Microtubule-targeted drugs are essential chemotherapeutic agents for various types of cancer. We have previously reported the synthesis of 3-vinyl-β-lactams (2-azetidinones) with potent antiproliferative activity against breast cancer MCF-7 cells ^[1]. As a continuation of our research work on tubulin polymerization inhibitors, we now present the synthesis and biochemical evaluation of a series of novel 3-isopropenyl-β-lactams (2-azetidinones) that are structurally related to the colchicine binding site tubulin inhibitor and vascular targeting agent, Combretastatin A-4. The 3-isopropenyl-β-lactams in this series contain 3,4,5-trimethoxyphenyl ring A, (required for CA-4), together with selected ring B substituents ^[2]. These compounds showed potent activity against breast cancer in MCF-7 and MDA-MB-231 cells and are minimally toxic to non-tumorigenic HEK-293T cells. Moreover, the compounds significantly arrested cell division during the G2/M phase and induced apoptosis in the MCF-7 cell line. Immunofluorescence studies in MCF-7 cells showed that the 3-isopropenyl-β-lactam caused mitotic catastrophe by targeting tubulin and inhibited tubulin polymerization. In conclusion, the 3-isopropenyl-2-azetidinones could be promising lead compounds for the development of anti-breast cancer drugs that target tubulin in future clinical trials.