2,5-Dimethylaniline → 2,5,8-Trimethylquinoline

In 50 ml of 6N hydrochloric acid, 10.0 g of 2,5-dimethylaniline were dissolved. To the resulting solution, 6.8 ml of crotonaldehyde were added dropwise under reflux. After the completion of the dropwise addition, the reaction mixture was heated under reflux for further one hour and the reaction was terminated. After the reaction mixture was allowed to cool down to room temperature, 5N sodium hydroxide was added to neutralize the reaction mixture, followed by extraction with ethyl acetate (200 ml×2). The combined organic layers were washed with saturated saline and dried over anhydrous magnesium sulfate. After filtration, the filtrate was concentrated and the product so obtained was purified by chromatography on a silica gel column, whereby 5.8 g of 2,5,8-trimethylquinoline were obtained. In 30 ml of ethanol were dissolved 3.6 g of 2,5,8-trimethylquinoline and 2.8 g of selenium dioxide, followed by heating under reflux for 6 hours. The residue obtained by distilling off ethanol was subjected to chromatography on a silica gel column, whereby 2.3 g of the product were obtained. Since the product was proved to be a mixture of the raw material, aldehyde and a diethylacetal derivative of the aldehyde from the result of 1H-NMR, it was treated with 6N-hydrochloric acid (10 ml)-THF (20 ml) and then, extracted with ethyl acetate (100 ml×2). The combined organic layers were washed with a saturated aqueous solution of sodium bicarbonate and dried over anhydrous magnesium sulfate, followed by filtration. The filtrate so obtained was concentrated and the residue was purified by chromatography on a silica gel column, whereby 1.9 g of the title compound were obtained.