Photoinduced cyclization of 3-acyl-2-halo-1-[(ω-phenylethynyl)alkyl] indoles to azaheterocyclo[1,2,3- lm ]-fused benzo[ c ]carbazoles

A one-pot synthesis of azaheterocyclo[1,2,3-lm]-fused benzo[c]carbazoles (2 and 3) has been developed by photocyclization of 3-acyl-2-halo-1-[(ω- phenylethynyl)alkyl] indoles (1) in good to excellent yields. All products are formed from 1 via two sequential photocyclization reactions. Two products, 9-chloro-7,8-dihydro-6H-benzo[c]pyrido[1,2,3-lm]carbazole (2-h) and 7,8-dihydro-6H-benzo[c]pyrido[1,2,3-lm]carbazole (3-h), are produced in the photocyclization of 2-halo-1-[(ω-phenylethynyl)alkyl]indole-3- carbaldehydes (1-h). In contrast, only products 2-h are produced in the photocyclization of 3-acetyl-2-chloro-1-[(ω-phenylethynyl)alkyl]indole-3- carbaldehydes (1o - t). The 9-H in 3-h (n = 2) does originate from the formyl group in 1-h via 1,5-hydrogen shift. The structures of three new products, 9-bromo-7,8-dihydro-6H-benzo[c]pyrido[1,2,3-lm]carbazole (2b), 9-chloro-10-methyl-7,8-dihydro-6H-benzo[c]pyrido[1,2,3-lm]carbazole (3h) and 12-chloro-7,8-dihydro-6H-benzo[c]pyrido[1,2,3-lm]carbazole (2w), have been corroborated by single-crystal X-ray structural analyses. © 2011 American Chemical Society.