Thioimidate Protection Enables Amide, Thioamide, and Amidine Installation at Aspartic Acid Residues

Peptide-bond isosteres provide an avenue to interrogate interactions along the peptide backbone. This study identifies why thioamides are especially prone to aspartimide formation at aspartic acid residues during peptide elongation and are difficult to access via conventional methods. Thioimidates have been described as effective protecting groups for thioamides during all three phases of solid-phase peptide synthesis (SPPS). The structure of the thioimidate completely prevents the formation of aspartimide and aspartimide-related side-products. Further, thioimidates can be selectively converted into amides or, more unexpectedly, amidines, enabling access to both the native control and backbone-modified analogues from a single SPPS precursor. The successful installation of amidines at aspartic acid residues establishes a rare and modular strategy to incorporate this underexplored isostere into peptides.