Thioamides are valuable peptide bond bio isosteres that offer a powerful means to probe interactions along the peptide backbone. However, their incorporation into peptides, particularly at aspartic acid residues—poses significant synthetic challenges. During solid-phase peptide synthesis (SPPS), thioamides are especially susceptible to Aspartimide formation, a common side reaction that reduces yield and purity. This reactivity complicates elongation steps and often limits access to thioamide-containing sequences using standard synthetic protocols.
In this work, we identify the underlying reasons for this heightened Aspartimide formation and present a strategy to overcome it. We demonstrate that converting thioamides into thioimidates provides an effective and versatile protecting approach during all stages of SPPS. The structural features of thioimidates completely suppress Aspartimide formation, eliminating this major synthetic obstacle. Importantly, this protection strategy is not merely temporary, thioimidates can be selectively transformed after peptide assembly into either amides or amidines. This post-synthetic flexibility allows the generation of both native (unmodified) peptide backbones and backbone-modified analogues from a single precursor synthesized on resin.
A key outcome of this study is the successful and modular installation of amidines at aspartic acid residues, representing a rare application of this underutilized isostere in peptide chemistry. This capability opens new opportunities for tuning hydrogen-bonding networks, modulating conformational stability, and exploring structure function relationships in peptides. Overall, the thioimidate approach addresses a long-standing barrier in thioamide chemistry, enabling efficient synthesis and structural diversification of peptides for biochemical and medicinal studies.
