The 3,4-fused tricyclic indole framework is a key structural motif in numerous bioactive natural products and has drawn considerable attention in organic synthesis. In this review, we highlight recent advances in synthetic strategies.
Unprecedented EtOH-mediated cascade C(sp3)–H alkylation reactions of 4-dialkylamino-indole-3-carbaldehydes have been realized for the green and divergent synthesis of spirocyclic azepino[4,3,2-cd]indole derivatives.
A regio- and stereoselective route to azepino[4,5-b]indoles is reported, with unprecedented formation of tetrahydrobenzo[f][1,5]diazecine-2,8(1H,3H)diones. The diazecine compounds show anti-cancer activity against HeLa and U87MG cell lines.
A Cu(I)-catalysed (4+3)-cycloaddition has been presented for obtaining azepino[c,d]indole motifs in good yields. The reaction follows a stereoselective pathway, and optically pure azepinoindoles could be synthesised in up to 96% ee.
Bicyclic N-dihalocyclopropylamide derivatives can participate in a cyclopropane ring-opening/intramolecular electrophilic aromatic substitution sequence, to produce various polycyclic systems having a nitrogen atom at a ring junction.