Acid-Regulated Selective Synthesis of Benzofuran Derivatives via Single-Component BDA Retro-Aldol/Michael Addition Cascade and [4 + 2] Cycloaddition Reactions.

The acid-controlled single-component retro-aldol/Michael addition cascade reaction and [4 + 2] cycloaddition of benzofuran-derived azadienes (BDAs) are reported for the first time. Under the conditions of trifluoromethanesulfonic acid as the catalyst and with the addition of water, BDAs initiate the retro-aldol reaction, followed by a 1,4-Michael addition, yielding (arylmethylene)bis(dibenzofuran) products with excellent yields and broad substrate applicability. This represents the first application of BDAs in a retro-aldol reaction. In contrast, in the absence of water and with boron trifluoride etherate as the catalyst, BDAs undergo a [4 + 2] cycloaddition reaction, constructing the spiro[benzofuran-2,3'-benzofuro[3,2-b]pyridine] framework with high yields and diastereoselectivity. The method features mild conditions and high atom economy, and provides a new approach for constructing benzofuran scaffold derivatives.