Stereocontrolled Synthesis of Left Halves of Halichondrins

A stereocontrolled synthesis of the left halves of halichondrins was reported. An intramolecular oxy-Michael reaction under basic conditions was used to construct the [6,6]-spiroketal in a stereocontrolled manner. With this approach, the left halves of halichondrins, homohalichondrins, and norhalichondrins were synthesized.