Generation and rearrangement of 4aH-carbazoles

Abstract

The benzotriazoles (12a–c) bearing ortho-blocked 1-aryl substituents have been decomposed thermally by flash vacuum pyrolysis, and photochemically by irradiation in acetonitrile at 254 nm. The pyrolyses are complex giving mainly mono- and di-methylcarbazoles from (12a), di- and trimethylcarbazoles from (12b), and a tetra- and a penta-methylcarbazole from (12c). All of these products could be formed by generation of 4aH-carbazole intermediates (24) with subsequent thermally-allowed sigmatropic rearrangements (Scheme 6). The photolyses give a different set of products consisting mainly of the cyclopentaquinolines (27a–c) from (12a–c), respectively. These products could be formed from the same 4aH-carbazoles (24) which now undergo photochemical, aza-di-π-methane, rearrangements to give intermediates (29) and hence products (27)(Scheme 7). The 4a-methyl intermediate (29c) is intercepted as the adduct (30) in an extended cycloaddition reaction with acrylonitrile. Key reaction products were synthesized by independent methods.