Rhodium-Catalyzed [3 + 1 + 2] Cycloaddition of Type II Diene-Vinylcyclopropanes and Carbon Monoxide for the Synthesis of 5/6 Skeletons with Two Adjacent Bridgehead Quaternary Centers

A rhodium-catalyzed [3 + 2 + 1] cycloaddition reaction of yne/ene-vinylcyclopropanes and carbon monoxide was developed previously to access 5/6 or 6/6 fused rings with one bridgehead quaternary carbon center. Herein, we report a rhodium-catalyzed [3 + 1 + 2] reaction of type II diene-vinylcyclopropanes and CO to construct 5/6 skeletons bearing two bridgehead quaternary carbons. The mechanism of the [3 + 1 + 2] reaction has also been investigated by quantum chemical calculations, showing that CO insertion ahead of alkene insertion can avoid a congested transition state forming two quaternary carbon centers in the traditional [3 + 2 + 1] pathway.