Cooperative activation of cyclobutanones and olefins leads to bridged ring systems by a catalytic [4 + 2] coupling

Bridged ring systems are widely found in natural products, and successful syntheses of them frequently feature intramolecular Diels–Alder reactions. These reactions are subclassified as either type I or type II depending on how the diene motif is tethered to the rest of the substrate (type I are tethered at the 1-position of the diene and type II at the 2-position). Although the type I reaction has been used with great success, the molecular scaffolds accessible by the type II reactions are limited by the strain inherent in the formation of an sp2 carbon at a bridgehead position. Here, we describe a complementary approach that provides access to these structures through the C–C activation of cyclobutanones and their coupling with olefins. Various alkenes have been coupled with cyclobutanones to provide a range of bridged skeletons. The ketone group of the products serves as a convenient handle for downstream functionalization. Although widely used to form bridged ring systems, certain intramolecular Diels–Alder reactions are hampered by the strain inherent in forming an sp2-carbon at a bridgehead position. Now, an alternative strategy has been described to access these bridged ring systems through the C–C activation and coupling of cyclobutanones with olefins.