Azithromycin impurity II (impurity B) [Azithromycin B] (2R,3R,4S,5R,8R,10R,11R,12S,13S,14R)-13-[[2,6-dideoxy-3-C-methyl-3-O-methyl-α-L-ribose-pyranohexyl]oxy]-2-ethyl-4,10-dihydroxy-3,5,6,8,10,12,14-(2,6-dideoxy-3-C-methyl-3-(dimethylamino-β-D-xyloxyhexyl)oxy]-1-oxa-6-azaspirochelate (2,6-dideoxy-3-(dimethylamino-β-D-xyloxyhexyl)oxy]-1-oxa-6-azaspirochelate (2R,6D) heptamethyl-11-[[3,4,6-trideoxy-3-(dimethylamino)-β-D-xylulose-hexopyranosyl]oxy]-1-oxa-6-azacyclopentadecan-15-one