Sundberg indole preparation

The previously unknown 2-nitroindoles have been conveniently prepared from o-nitroben-zaldehyde via the Sundberg indole synthesis fEq 10 61 ... 

The Sundberg indole synthesis using aromatic azides as precursors of nitrenes has been used in synthesis of various indoles. Some kinds of aryl azides are readily prepared by SNAr reaction of aromatic nitro compounds with an azide ion. For example, 2,4,6-trinitrotoluene (TNT) can be converted into 2-aryl-4,6-dinitroindole, as shown in Eq. 10.60.83... 

An indole alkaloid synthesis employing a bona fide intramolecular Heck reaction was documented in Sundberg s preparation of 5,6-homoiboga derivatives [90]. Several attempts to construct S,6-homoiboga derivative 191 using inter- or intramolecular Heck reaction conditions with phosphine ligands led to poor yields. Application of Jeffery s ligand-free" phase-transfer... 

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." ... 

Li and colleagues took advantage of the Cadogan-Sundberg indole synthesis to prepare botulinum neurotoxin A light chain (BoNT/A LC)... 

Although the cyclization of nitrenes to indoles via ortho-azidostyrenes justifiably is associated with Sundberg, several earlier workers described and studied this reaction. Notably, the researches of Smith [1-5], SmoUnsky [6-8], Abramovitch [9], Coffin and Robbins [10], and Isomura colleagues [11] describe the pyrolysis of ort/io-azidobiphenyls to carbazoles and, in a few cases, ort/io-azidostyrenes to indoles. A sampling of these pre-Sundberg reactions from the work of Smith and coworkers is shown in Scheme 1. The typical solvent used was kerosene, and the conversion of azides to carbazoles was also accomplished photochemically, albeit in lower yield. Other carbazoles prepared thermally were 1-4 in excellent yields. These workers also synthesized 4H-thieno[3,2-ii]indole (5) from 2-(o-azidophenyl)thiophene [3]. 

The Classical Cadogan-Sundberg Conditions to Prepare Indole 52... 

As the first application of Heck reaction in making heterocyclic compounds — namely indole derivatives, the Mori-Ban reaction has been widely used in variety synthesis of indoles. In a program to synthesize CC-1065 analogs, Sundberg prepared indole 19 from o-bromo-JV-allylaniline 18 in an excellent yield using the Jeffrey s conditions. Silver carbonate and sodium carbonate were less effective than triethylamine. 

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