Organo NHC catalyzed aqueous synthesis of 4β-isoxazole-podophyllotoxins: anticancer, caspase activation, tubulin polymerization inhibition and molecular docking studies.

We present, for the first time, the organo--heterocyclic carbene (NHC) catalyzed 1,3-dipolar cycloaddition of 4β--propargyl podophyllotoxin (1) with aromatic nitrile oxides to afford regioselective 4β-isoxazolepodophyllotoxin hybrids (6a-n) in benign aqueous-organic media. Preliminary anticancer activity results showed that compound 6e displayed superior activity against MCF-7, HeLa and MIA PaCa2 human cell lines compared with podophyllotoxin. Compounds 6j and 6n showed greater activity against the MCF-7 cell line than the positive control. Caspase activation studies revealed that compound 6e at 20 μg ml concentration had greater caspase 3/7 activation in MCF-7 and MIAPaCa2 cells than podophyllotoxin. Furthermore, tubulin polymerization inhibition studies revealed that compound 6e showed comparable activity with podophyllotoxin. Finally, molecular docking studies of compounds 6e, 6j, 6n and podophyllotoxin on α,β-tubulin (pdb id 1SA0) revealed that compound 6n showed excellent binding energies and inhibition constants compared with podophyllotoxin.