Total synthesis of daptomycin and other cyclic peptides via Ser/Thr ligation-mediated peptide cyclization

Abstract

Head-to-tail cyclic peptides with a wide range of ring sizes have been discovered in various organisms including bacteria, fungi, plants and animals. Many of them exhibit remarkable biological activities with high potency. Daptomycin, a cyclic lipodepsipeptide isolated from soil bacteria Stretomyces roseoporus, is the first natural product antibiotic launched in a generation. Daptomycin has potent bactericidal activity against otherwise antibiotic-resistant Gram-positive pathogens including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE) and vancomycin-resistant S. aureus. Daptomycin contains a 31-membered ring made up of 10 amino acids and a linear 3-amino acid side chain modified with an n-decanoyl lipid at the N-terminus. The complex structure of daptomycin, the presence of two non-proteinogenic amino acids (kynurenine and 3-methyl glutamic acid) and the macrolactamization of a 31-membered ring render daptomycin a challenging target for total synthesis. We recently developed a chemoselective serine/threonine ligation (STL) allowing peptide ligation at Ser/Thr site using side chain unprotected segments. We have successfully applied STL intramolecularly in the key cyclization step for the synthesis of daptomycin molecule, which allows us to finally achieve the first total synthesis of daptomycin. With this technique in hand, the chemical synthesis of daptomycin analogues, which are difficult to obtain otherwise becomes possible and is now ongoing. This would allow for the search for optimized analogues. We further investigated if STL could be applied for the synthesis of cyclic tetrapeptides, which are extremely difficult to be synthesized in the absence of turninducing components due to their rigid structural framework. To our delight, a series of cyclic tetrapeptides without any turn-inducing component, like Gly, Pro or Damino acids have been successfully synthesized by intramolecular STL. The synthesis of cyclic tetrapeptides with drug-like scaffold would be useful for the therapeutic development. We also applied intramolecular STL to successfully synthesize some natural cyclic peptides with different ring sizes, including anticancer stylopeptide 1, phakellistatin 4 and anti-inflammatory cyclosquamosin D.