SEA Ligation Assisted Preparation of Cyclic Peptides Under microfluidic Conditions

The intramolecular SEA ligation for the preparation of cyclic peptides of various sizes was studied under microfluidic conditions. Two type of pep- tides, namely POL (10 residues) and RTD-1 (18 residues) as well as difficult junctions (e.g. valine, isoleucine, and threonine) were chosen as the main targets of this research. The microfluidic setup for the intramolecular SEA ligation formally telescoped two process steps. In the first microreactor, the reversible N,S-acyl shift reaction lead to the reactive SEA thioester species in ~90% conversion for all studied peptides. The telescoping of the first step and the use of the reactive SEA thioester species in the capture step of the N- terminal cysteine mediated by an exogenous arylthiol catalyst produced a cyclic thioester intermediate which upon irreversible S,N-acyl shift gave the final cyclic product. This fully telescoped process of 64 (POL and analogs) to 75 min (RTD-1 and analogs) total residence time gave the associated de- sired cyclic product with conversions ranging from 60 to 96% depending on the peptide sequence and the ligation site.