Transition-metal-free, base-promoted annulation/ring-cleavage/ring-reconstruction cascades: a facile access to N-protection free indole-indenones

A one-step, transition-metal-free, base-promoted cascade reaction of 2-halogenated arylglyoxals with 2-oxindoles is reported herein, which provides a straightforward approach to structurally diverse free (NH)-indeno[2,1-b]indol-6(5H)-ones in satisfactory yields. Control experiments indicated that geminal bisindolin-2-ones intermediate played a crucial role in streamlining the downstream annulation/ring-cleavage/ring-reconstruction sequences. This novel reaction not only furtherly enriches reactivity of indolin-2-ones as recursive enolate anion source, but also achieves thermal-induced α-arylation of oxindoles without a transition-metal catalyst.