Eribulin Impurity 28

(2R,3R,3aS,7R,10S,11S,12S,13R,14S,15S,21S,24S,27S,29R,31R,32aS, E) -2- ((S) -2,3-bis ((tert-butyldimethylsilyl) oxy) propyl) -12,13,15-tris ((tert-butyldimethylsilyl) oxy) -3-methoxy-29-methyl-23,30-dimethylene-2,3,3a,6,7,8,9,10,11,12,13,14,15,19,20,21,22,23,24,25,26,27,28,29,30,31,32,32a-octacosahydro-5H-7,11:10,14:21,24:27,31-tetraepoxycyclohentriaconta[b]furan-5,18 (4H) -dione