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Background   α -Bromination of the side chain of aromatic ketones using NBS in the presence of  p -toluenesulfonic acid ( p -TsOH) in acetonitrile is very common. However, regioselective bromination of bis and tris( ω  - bromoacetophenones) with NBS in the presence of  p -TsOH in acetonitrile under microwave irradiation is quite novel. The bis- and tris( ω  - bromoacetophenones) are used in synthesis of bis and tris(heterocycles). bis(heterocycles) have received a great deal of attention, because many biologically active natural and synthetic products have molecular symmetry. The use of the pressurized microwave irradiation is very advantageous to many syntheses and provide a large rate enhancement.    Results  Bis and tris( ω  - bromoacetophenones) were obtained as single monobrominated derivatives in a shorter time than the conventional conditions. The results clearly demonstrate the better reactivity and selectivity of NBS/ p -TsOH/CH 3 CN as a brominating mixture under microwave conditions. The reaction of bis and tris( ω -bromoacetophenone) with 2-aminopyridine and 2-aminopyrimidine proceeded smoothly in a mixture of anhydrous ethanol and DMF under reflux or using 300 W/105°C/ 20 min microwave irradiation conditions to afford the corresponding bis(imidazo[1,2- a ]pyridine), bis(imidazo[1,2- a ]pyrimidine) and tris(imidazo[1,2- a ]pyridine) derivatives in moderate to excellent yields. The carbonyl analogue of the targeted bis(imidazopyridines) could be synthesized by the reaction of  N , N -dimethyl- N' -(pyridin-2-yl)formimidamide with bis( ω -bromoacetophenone) in refluxing ethanol. The structures of the newly synthesized compounds were confirmed by their spectral data as well as their elemental analyses.    Conclusion  In conclusion, selective  α -bromination of bis- and tris(acetophenones) has been accomplished efficiently utilizing NBS/ p -TsOH/CH 3 CN under microwave irradiation. In addition, a facile synthesis of novel series of bis- and tris(imidazopyridine) and bis(imidazopyrimidine) derivatives.

chemistry central journal

Microwave assisted synthesis of bis and tris(ω-bromoacetophenones): versatile precursors for novel bis(imidazo[1,2-a]pyridines), bis(imidazo[1,2-a]pyrimidines) and their tris-analogs