General Methods for Carbazole Construction

From the initial discovery till today, biologically active carbazole alkaloids isolated from nature may have quite simple but in some cases also structurally complex substitution patterns. Therefore, a large number of classical and non-classical methods has been developed for the synthesis of the carbazole framework. 

Synthesis of Carbazoles by Deoxygenation of Nitrobiphenyls 1 Watermann and Vivian s Method 

An important route to the carbazoles is the thermal or photolytic decomposition of orf/zo-azidobiphenyls. Of the two procedures, thermolysis proceeds in higher yields. One advantage of photolysis is that it proceeds at room temperature and would therefore be compatible with thermally labile functional groups. 

Among the two cyclization procedures that utilize 2,2 -diaminobiphenyls, the Tauber synthesis has been most widely used and proceeds in higher yields than the diazotization reaction. The application of this method to the synthesis of carbazoles is limited by the availability of 2,2 -diaminobiphenyls. 

A large rtumber of carbazole syntheses involve the preparation and dehydrogenation of hydrocarbazoles, mainly 1,2,3,4-tetrahydrocarbazoles. They are usually prepared by either the Fischer-Borsche synthesis or the Japp-Klingemann reaction. The most commonly used dehydrogenating agents are palladium on charcoal or chloranil (495,496). 

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