Part IV: design, synthesis and antitumor activity of fluoroquinolone C-3 heterocycles: bis-oxadiazole methylsulfide derivatives derived from ciprofloxacin

To explore an efficient strategy for further development of anticancer fluoroquinolone candidates derived from ciprofloxacin, a heterocyclic ring as the bioisosteric replacement of C3 carboxyl group led to a key intermediate, oxadiazole thiol (5), which was further modified to the bis-oxadiazole methylsulfides (7a–7h) and the corresponding dimethylpiperazinium iodides (8a–8h), respectively. Structures were characterized by elemental analysis and spectra data, and their anticancer activities in vitro against CHO, HL60 and L1210 cancer cells were also evaluated by MTT assay.  The preliminary results show that piperazinium compounds (8) possess more potent activity than that of corresponding free bases (7).