Azithromycin EP Impurity K/(2S,4R,4a'R,5S,6S,7'R,8'S,9'R,10'R,13'R,15'R,16'R,17'S,17a'S)-16'-(((2S,3R,4S,6R)-4-(dimethylamino)-3-hydroxy-6-methyltetrahydro-2H-pyran-2-yl)oxy)-7'-ethyl-5,8',9',15'-tetrahydroxy-4-methoxy-4,6,8',10',11',13',15',17'-octamethy
