Toward the total synthesis of onchidin, a cytotoxic cyclic depsipeptide from a mollusc

The total synthesis of onchidin (1), a cytotoxic, C₂-symmetric cyclic decadepsipeptide from a marine mollusc, according to the published structure, is described. A novel β-amino acid, (2S,3S)-3-amino-2-methyl-7- octynoic acid (AMO), was efficiently prepared in high yield with high diastereo- and enantioselectivity based on a catalytic asymmetric three-component Mannich-type reaction with a chiral zirconium catalyst. The formation of sterically unfavorable N-methyl amide and hindered ester bonds were successfully demonstrated, and final macrocyclization was achieved at a secondary-amide site. Completion of the synthesis of 1 suggested that a revision of the structure of the natural product is required. Two diastereomers were also synthesized as candidates for the actual structure of onchidin. Furthermore, efficient solid-phase methods were employed for the combinatorial synthesis of other derivatives to clarify the real structure of onchidin. The solid-phase assembly of a pentadepsipeptide containing all the building blocks was established followed by dimeric cyclization in solution.