Scheme 4a.
aReagents and conditions: (i) isobutanol, H2SO4, 120 °C, reflux, 16 h, dark, 79%; (ii) tert-butyl hydroxybutanoate, Ph3P, DIAD, THF, 25 °C, 4 h, dark; (iii) TFA, CH2Cl2, 25 °C, 3 h, dark, 85% over 2 steps; (iv) 2’-TBDMS-paclitaxel, DIC, DMAP, CH2Cl2/DMF, 25 °C, 24 h, dark, 70%; (v) HF/pyridine, CH3CN/pyridine; 0 °C to 25°C, 48 h, dark, 87%.