[CH3:1][C@H:2]1[O:7][C@@H:6]([CH3:8])[CH2:5][N:4]([C:9]2[S:10][CH:11]=[C:12]([C:14]#[N:15])[N:13]=2)[CH2:3]1.CN(C=O)C.[Br:21]N1C(=O)CCC1=O.[OH-].[Na+]>O>[Br:21][C:11]1[S:10][C:9]([N:4]2[CH2:3][C@H:2]([CH3:1])[O:7][C@H:6]([CH3:8])[CH2:5]2)=[N:13][C:12]=1[C:14]#[N:15]A flask charged with Intermediate 2 (25.61 g, 114.69 mmol) and DMF (250 mL) was cooled in an ice/water bath. N-bromosuccinimide (24.49 g, 137.63 mmol) was added portionwise (slightly exothermic reaction). The reaction mixture was stirred at room temperature for 10 min. Water (400 mL) was then added dropwise leading to a yellow precipitate, followed by sodium hydroxide 1 M (115 mL). The precipitate was filtered, washed with water (50 mL) and DIPE (10 mL). This solid was recrystallised from acetonitrile. Filtration and overnight drying in a vacuum oven at 50° C. yielded 25.5 g (73%) of Intermediate 3 as a yellow solid.
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