Synthesis of a novel polycyclic ring scaffold with antimitotic properties via a selective domino Heck-Suzuki reaction

Abstract

The synthesis of a previously undescribed sp³-rich 6-5-5-6 tetracyclic ring scaffold using a palladium catalysed domino Heck-Suzuki reaction is reported. This reaction is high-yielding, selective for the domino process over the direct Suzuki reaction and tolerant towards a variety of boronic acids. The novel scaffold can also be accessed via domino Heck-Stille and radical cyclisations. Compounds based around this scaffold were found to be effective antimitotic agents in a human cancer cell line. Detailed phenotypic profiling showed that the compounds affected the congression of chromosomes to give mitotic arrest and apoptotic cell death. Thus, a novel structural class of antimitotic agents that does not disrupt the tubulin network has been identified.

Scheme 1. Synthesis of novel 6-5-5-6 tetracyclic ring scaffold 4.

Scheme 2. Domino Heck–Suzuki reactions of boronic acids 2a–m with bicyclo[3.2.1]octadiene 1 to form compounds 4a–m. MW = microwave irradiation. 2 equivalents of boronic coupling partner (2a–m) were used in all reactions. Yields of isolated products given in all cases. X-ray crystal structure of compound 4d shown.

Scheme 3. Proposed reaction mechanism pathways for the domino Heck–Suzuki process (black) and the direct Suzuki reaction (competing reaction pathway shown in blue).

Scheme 4. Alternative domino Heck–Stille and radical cyclisations. TFP = tri-(2-furyl)phosphine.

Fig. 1. Antimitotic and growth inhibitory effects of fused ring analogues 4: (a) mitotic arrest induced by 4b–d, and 4h; U2OS cells were incubated with compounds for 20 h before being fixed and stained with Hoechst (a DNA dye) and a pH3 specific antibody. Cells were imaged and analysed on a Cellomics Arrayscan. Data shown as the mean ± sem for an experiment conducted in triplicate. CPD = compound; (b) Growth inhibition of U2OS cells by 4b–d, and 4h. U2OS cells were incubated with compounds for 72 h before fixing and staining with sulforhodamine B. Absorbance was then measured at 510 nm. Data is shown as the mean ± sem for an experiment conducted in triplicate.

Fig. 2. Induction of apoptosis by 4d. U2OS cells were incubated with 4d for 72 h before fixing and staining with Hoechst and a cleaved PARP specific antibody. Cells were imaged and analysed on a Cellomics Arrayscan. Data shown as the mean ± sem for an experiment conducted in triplicate.

Fig. 3. Analysis of mitotic phenotype of U2OS cells with 20 μM 4d shows chromosome congression defects. Cells were treated with 4d for 20 h before being fixed and stained with DAPI (blue), α-tubulin (green) and pericentrin (red); scale bar = 6 μm.

Fig. 4. Analysis of interphase U2OS cells treated with 20 μM 4d or DMSO (control) shows no difference in the tubulin network. Cells treated with 5 μM (S)-Dosabulin show a clear disruption in the tubulin network. Cells were treated with 4d or (S)-Dosabulin for 20 h before being fixed and stained with DAPI (blue), α-tubulin (red) and pericentrin (green). Scale bar = 10 μm.