1,6-Reduction-Type Hydride Shift/Cyclization Process without Adjacent Heteroatom: Stereoselective Synthesis of Hexahydrofluorene Derivatives via Internal Redox Reaction/Friedel–Crafts Reaction Sequence

We report stereoselective access to hexahydrofluorene derivatives with three contiguous stereogenic centers based on a hydride-shift-triggered C(sp³)-H bond functionalization (internal redox reaction). The key process is an unprecedented 1,6-reduction-type hydride shift at the position without an adjacent heteroatom. When alkenylidene malonates having a methine group adjacent to a para-methoxyphenyl group were treated with a catalytic amount of SnCl₄, an internal redox reaction and an intramolecular Friedel-Crafts reaction proceeded successively to afford hexahydrofluorene derivatives with three contiguous stereogenic centers in good chemical yields with excellent diastereoselectivities. Additional experiments and DFT calculations revealed that the interconversion between the cis- and trans-isomers was responsible for the high yields and diastereoselectivities.