The synthesis of pyrido [4,3-b]carbazoles from diphenylamine derivatives: alternative routes to and relay syntheses of ellipticines and olivacines

Abstract

Two new synthetic routes to pyrido [4,3-b]carbazoles are described. In the first, Goldberg-type coupling of various aryl sulfonamides with aryl bromides in the presence of copper and potassium carbonate gives N,N-diaryl sulfonamides. UV irradiation of these, in ethanol, removes the toluene-p-sulfonyl protecting groups and cyclises the diphenylamine moiety to the corresponding carbazoles. These carbazoles are established intermediates in the synthesis of several ellipticines (5,11 -dimethyl-pyrido[4,3-b]carbazoles).

In a complementary route, a series of substituted acetanilides are similarly coupled under Goldberg conditions with 2-bromo-5-cyanotoluene to give the corresponding cyanodiphenylamines and diphenylamides. Hydrolysis of the latter gives the diphenylamines which are then oxidatively cyclised to 3-cyano-1 -methylcarbazoles. Reduction of the cyanocarbazoles leads to 3-formylcarbazoles which are known intermediates for the synthesis of 5-methylpyrido[4,3-b]carbazoles.