Concise Total Synthesis and Biological Evaluation of Pargamicin A and its Diastereomer, Piperazic Acid‐containing Cyclopeptides

Abstract

We have accomplished the total synthesis, structure determination, and biological evaluation of pargamicin A and one of its diastereomers. Two key tripeptide segments were synthesized using a linear peptide elongation process that includes the direct coupling of a poorly nucleophilic piperazic acid derivative. The resulting tripeptides were coupled using triphosgene/collidine at ambient temperature leading to a precursor for the final cyclization step. T3P‐promoted macrolactamization under high‐dilution conditions, followed by the removal of the benzyl protecting group was used to furnish two putative structures of pargamicin A. Comparison of the ¹H and ¹³C NMR spectra and the antibacterial activity of the natural and synthetic products successfully revealed that the absolute configuration of the N‐hydroxy‐Ile residue of pargamicin A is 2S,3S. A biological evaluation of synthetically obtained pargamicin A and its diastereomer suggested that the stereostructure of the cyclic peptide scaffold of the natural product plays a crucial role in determining the strength of its antibacterial activity.