Thermolysis of 3-(Carbazol-3-yl)-2-azidopropenoates

Thermal cyclisation of 2-azido-3-(carbazol-3-yl)propenoic esters produces the 1,6-dihydropyrrolo[3,2-c]carbazole and not the 1,9-dihydropyrrolo[2,3-b]carbazole whereas, in the case of the 4-methylcarbazolyl derivative, the 1,9-dihydropyrrolo[2,3-b]-carbazole is formed in preference to the alternative pyrido[4,3-c]carbazole