[Br:1][C:2]1(C)[CH:7]=[CH:6][C:5](Br)=[CH:4][CH:3]1[CH3:9].CN([CH:14]=[O:15])C.[CH2:16]([Li])CCC>C(OCC)C.CCCCCC>[Br:1][C:2]1[C:3]([CH3:9])=[CH:4][C:5]([CH:14]=[O:15])=[CH:6][C:7]=1[CH3:16]A solution of 2,5-dibromoxylene (8.0 g, 30.3 mmol) in diethyl ether (120 mL) is cooled to −78° C. and then treated with n-buthyllithium (20 mL, 1.6 M in hexane). After stirring for 40 min, DMF (6 mL) is slowly added. The mixture is warmed to rt and stirred for 1 h. The mixture is cooled again to −78° C. before another portion of n-butyllithium (5 mL) is added. The reaction mixture is allowed to warm to rt and stirred for another hour. The reaction is quenched by adding 5% aq. HCl. The mixture is extracted with EA, and the extract is concentrated in vacuo. The crude product is purified by CC on silica gel eluting with heptane:EA 5:1 to give 4-bromo-3,5-dimethyl-benzaldehyde (8.2 g) as a white soft solid.
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