A cross-coupling/reductive cyclization protocol has been employed in a unified approach to all four carbolines. So, for example, the 2-nitropyridine 8, which is readily prepared through an efficient palladium-catalyzed Ullmann cross-coupling reaction, is reductively cyclized under conventional conditions to give 6,7,8,9-tetrahydro-α-carboline that is itself readily aromatized to give α-carboline (1).
The title natural products 2-7 have been prepared by reductive cyclization of the relevant 2-arylcyclohex-2-en-1-one (e.g. 20) to the corresponding tetrahydrocarbazole and dehydrogenation (aromatization) of this to give the target carbazole (e.g. 4). Compounds such as 20 were prepared using a palladium-catalyzed Ullmann cross-coupling reaction between the appropriate 2-iodocyclohex-2-en-1-one and o-halonitrobenzene.
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