Total Synthesis of Dolastatin 16 and the Silstatins: Cyclic Depsipeptides from the Sea

The 23-step total synthesis of dolastatin 16, a cyclic depsipeptide of marine origin, is presented. Included are syntheses of nonnatural amino acids dolamethylleuine and dolaphenvaline. The biological activity of the synthetic product differed from naturally isolated dolastatin 16, which may indicate the initial screening identified an inactive compound and the active one was not detected initially, or may be a result of the conformational dynamics induced by the proline residues. Additionally, a family of structural analogues to the bacillistatins, another cyclic marine depsipeptide, were synthesized. These were deemed the silstatins. 8 modifications were produced. The alterations aimed to introduce a heteroatomic residue for further derivatization, such as producing an antibody-drug conjugate. This introduction did in general decrease the neoplastic activity of these agents, as expected, but by modulating the lipophilicity of the compound we were able to salvage much of the potency of the bacillistatins while potentially allowing prodrug development.