The first total synthesis of halichondrin C has been completed, highlighted by development of the synthetic method to construct the C8–C14 polycycle. Cr-mediated coupling reactions are used seven times to form a new C–C bond. The acid stability of halichondrin C is studied, demonstrating that the macrolactone stabilizes the C8–C14 polycycle, relative to the one present in the C1–C16 model.