Eribulin (marketed under the trade name HALAVEN® as eribulin mesylate), a nontaxane microtubule dynamics inhibitor, is a structurally simplified, synthetic analog of the marine natural product halichondrin B. Methods for the synthesis of eribulin and other halichondrin B analogs are described in U.S. Pat. Nos. 6,214,865, 6,365,759, 6,469,182, 7,982,060, and 8,148,554, the syntheses of which are incorporated herein by reference. New methods for the synthesis of halichondrin B analogs, in particular eribulin and eribulin mesylate, are desirable.