88 Indoles
Vilsmeier reactions in an intramolecular sense, at an indole -position, generate 3,4-dihydro--carbolines.
Even indoles carrying electron-withdrawing (and therefore deactivating) groups on nitrogen will undergo
-electrophilic substitution, the 3-nitration of 1-phenylsulfonylindole being an example. Actually, it can be of benefi t
to stability if an indole is N-substituted, with a temporary protecting group if required – this applies particularly to
halo-indoles – so the product of bromination of 1-tri-isopropylsilylindole is more easily handled than 3-bromoindole
itself.
Alkylation of indoles can be achieved with , -unsaturated ketones or aldehydes via conjugate addition, usually using
acidic conditions. The second example shows how the alternative use of a chiral organo-catalyst can induce optical
activity in the product.