Serine-, threonine-, and cysteine-derived cyclic building blocks (pseudo-prolines, ΨPro) serve as reversible protecting groups for Ser, Thr, and Cys and prove to be versatile tools for overcoming some intrinsic problems in the field of peptide chemistry. The presence of ΨPro within a peptide sequence results in the disruption of β-sheet structures considered as a source of intermolecular aggregation during chain elongation, thus increasing solvation and coupling kinetics in peptide assembly. Due to their easy synthetic access and variability in the chemical stability by modifications introduced in the C-2 position of the oxazolidine/thiazolidine ring system, this protection technique is adaptable to all common strategies in peptide synthesis. We describe new types of ΨPro building blocks suitable for standard Fmoc/tBu-based solid phase peptide synthesis, convergent strategies, and chemoselective ligation techniques as well as their use as a structure-disrupting, solubilizing protection technique for the example of peptides generally considered as “difficult sequences”.

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