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Acid-promoted intra- and intermolecular [2+2] cycloaddition of indoles to aryl alkynes to access cyclobutene-fused indolines.

Herein, we have reported the first example of both intra- and intermolecular [2+2] cycloaddition of the electron-rich indoles and unactivated aryl alkynes promoted by the combination of Fe(NO3)3 and HNO3, which highlights efficient and selective access to several different types of functionalized cyclobutene-fused indolines from readily available starting materials with cheap catalysts and simple operations.