A solution of the product of Step 5 (1.4 g, 3.5 mmol) in water (70 ml) and 2N hydrochloric acid (5.25 ml, 10.5 mmol) was treated with sodium tungstate dihydrate (100 mg, 0.36 mmol) and 30% hydrogen peroxide solution (2.4 g, 21 mmol) and stirred at room temperature for 18 hours. The reaction mixture was poured into aqueous sodium thiosulphate solution, basified with saturated aqueous sodium hydrogen carbonate solution, extracted into dichloromethane, dried (MgSO4) and evaporated in vacuo to afford the title compound (0.70 g, 46%); MS(ES+) m/e 432 [M+H]+.
