Azithromycin impurity IV (impurity I) [Azithromycin I (3'-N-desmethylazithromycin] (2R, 3S, 4R, 5R, 8R, 10R, 11R, 12S, 13S, 14R)-13-[[2,6-dideoxy-3-C-methyl-3-O-methyl-α-L-ribulose-cohexylglycopyranosidyl]oxy]-2-ethyl-3,4,10-trihydroxy-3. 5,6,8,10,12,14-heptamethyl-11-[[3,4,6-trideoxy-3-methylamino-beta-D-xylulose-hexopyranosyl]oxy]-1-oxa-6-azacyclopentadecan-15-one