Synthesis of cyclic peptide natural products and inhibitors of histone modifying enzymes

Natural products have been the source of numerous leads for several drugs. As these natural products are often isolated in small quantities, it is necessary to produce them synthetically to allow testing for biological activity. Furthermore, synthesis allows the preparation of unnatural analogues for SAR studies. Cyclic peptides represent an important family of biologically active natural products. The hepta- and octacyclopeptides sanguinamide A and sanguinamide B were recently isolated in submicromolar amounts by the Molinski group. The lack of material prevented biological evaluation of the natural products. For this reason and to confirm the structural elucidation we have targeted the total synthesis of sanguinamides. In addition to two proline residues, sanguinamides A and B include heterocycles and natural L-amino acid residues. We have completed the total syntheses of sanguinamides A and B; however the synthetic rotamers differed in both cases from the natural rotamers. We have investigated the influence of macrocyclisation on cis/trans conformational preference of the proline residues for the synthesis of sanguinamide A. We attempted several isomerisations and calculated the relative energies of the different sanguinamide conformers. [D-Ile]-Sanguinamide A, Cys(tBu) analogue of sanguinamide A and the synthetic sanguinamide B displayed antibacterial activity while the synthetic trans, trans-sanguinamide A displayed mild tyrosine kinase inhibitory activity. While extracted stylissamide A showed inhibition of translation during the elongation step, even though being structurally identical to the natural product, the synthetic compound prepared by macrocyclisation from a linear precursor was found to be totally inactive. Histones undergo different types of covalent modifications on the N-terminal tails such as acetylation, phosphorylation and methylation. Histone modification is a major mechanism of regulation in gene expression, replication and repair. Deregulation of histone modifications leads to cancer progression and therefore, inhibitors of enzymes which are able to catalyse the addition and removal of these epigenetic marks have therapeutic potential for treating cancer. An enzyme of particular interest is the family of zinc-dependent histone deacetylases (HDACs) that remove acetyl groups from acetylated lysine residues. Depsipeptides were prepared as HDAC inhibitors. We will ii present our total synthesis of largazole along with a range of analogues and discuss the SAR obtained from HDAC and cell proliferation assays. We elucidated the stereochemistry of burkholdac B by total synthesis of three diastereomers. The diastereomers made along with the natural product were tested as HDAC inhibitors. We are interested in inhibitors of lysine-specific demethylase 1 (LSD1) which is a different kind of epigenetic enzyme involved in demethylation of histone proteins in chromatin. Tranylcypromine is known to be an LSD1 inhibitor. Analogues of PCPA have been synthesised in order to explore the structure-activity relationships of this inhibitor. Analogues were also prepared and tested as LSD1 inhibitors