A suspension of Intermediate 77 (0.65 g, 1.0 mmol) and carbon tetrabromide (1.0 g, 3.0 mmol) in dichloromethane (14 mL) was sonicated for 5 minutes and then cooled in an ice-water bath. Following the addition, via syringe, of a triphenylphosphine (0.52 g, 2.0 mmol) solution in tetrahydrofuran, the cooling bath was removed, allowing the mixture to stir overnight at room temperature. The mixture was quenched with ethanol and concentrated to foam under reduced pressure. The foam was taken up in tetrahydrofuran (15 mL) and treated as before with carbon tetrabromide (3.0 mmol) and triphenylphosphine (2.0 mmol). After stirring overnight at room temperature, the mixture was concentrated under reduced pressure. The crude material was again taken up in tetrahydrofuran (15 mL) and treated as before with carbon tetrabromide (3.0 mmol) and triphenylphosphine (2.0 mmol). After stirring overnight at room temperature, the mixture was concentrated under reduced pressure. The residue was taken up a suspension in acetone and filtered. The concentrated filtrate was purified via automated flash silica gel chromatography, using a 40 g Silicycle SiliSep flash column (hexanes/ethyl acetate). Concentration of the desired fractions under reduced pressure provided the title compound as straw-colored foam (0.35 g, 48%). ES/MS calcd. for C36H32BrN4O5S+ 711.1. Found m/z=711.2 (M+H)+.
