Two series of novel 3-O-carbamoyl derivatives of clarithromycin and 11, 12-cyclic carbonate azithromycin were designed, synthesized and evaluated for their in vitro antibacterial activities. Compounds 4j and 4k were the most potent activity against erythromycin- susceptible Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae, which were comparable to those of clarithromycin and azithromycin. ...