Total synthesis of the large non-ribosomal peptide polytheonamide B

Polytheonamide B is by far the largest non-ribosomal peptide known at present, and displays extraordinary cytotoxicity (EC₅₀ =68 pg ml ^-1 , mouse leukaemia P388 cells). Its 48 amino-acid residues include a variety of non-proteinogenic d- and l-amino acids, and the absolute stereochemistry of these amino acids alternate in sequence. These structural features induce the formation of a stable β-strand-type structure, giving rise to an overall tubular structure over 30Å in length. In a biological setting, this fold is believed to transport cations across the lipid bilayer through a pore, thereby acting as an ion channel. Here, we report the first chemical construction of polytheonamide B. Our synthesis relies on the combination of four key stages: syntheses of non-proteinogenic amino acids, a solid-phase assembly of four fragments of polytheonamide B, silver-mediated connection of the fragments and, finally, global deprotection. The synthetic material now available will allow studies of the relationships between its conformational properties, channel functions and cytotoxicity.