Under argon atmosphere, sodium carbonate (11.2 mg, 0.11 mmol), water (0.7 mL), and 6-bromo-3,4-dihydro-2H-1,4-benzoxazin-3-one (24.2 mg, 0.11 mmol) were added to a solution of ethyl 2-(tert-butoxy)-2-[2,5-dimethyl-4-(tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-3-yl]acetate (28f) (48 mg, 0.10 mmol) in N,N-dimethylformamide (2.1 mL). The solution was degassed under argon and palladium tetrakis(triphenylphosphine) (23.4 mg, 0.02 mmol) was added. The mixture was heated at 100° C. for 15 hours. The mixture was then cooled at room temperature, water was added and the aqueous layer was extracted with ethyl acetate. The organic layer was washed with brine, dried over sodium sulfate, filtered, and evaporated under reduced pressure. The residue was purified by preparative TLC (cyclohexane/ethyl acetate 70/30) to afford the desired product (35a) (14 mg, 0.03 mmol, 33%).
