One-Step Synthesis of Hydropyrrolo[3,2-b]indoles via Cascade Reactions of Oxindole-Derived Nitrones with Allenoates.

Hydropyrrolo[2,3-b]indole is a privileged indoline motif, while its analogue, hydropyrrolo[3,2-b]indole, has been much less explored. Herein, we developed a cascade reaction of oxindole-derived nitrones with allenoates, providing straightforward access to the tetracyclic hydropyrrolo[3,2-b]indole scaffolds. Formation of multiple C-C/C-X bonds and cleavage could be achieved within one synthetic step using a simple catalyst (Gimeracil) under mild conditions. The reaction pathway may involve the generation of spiro-hydroazepinone as the key intermediate.