The invention provides methods for the synthesis of eribulin or a pharmaceutically acceptable salt thereof (e.g., eribulin mesylate) through a macrocyclization strategy. The macrocyclization strategy of the present invention involves subjecting a non-macrocyclic intermediate to a carbon-carbon bond-forming reaction (e.g., an olefination reaction (e.g., Horner-Wadsworth-Emmons olefination), Dieckmann reaction, catalytic Ring-Closing Olefin Metathesis, or Nozaki-Hiyama-Kishi reaction) to afford a macrocyclic intermediate. The invention also provides compounds useful as intermediates in the synthesis of eribulin or a pharmaceutically acceptable salt thereof and methods for preparing the same.