The present invention provides halichondrin analogs, such as compounds of Formula (I). The compounds may bind to microtubule sites, thereby inhibiting microtubule dynamics. Also provided are methods of synthesis, pharmaceutical compositions, kits, methods of treatment, and uses that involve the compounds for treatment of a proliferative disease (e.g., cancer). Compounds of the present invention are particularly useful for the treatment of metastatic breast cancer, non-small cell lung cancer, prostate cancer, and sarcoma. The included methods of synthesis are useful for the preparation of compounds of Formula (I)-(III) along with naturally occurring halicondrins (e.g., halichondrin B & C, norhalichondrin A, B, & C, and homohalichondrin A, B, & C). Also included are methods for interconverting between the halichondrins, norhalichondrins, and homohalichondrins and their unnatural epimers at the C38 ketal stereocenter through the use of an acid-mediated equilibration.