Abstract: Improved single-step process for preparing 7,16-deoxy-2-aza-10-O-cladinosyl-12-O-desosaminyl-4,5-dihydroxy-6-ethyl-3,5,9,11,13,15-hexamethylbicycle[11.2.1]hexadeca-1(2)-en-8-ona from erythromycin A, with high yield and under soft conditions suitable for its industrial production. The transformation of erythromycin A into an intermediate compound, called 6,9-iminoether, which is obtained in a single step, is achieved by forming the mesitylenesulfonyloxime “in situ” from erythromycin, which in the presence of a base in aqueous acetone undergoes a Beckmann's transposition creating the iminoether with the help of the hydroxyl in position 6 of the macrolide ring; this intermediary is transformed into the antibiotic 9-deoxo-9a-methyl-9a-aza-9a-homoerythromycin A, which is obtained by precipitation in hexane, thereby obtaining an innovative form, with an anhydrous crystalline structure and physical characteristics different from the forms known to date.