offer
mesh_terms: D000215:Acylation; D000577:Amides / Q000737:chemistry*; D000595:Amino Acid Sequence; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000737:chemistry; D015394:Molecular Structure; D009578:Nitrobenzenes / Q000737:chemistry; D010455:Peptides / Q000138:chemical synthesis* / Q000737:chemistry; D060327:Solid-Phase Synthesis Techniques
chemical_list: D000577:Amides; D009578:Nitrobenzenes; D010455:Peptides; D001224:Aspartic Acid; C075871:aspartimide
keywords: Fmoc SPPS; Hmb; O,N acyl transfer; aspartimide formation; backbone amide protection; difficult sequence; nitro reduction; reductive amination