Fischer Indolization and Its Related Compounds. XVI. Vilsmeier-Haack Reaction of N-Methyl-1, 2, 3, 4-tetrahydrocarbazole Derivatives

Vilsmeier-Haack reaction of 6-chloro-1, 2, 3, 4-tetrahydro-9-methylcarbazole (1b) with 3 mol eq. POCl3 at 70°gave three compounds, 6-chloro-1, 2, 3, 4-tetrahydro-9-methyl-carbazole-1, 1-dicarboxaldehyde (4), 6-chloro-3, 4-dihydro-9-methylcarbazole-1 (2H)-one (5b), and 6-chloro-1, 2, 3, 4-tetrahydro-1-hydroxy-9-methylcarbazole-1-carboxaldehyde (6), while no 4a-formylated product1) was obtained. On the other hand, the reaction of the same compound (1b) with 1.3 mol eq. POCl3 at 100°gave 6-chloro-1, 2, 3, 4-tetrahydro-9-methyl-carbazole-1-carboxaldehyde (20), 5b, 6-chloro-1, 2, 3, 4-tetrahydro-9-methylcarbazole-7-carboxaldehyde (3b), and an aromatized compound, 6-chloro-1, 9-dimethylcarbazole-3-carboxaldehyde (2b). The formation mechanism of the aromatized compound (2b) is considered to involve elimination of a dimethylamino group and formation of 6-chloro-1, 9-dimethylcarbazole (21) as an intermediate.