All‐Carbon Quaternary Center Containing Tetrahydro‐5 <i>H</i> ‐benzo[ <i>c</i> ]fluorenes via Highly Stereoselective Transannular Carbocation Cyclization

Transannular carbocation cyclization of the achiral benzannulated cyclononene tertiary alcohols under acid catalysis furnished the [5/6]-fused tetracycles with three contiguous stereogenic centers, one of which is an all-carbon C11b quaternary center, in good-to-excellent yields and diastereoselectivity. Hydride, water, azide, allyl, and electron-rich (hetero)arenes could be employed as nucleophiles. The reaction is under both substrate- and condition-control. Further, functionalization of some selected substrates was also demonstrated. Computational chemistry delineated different competing reaction pathways such as direct substitution at the bisbenzylic position for other related systems-the benzannulated cyclooctene/cyclononene secondary alcohols and provided further evidence for the observed stereoselectivity.