Metal-Mediated [6 + 3] Cycloaddition Reactions of Fulvenes. A Novel Approach to Indan Systems

The [6 + 3] cycloaddition of 2-oxyallyl cations to the electron rich fulvene ketene acetal provides an efficient route to the indan skeleton. The structures of these indan adducts were assigned by extensive 2D NMR experiments. Direct hydrolysis of these ketal adducts affords the corresponding diketones or indens. Reaction of the fulvene ketene thioacetal as well as the tandem [6 + 3]−[4 + 3] cycloadditions were also studied. A mechanism is proposed which may account for the origin of stereo- and regioselectivity in this cycloaddition.