An Example of Regioselective Fischer Indole Annulation

Bicyclo[n.4.0]alkan-3-ones (n = 3, 4, 5) with a trans bridge-head configuration give linear annulated tetrahydro­carbazole derivatives in Fischer indole synthesis. The structure of two tetrahydro­carbazoles was confirmed by X-ray crystal analysis. The hexa­hydroindanone derivative with a cis bridge-head configuration gives, contrastingly, the angular annulation product under the same reaction conditions.