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

Abstract magnified image 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°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°C. Application of this approach to the closure of five‐ and seven‐membered rings was also successful. J. Heterocyclic Chem., (2009).