Erythromycin Impurity: (3R,4S,5S,6R,7R,9R,11R,12R,13S,14R)-4-(2,6-dideoxy-3-C-methyl-alpha-L-ribo-hexopyranosyloxy)-14-ethyl-7,12,13-trihydroxy-6-[3,4,6-trideoxy-3-(dimethylamino)-beta-D-xylo-hexopyranosyloxy]-3,5,7,9,11,13-hexamethyloxacyclotetradec.