Erythromycin Impurity: (1Z,3R,4R,5S,6R,9R,10S,11S,12R,13R,15R)-10-[(2,6-Dideoxy-3-C-methyl-3-O-methyl-a-L-ribo-hexopyranosyl)oxy]-6-ethyl-4,5-dihydroxy-3,5,9,11,13,15-hexamethyl-12-[[3,4,6-trideoxy-3-(dimethylamino)-ß-D-xylo-hexopyranosyl]oxy]-7,16-d.