Synthesis and Biological Activity of C-6 and C-7 Modified Paclitaxels

Structure modification of paclitaxel at the C-6 and C-7 positions has been achieved using the readily available intermediate 6α-hydroxy-7-epipaclitaxel (2). While a single diastereomer of the cyclic sulfite of 2 was prepared at lower temperature, both diastereomers were obtained at room temperature. The cyclic sulfate was found to be inert toward nucleophilic attack, which confirms the great steric hindrance of the β-face of the C-6 and C-7 region of paclitaxel. Derivatization at the 6β-position with a nitrogen-containing function was accomplished using the alternate substrate 6α-trifluoromethanesulfonate (9), with the 7-epi hydroxyl group protected to avoid the formation of C-ring rearranged product. Hydrogenation of the 6β-azide was difficult but could be achieved in moderate yield under high pressure. Similar to other C-6 and C-7 modified analogs reported earlier, these new compounds displayed comparable in vitro cytotoxicity to paclitaxel against the HCT116 human colon cancer cell line and the A2780 human breast cancer cell line.