Synthesis of diarylazolo[a]pyridines from [(Z)-2,4-diaryl-4-oxo-2-butenyl]azolium salts

Abstract

Some examples of azolo[a]pyridine system construction are known in which heterocyclization based on azoles unsubstituted at position 2 is used as a method of annelation of the pyridine ring [1][2][3][4]. Only one of these [2] has a general nature and was extended to different azoles. We have previously [5, 6] found a suitable method for the synthesis 1-R-6,8-diaryl-1H-imidazo[1,2-a]pyridin-4-ium (1) and 5-R-2,4-diaryl-5H-pyrido[1,2-a]benzimidazol-1-ium bromides (2) which include base initiated cyclization of (Z)-3-R-1-[2,4-diaryl-4-oxo-2-butenyl]-3H-benz-imidazol-1-ium and (Z)-1-R-3-[2,4-diaryl-4-oxo-2-butenyl]-1H-imidazol-3-ium quaternary salts. The latter were obtained by alkylation of 1-alkyl-1H-imidazoles and -benzimidazoles with derivatives of 4-bromo-1,3-diphenyl-2-buten-1-one ( -bromodypnone) 3a-d. In this work we extend the range of azoles used in these conversion schemes.It was shown earlier [6] that the structure of the azole the stage of the formation of the quaternary salts preeminently affected the effectivness of the conversion of the azoles into the polycyclic systems 1 and 2. For example the rates of alkylation and the yields of reaction products decreased on going from 1-alkyl-1H-imidazoles (80-90%, 1-4 h) to 1-alkyl-1H-benzimidazoles (60-80%, 6-12 h). On introducing electron-acceptor substituents into the imidazole ring a still larger decrease in rate and yield of reaction products was observed. Thus on subjecting solutions of 5-chloro-1-methyl-1H-imidazole with derivatives of -bromodypnone 3a-c in benzene at room temperature [6] (Z)-3-[2,4-bis(aryl)-4-oxo-2-buten-yl]-5-chloro-1-methyl-1H-imidazol-3-ium bromides 4a-c