Base-Assisted and Silica Gel-Promoted Indole-Substituted Indene Synthesis

Herein, we report for the first time a transition-metal-free and mild protocol that requires inexpensive K₂CO₃ and silica gel for direct access to polysubstituted indenes from readily obtainable starting precursors. Notably, sequential Michael addition, intramolecular cyclization, and silica gel-promoted nucleophilic substitution reactions afford the desired products. A broad range of indoles and other aromatic nucleophiles are well-tolerated, affording indenes in moderate to good yields. Gratifyingly, indene could be easily converted into synthetically useful 3-indole-substituted indanone and indanol. Nonetheless, the successful isolation of a reaction intermediate highlights the crucial role of methanol in this reaction.