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Compound 156 (3.8 g, 9.6 mmol) was dissolved in DMSO (115 mL). The solution was cooled on an ice bath and 48% aqueous HBr (32 mL, 285 mmol) was added slowly. The solution was stirred in a closed vessel at room temperature for 5 days. The reaction mixture was cooled on an ice bath and 27% aqueous NaOH (30 mL) was added to give a basic solution. The water phase was extracted three times with EtOAc, the combined organic phases were washed with brine, dried (MgSO4), filtered and evaporated in vacuo. The crude product was purified by flash chromatography using EtOAc/petroleum ether 1:1 as the eluent to afford the title compound as a slightly coloured crystalline compound.

1. temperatureThe solution was cooled on an ice bath
2. temperatureThe reaction mixture was cooled on an ice bath
3. customto give a basic solution
4. extractionThe water phase was extracted three times with EtOAc
5. washthe combined organic phases were washed with brine
6. dry with materialdried (MgSO4)
7. filtrationfiltered
8. customevaporated in vacuo
9. customThe crude product was purified by flash chromatography