Beckmann rearrangement of erythromycin A 9(E)-oxime with toluenesulfon
yl chloride in ethyl ether at -45 degrees C generates 9,11-imino ether
IV which leads to azithromycin. The 9,11-imino ether can also be read
ily obtained from isomerization of its isomer 6,9-imino ether III.