Cabazitaxel Impurity 51

5- ((2aR,4S,4aS,6R,9S,11S,12S,12aR,12bS) -12b-acetoxy-12- (benzoyloxy) -11-hydroxy-4,6-dimethoxy-4a,8,13,13-tetramethyl-5-oxo-2a,3,4,4a,5,6,9,10,11,12,12a,12b-dodecahydro-1H-7,11-methanocyclodeca[3,4]benzo[1,2-b]oxet-9-yl) 3- (tert-butyl) (4R,5S) -2- (4-methoxyphenyl) -4-phenyloxazolidine-3,5-dicarboxylate