Novel synthesis of 3-fluoro- and 3,3-difluoro-substituted β-lactams: evaluation as potential antiproliferative and tubulin destabilizing agents

Combretastatin A-4 (CA-4), a natural stilbene is a potent microtubule-disrupting agent, which binds at the colchicine-binding site of tubulin. The design, synthesis and biochemical evaluation of a series of analogues of the microtubule-destabilising agent CA-4 is described. The monocyclic β-lactam CA-4 analogues containing halogen substituents at the C-3 position of β-lactam ring were synthesized using the Staudinger reaction. Previous investigations described two approaches for the construction of 3-fluoro-β-lactams using the ketene-imine condensation or the enolate-imine condensation method. In the present work, a ready synthesis of 3-fluoro- and 3,3-difluoro-substituted β-lactams was developed by a convenient microwave-assisted Reformatsky reaction using ethyl bromofluoroacetate and ethyl bromodifluoroacetate respectively. To the best to our knowledge, this is the first report of this new synthetic approach for 3-fluoro- and 3,3-difluoro-β-lactams as CA-4 analogues. The reaction was successful with short reaction time compared to the conventional Staudinger reaction, moderate yields and few steps. It is demonstrated to be a convenient and facile method for the preparation of a variety of 3-fluoro- and 3,3-difluoro-β-lactams. The 3-fluoro-β-lactams (1-8) and 3,3-difluoro-β-lactams (9-16) in this series contain the 3,4,5-trimethoxyphenyl ring A, (required for potent activity of CA-4), together with various ring B substituents. The structure of β-lactams 6 and 14 was confirmed by X-ray crystallography. Preliminary cell viability studies demonstrated the antiproliferative effects of these novel compounds, with IC₅₀ value of 0.153 µM for 6 in MCF-7 human breast cancer cell line.