Functionalized Dihydro/Tetrahydrobenzo[a]Fluorenes via Interrupted Iso-Nazarov Reaction.

Under the catalysis of p-TsOH immobilized on silica (PTS-Si), the iso-Nazarov reaction of (E)-(2-stilbenyl)methanol/(E)-(2-stilbenyl)imine tosylate bearing a styrene moiety forged the corresponding [5/6] systems with the exclusive trans relationship at the two newly formed adjacent stereogenic centers. The ensuing "interrupted" nucleophilic addition by the pendent styrenyl olefin furnished the dihydro-11H-benzo[a]fluorene with the exclusive cis stereocontrol at the two-carbon ring junction. Thus, from non-chiral starting materials, the interrupted iso-Nazarov reaction formed two rings and provided the olefin moiety, which can be functionalized to other groups/systems possessing one to two additional chiral centers. Up to five contiguous stereogenic centers on the tetrahydrobenzo[a]fluorene could be constructed efficiently.