[4 + 2] Annulation of 3‑Nitroindoles with Alkylidene Malononitriles: Entry to Substituted Carbazol-4-amine Derivatives

A general and transition-metal-free
method for the construction
of the carbazol-4-amine motif via a vinylogous Michael addition/cyclization/isomerization/elimination
reaction of 3-nitroindoles with alkylidene malononitriles has been
developed. This novel methodology allows the facile synthesis of a
series of di- and trisubstituted carbazol-4-amine derivatives in moderate
to good yields. A gram-scale experiment was successfully performed,
highlighting the practicability of this method. Moreover, this strategy
is also applicable to 3-nitrobenzothiophene, affording the corresponding
dibenzo­[b,d]­thiophen-1-amine derivatives
in moderate yields.