Synthesis of highly substituted benzo-fused nitrogen heterocycles via tandem benzannulation/cyclization strategies

Abstract

Benzannulations employing ynamides and vinylketenes (generated in situ from [alpha]-diazo ketones) were investigated. Irradiation of the diazo ketones using a batch or continuous-flow reactor leads to the formation of vinylketenes via a photo-Wolff rearrangement. The vinylketenes then react with ynamides via a pericyclic cascade process to produce highly substituted aniline derivatives. Using this vinylketene-based benzannulation, tandem strategies for the synthesis of highly substituted benzo-fused nitrogen heterocycles were investigated. A tandem benzannulation-iodocyclization method for the synthesis of polysubstituted quinolines was established. In addition, a tandem strategy for the synthesis of carbazoles was developed and applied in the total synthesis of the carbazole alkaloid carazostatin as well as formal syntheses of the alkaloids carbazoquinocin C and antiostatin A₄.