Compounds of the formula and the salts, esters and amides thereof wherein Z is S, SO, SO2 or O; each of R1, R2 and R3 is independently H or substituted or unsubstituted alkyl (1-10C), substituted or unsubstituted alkenyl, substituted or unsubstituted acyl (2-11C), substituted or unsubstituted aryl (6-14C), substituted or unsubstituted arylcarbonyl (7-15C), substituted or unsubstituted arylalkyl (7-15C), substituted or unsubstituted pyridyl wherein substituents on any alkyl, alkenyl, or acyl moiety are selected from the group consisting of halo, and RO, wherein R is H or alkyl (1-6C), and substituents on any aryl or pyridyl moiety are selected from the group consisting of halo, and RO, where R is H or alkyl (1-6C), —CN and —CF3; with the proviso that R1 is not H and R1 and R3 are not identical and with the proviso that in formula (1), the B ring may contain one double bond that is located between positions 2 and 3, and in formula (2), the B ring may contain one double bond that can be located between positions 2 and 3 or positions 3 and 4 are useful as antibacterial agents.