Scope and limitations of pseudoprolines as individual amino acids in peptide synthesis

Protected 4-carboxyoxazolidines and thiazolidines (pseudoprolines) are derivatives of serine, threonine or cysteine amino acids. Such compounds are used in peptide synthesis among the other protected amino acids. They are usually practiced when a peptide sequence is readily aggregating during synthesis due to their ability to disrupt secondary structure formation. Such compounds are usually applied as dipeptides. In present work Fmoc-protected pseudoprolines were synthesized and applied in peptide synthesis not as dipeptides but as individual amino acids. Different acylation protocols and amino acids were tested to acylate pseudoprolines. Several "difficult" peptides were synthesized to confirm the efficacy of such constructions. It was shown that pseudoprolines could be easily synthesized and used in automated or manual synthesis not as dipeptides but as ordinary amino acids.