To a suspension of sodium hydride (452 mg, 13.4 mmol, 60% in mineral oil) in DMF (25 ml) under nitrogen was added dropwise a solution of 6 (3.7 g, 10.35 mmol) in DMF (50 ml) and the mixture was stirred at 60° C. for 1 h. The solution was then cooled to room temperature and to this mixture was added a solution of tert-butyl bromoacetate (2.62 g, 13.4 mmol) in DMF (25 ml). The mixture was stirred at 60° C. for 3 h. The DMF was removed under high vacuum and the crude product was suspended in water (100 ml) and the mixture was extracted by ethyl acetate (2×100 ml). The combined ethyl acetate phases were washed by water (1×50 ml), brine (1×50 ml), dried over sodium sulphate filtered and concentrated to dryness. The crude product obtained was purified by chromatography on silica gel using hexane/ethyl acetate (9/1) to give the title compound 7 (3.5 g, 72%).
