Aromatization of 5,5-disubstituted 2-imidazolines

Method A. The imidazoline (2 mmol) is heated at 130°C in 50% aqueous H2SO4 (10 ml) for 24 h, ITie cooled solution is diluted with water (50 ml), and the resultant precipitate is filtered and then crystallized from ethanol. 

Method B. The imidazoline (2mmol) is refluxed (15h) in 1,1,2-trichloroethane (20ml) with triethylamine hydrochloride (2mmol). After cooling to room temperature the solvent is rotary evaporated, and the crude residue is washed with water and extracted twice with chloroform. Ihe organic extracts are dried (Na2S04) and evaporated. The residue is crystallized from ethanol. 

Prepared in these ways are 5-methyl-1,2,4-triphenyl- (85% (method A) 65% (method B)), 5-methyl-2,4-diphenyl-l-p-tolyl- (70% (method A) 90% (method B)), 5-methyl-2,4-diphenyl-l-p-fluorophenyl- (60% (method B)) and 1,2,4,5-tetraphenylimidazoles (90% (method A) 85% (method B)). 

To a mixture of absolute ethanol (800 ml) and dry toluene (175 ml) is added 2-bromoacetoacetaldehyde (41.2 g, 0.25 mol). This mixture is refluxed (2.5 h) while solvent is gradually removed, then after cooling, the remaining solution is concentrated in vacuo to an oil (yield quoted as virtually quantitative by NMR). 

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