Benzo-b-Carbazoles and Carbazoloquinones

Methods for the synthesis of tetracyclic benzocarbazoles and benzo-b-carbazoloquinone have become relevant to alkaloidal synthesis due to the progress in the chemistry of the kinamycin group antibiotics. 

Indole-2,3-quinodimethane methodology has been utilized in the synthesis of the above groups of carbazoles 108). The N-sulphophenyl indole-2,3-quinodimethane anologue (136) undergoes the Diels-Alder reaction with benzyne generated from (137). In three steps (138-138B) it furnishes the simple benzo-b-carbazole (139). 

On the other hand a benzocarbazoloquinone (141) has recently been synthesised through oxidative cyclisation of a 2-benzoylindole containing benzylic alcohol group in the ortho position (140) with tetra-n-butyl-ammonium perruthenate (TBAP) (57). 

Since the hexacyclic and pentacyclic bases synthesised so far are cyclomers of C23-alkaloids of the mahanimbine group, their syntheses are discussed together with syntheses of tetracyclic alkaloids of C23 skeleton. 

Syntheses of nor-mahanimbine and its regioisomers have been effected (65) by reacting 2-hydroxycarbazole (142) with citral (148) in boiling pyridine showing that the reaction is regioselective, not regiospecific. 

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