The compound of step 3 (0.293 g, 0.73 mmol) was suspended in 4 ml of tert-butanol and cooled to 0° C. To this suspension were added a solution of sodium dihydrogenphosphate (0.263 g, 2.19 mmol) in 1 ml of water, a 2 M solution of 2-methyl-2-butene in THF (2.19 ml, 4.38 mmol) and sodium chlorite (0.124 g, 1.09 mmol) in small portions. After stirring at room temperature for 18 h, once more a solution of sodium dihydrogenphosphate (0.263 g, 2.19 mmol) in 2 ml water, a 2 M solution of 2-methyl-2-butene in THF (2.19 ml, 4.38 mmol) and sodium chlorite (0.124 g, 1.09 mmol) were added again and stirring was continued for an additional 48 h. Then 30 ml of a 1:1 mixture of brine and EA were added, the organic phase was separated, and the aqueous phase was extracted with EA. The combined organic phases were washed with brine, dried over sodium sulfate, filtered and evaporated to dryness under reduced pressure to give 0.420 g of the crude title compound as a yellow powder which was directly used in the next step.
