Accelerated Microfluidic Native Chemical Ligation at Difficult Amino Acids Towards Cyclic Peptides

The intramolecular SEA ligation for the preparation of cyclic peptides of various sizes was studied under microfluidic conditions. Three types of peptides, namely POL (10 residues), RTD-1 (18 residues) and F2-K1 (28 residues) as well as specific difficult junctions 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 leading to a super reactive SEA thioester species (3) proceeded in ~90% conversion for all studied peptide sequences. Extremely fast cyclative ligations (< 5 min) at difficult junctions that are notoriously time-consuming were achieved accordingly in a second, telescoped microfluidic module. Efficient ligations at difficult (valine, threonine or isoleucine) and even intractable junctions (such as proline) were achieved with extremely short ligation times down to 2 min (vs 48 h under conventional conditions). Excellent conversions (60 to 96%) and overall isolated yields were obtained for the cyclization of POL and analogs (64 min total production time) and for RTD-1 and analogs (75 min total production time)