Indoles 97Synthesis of isatins from anilines (1,2- and 3,3a-bonds made)
Isatins can be readily prepared via the reaction of an aniline with chloral, the resulting product being converted into an
oxime, which is then cyclised in strong acid.
Synthesis of oxindoles from anilines (1,2- and 3,3a-bonds made)
The main synthesis of oxindoles is simple and direct and involves an intramolecular Friedel–Crafts alkylation reaction
as the cyclising step.
Synthesis of indoxyls from anthranilic acids (1,2- and 2,3-bonds made)
Indoxyls are normally prepared from anthranilic acids; fi rst, the nitrogen is alkylated with a 2-haloacetic acid and then
cyclisation is achieved with loss of carbon dioxide; the resulting acetate of the indoxyl is easily hydrolysed.
Azaindoles
The (mono) azaindoles (more correctly pyrrolopyridines), where one of the carbons of the six-membered ring of
indole has been notionally replaced by nitrogen, are of chemical interest as typical bicyclic systems comprising an
electron-rich fused to an electron-poor system. Although they do not occur in nature, they have a number of potential
applications, including in medicinal chemistry, as isosteres or substitutes for various indole-based moieties. The pyrrole
enamine system and the pyridine imine system can interact mesomerically in all four isomers.