Synthesis of Carbazoles by a Diverted Bischler–Napieralski Cascade Reaction

An unforeseen twist
in a seemingly trivial Bischler–Napieralski
reaction led to the selective formation of an unexpected carbazole
product. The reaction proved to be general, providing access to a
range of diversely substituted carbazoles from readily available substrates.
Judicious variation of substituents revealed a complex cascade mechanism
comprising no less than 10 elementary steps, that could be diverted
in multiple ways toward various other carbazole derivatives.