the invention features compounds of the formula : or a pharmaceutically acceptable salt thereof , wherein a is cr 2 and b is n ; wherein n is 0 - 3 ; wherein r 1 is m — o — p } q — l , in which g is ch 2 , ch ═ ch , c ≡ c , co , o , s , or nr 5 , where r 5 is h or c 1 - 6 alkyl , chr 6 , where r 6 is oh or c 1 - 6 alkyl , ch , where each of r 7 and r 8 is , independently , h , halo , or c 1 - 6 alkyl , oco , conr 9 , nr 10 co , where each of r 9 and r 10 is , independently , h or c 1 - 6 alkyl , so 2 nh , or nhso 2 ; in which m is 1 - 4 , o is 0 - 4 , p is 0 - 4 , and q is 0 - 4 . the compounds disclosed herein have potent anti - microbial , e . g ., both gram - positive and gram - negative anti - bacterial properties . the compounds inhibit dna polymerase iiic and dna polymerase iiie enzymes and thus act therapeutically by inhibiting the growth of a broad array of bacteria . the compounds can be administered to prevent or to treat gram - positive or gram - negative bacterial infections , e . g ., in eukaryotic cell cultures , animals , or humans .