Functionalized platforms based on marine cyclopeptides: different pathways to the hexapeptide

The synthesis of macrocyclic, roughly planar, exclusively syn-functionalized hexapeptides, related to dolastatin I and bistratamide C from enantiomerically pure oxazole precursors is reported. The platforms can either be synthesized in a stepwise procedure by final cyclization of the linear oxazole trimers or in a direct ‘one-pot’ reaction. The investigation on the coupling of these building blocks into linear dimers and trimers with modern peptide coupling reagents is reported.