An efficient method for the reconstruction of the 9-dihydroerythromycin A macrolactone skeleton has been established. The key steps are oxidative cleavage at the 11,12-position and reconstruction after insertion of an appropriate functionalized amino alcohol. Novel 15-membered macrolides, we named as "11a-azalides", were synthesized based on the above methodology and evaluated for their antibacterial activity. Among them, (13R)-benzyloxymethyl-11a-azalide showed the most potent Streptococcus pneumoniae activity, with improved activity against a representative erythromycin-resistant strain compared to clarithromycin (CAM).