Photocyclisation of 2-aroylquinoxalines; formation of coloured indolo[1,2-a]quinoxalines

Article abstract of DOI:10.1016/S0040-4020(01)96016-8

Irradiation of 2-benzoylquinoxaline (10) and 2-benzoyl-3-methylquinoxaline (11) in the presence of trifluoroacetic acid resulted in cyclisation to the orange-red 7-hydroxyindolo-[1,2-a]quinoxalin-5-ium trifluoroacetate (16) and the 6-methyl homologue 17 respectively. Similarly, irradiation of quinoxaline 11, 2-benzoyl-3-benzylquinoxaline (12), 2-(2,4-dimethylbenzoyl)-3-methylquinoxaline (23), and 2-benzoyl-3,6,7-trimethylquinoxaline (24) in methanol, or in ether, containing p-toluenesulphonic acid gave orange-red salts; 6-methyl-(18), 6-benzyl- (19), 6,8,10-trimethyl- (26), and 2,3,6-trimethyl-7-hydroxyindolo[1,2-a]-quinoxalin-5-ium p-toluenesulphonate (27) respectively. The purple photoproduct, 6-methyl- indolo[1,2-a]quinoxalin-7(5H)-one (15) was formed on irradiation of quinoxaline 11 in methanol. The 7-hydroxyindolo[1,2-a]quinoxalinium salts 16, 18, 19 and 27 formed yellow 7-acetoxyindolo[1,2-a]quinoxalines 20,21,22, and 29 respectively, and quinoxalinium salt 26 was converted into 6,8,10-trimethyl-7-propionoxyindolo[1,2-a]quinoxaline (28). In aerated methanolic sodium methoxide, the purple photoproduct 15 was converted into 1-(2-carboxyphenyl)-3-methylquinoxalin-2(1H)-one (34).