Exploiting the MeDbz Linker To Generate Protected or Unprotected C‐Terminally Modified Peptides

Abstract

C-terminally modified peptides are important targets for pharmaceutical and biochemical applications. Known methods for C-terminal diversification are limited mainly in terms of the scope of accessible modifications or by epimerization of the C-terminal amino acid. In this work, we present a broadly applicable approach that enables access to a variety of C-terminally functionalized peptides in either protected or unprotected form. This chemistry proceeds without epimerization of C-terminal Ala and tolerates nucleophiles of varying nucleophilicity. Finally, unprotected peptides bearing nucleophilic side chain groups can be selectively functionalized by strong nucleophiles, while macrocyclization is observed for weaker nucleophiles. The potential utility of this method is demonstrated through the divergent synthesis of the conotoxin conopressin G and GLP-1(7-36) and analogs.

Sequence Diversification by Divergent C-Terminal Elongation of Peptides

“…Recently, we reported a method for C-terminal derivatization, as outlined in Scheme 1. 5 First, peptide 1 is generated on solid support using a methyl diaminobenzoyl (MeDbz) linker. 6 After activation to form the N -acyl urea (MeNbz, 2 ), the peptide was either cleaved with a nucleophile 7 to generate protected peptide 3 , or it was cleaved at Gly using trifluoroacetic acid (TFA), and then the unprotected peptide ( 4 ) was functionalized.…”

“…In our previous work, 5 we reported efficient addition of primary amine nucleophiles to MeNbz-appended peptides to generate secondary C-terminal amides. We observed reduced reactivity for secondary amines and α -branched alcohols during resin-cleaving reactions.…”

Epimerization-free access to C-terminal cysteine peptide acids, carboxamides, secondary amides, and estersviacomplimentary strategies

“…We recently reported a strategy for C-terminal functionalization of non-cysteine peptides involving activation of the methyl-diaminobenzoyl (MeDbz) linker ( 1 → 2 ) 21 followed by nucleophilic cleavage of the N -acyl urea (MeNbz) group 22 to yield various protected ( 3 ) or unprotected ( 4 ) peptides ( Scheme 1 ). 23 If this approach were to prove mild enough to enable preparation of challenging C-terminal cysteine peptide derivatives, it would establish the MeNbz group as one of the mildest known activated carbonyl intermediates. 24 We were report herein the exploitation of this reactivity to access C-terminal cysteine peptide acids, primary and secondary amides, and esters without epimerization.…”

“…The unprotected peptide H-AKTWA- MeNbz-Gly ( 15b ) was functionalized in solution to afford H-AKTWAC-OH ( 15c ) with complete conversion in 30 min with no observed side-chain macrocyclization. 23 To enable comparison with the crypto-thioester approach, 27 C-terminal proline-containing peptide 16a was cleaved from resin using H-Cys-OH to afford protected H-AKTWPC-OH ( 16c ) with 10% conversion over 4 h. 38 Repeating this reaction in solution on unprotected peptide ( 16b ) led to complete conversion after 1 h at ambient temperature. Elongation of Boc-LYRAGLRAY ( 17a ) proceeded with resin cleavage and complete conversion in the presence of DMF and NCL buffer.…”

Solid-Phase Synthesis of Head to Side-Chain Tyr-Cyclodepsipeptides Through a Cyclative Cleavage From Fmoc-MeDbz/MeNbz-resins

“…This MeNbz-resin has also been used by Stockdill's group to synthesize C-terminal-modified peptides (Arbour et al, 2017(Arbour et al, , 2018a. In this case, the linker is released through an intermolecular reaction by the action of nucleophiles such as alcohols, thiols, amines, and even amino acids.…”