Synthesis of Polysubstituted Cyclohexenes via Pyridine-Boryl Radical-Catalyzed Formal [4 + 2] Cycloadditions of Cyclobutanes with Alkynes

Polysubstituted cyclohexene derivatives widely exist in pharmaceuticals and bioactive natural products. Herein, we report the first pyridine-boryl radical-catalyzed intermolecular formal [4 + 2] cycloadditions between cyclobutanes and alkynes, producing tri- and tetrasubstituted cyclohexene derivatives with a broad substrate scope and moderate to high yields. Moreover, the intramolecular formal [4 + 2] cycloaddition of alkyne-tethered cyclobutanes has also been achieved.