1,2,3,9-Tetrahydro-4H-carbazol-4-one and 8,9-Dihydropyrido-[1,2-a]indol-6(7H)-one from 1H-Indole-2-butanoic Acid

Efficient syntheses of the title ring systems have been developed from 1H-indole-2-butanoic acid, which was easily prepared from 2-fluoro-1-nitrobenzene in four steps. Heating 1H-indole-2-butanoic acid in toluene containing p-toluenesulfonic acid at 110 degrees C furnished 1,2,3,9-tetrahydro-4H-carbazol-4-one in 88% yield. Heating this same acid in toluene with no added acid gave 8,9-dihydropyrido[1,2-a]-indol-6(7H)-one in 90% yield. The tetrahydro-4H-carbazol-4-one was also prepared directly in 92% yield from methyl 6-(2-nitrophenyl)-5-oxohexanoate by a tandem reduction-cycloaromatization-acylation reaction with iron in concentrated hydrochloric acid at 110 degrees C. Application of this approach to the closure of five- and seven-membered rings was also successful.