Synthesis
The general procedure for the synthesis of 4-methyl-3,4-dihydro-2H-1,4-benzoxazine-7-carbaldehyde from N,N-dimethylformamide and 7-bromo-4-methyl-3,4-dihydro-2H-1,4-benzoxazine was as follows: n-butyllithium (5.79 mL, 14.47 mmol) was slowly added to a stirring 7-bromo-4-methyl-3,4 -dihydro-2H-1,4-benzoxazine (Int-81a, 3 g, 13.15 mmol) in a solution of tetrahydrofuran (24 mL). After keeping stirring at -78 °C for 1 h, N,N-dimethylformamide (2.037 mL, 26.3 mmol) was slowly added dropwise, followed by allowing the reaction mixture to gradually warm up to room temperature over a period of 2 hours. Upon completion of the reaction, the reaction was quenched with aqueous ammonium chloride and the product was extracted with ethyl acetate. The organic phases were combined, washed with brine, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to afford the target product Int-81b (4-methyl-3,4-dihydro-2H-1,4-benzoxazine-7-carbaldehyde, 2.32 g, 13.09 mmol, 100% yield) as a green solid.
