Indoles Cadogan-Sundberg synthesis

The Cadogan-Sundberg indole synthesis, often known as the Cadogan reaction, refers to the deoxygenation of o-nitrostyrenes or o-nitrostilbenes... 

Thankfully, some of the conventional and modem indole synthesis methods do apply to make azaindoles. They include the Larock reaction, Mori-Ban indole synthesis, Bartoli indole synthesis, Batcho-Leimgmber indole synthesis, and Cadogan-Sundberg indole synthesis. 

This reaction can be traced back to the initial work of Cadogan for the preparation of car-bazole by the reductive cyclization of aromatic nitro compounds with triethyl phosphite. This reaction was then extended by Sundberg to the fornnation of indoles by the treatment of o-nitrostyrene with triethyl phosphite or the formation of indolines from the o-alkyl nitrobenzenes. Thus the formation of indole from the reduction of o-nitfostyrenes is referred to as the Cadogan-Sundberg indole synthesis." ... 

The Cadogan-Sundberg indole synthesis involves the insertion of phosphorus atom to the N=0 double bond of the nitro group followed by the cleavage of triethyl phosphate to give IV-hydroxyl indole. Subsequently, the IV-hydroxyl indole was reduced by a second triethyl phosphite to afford indole as shown here. 

The reductive cyclization of 6>-nitrostryene (i.e., the Cadogan-Sundberg indole synthesis) has been improved by the transition metal catalyzation, selenium-based reductive cyclization, or hydrogen/Raney nickel reduced cyclizationJ... 

Other references related to the Cadogan-Sundberg indole synthesis are cited in the... 

Table 1 Early Applications of the Cadogan-Sundberg Indole Synthesis...

Scheme 8 The Cadogan-Sundberg Indole Synthesis Applied to Carbazoles...

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