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Compound 5c (680 mg, 2.20 mmol) and carbon tetrabromide (947 mg, 2.86 mg) were dissolved in dichloromethane (20 ml). The solution was cooled down to 0° C. with an ice bath and pyridine (2.0 ml) was added followed by triphenylphosphine (749 mg, 0.286 mmol). The solution was stirred at 0° C. for half an hour and at room temperature for 2 hours. After standard workup with dichloromethane, the residue was purified by silica gel column chromatography (20% ethyl acetate in hexane) to afford compound 6c (680 mg, 79.6%): 1H NMR δ 3.79 (m, 4H), 3.66 (m, 8H), 3.56 (t, 2H), 3.47 (t, 2H), 0.89 (s, 9H), 0.07 (s, 6H).

1. customAfter standard workup with dichloromethane, the residue was purified by silica gel column chromatography (20% ethyl acetate in hexane)