Interplay of Twisting and Folding in Overcrowded Heteromerous Bistricyclic Aromatic Enes

Fluorenylidenexanthenes <b>5</b>−<b>7</b> were synthesized by 2-fold extrusion diazo−thione couplings. <b>7</b> exhibited yellow crystals and purple (560 nm) solutions. <sup>1</sup>H NMR of <b>5</b> and <b>7</b> indicated subtle equilibria twisted (t) ⇌ <i>anti</i>-folded (a) major/minor conformations. <sup>13</sup>C DNMR of <b>6</b> gave Δ<i>G</i><sub>c</sub><sup>‡</sup>(enantiomerization/inversion) = 26.5 kJ/mol and Δ<i>G</i><sub>c</sub><sup>‡</sup>(<i>E,Z</i>-topomerization) = 82.0 kJ/mol. PM3 calculations of <b>5</b> revealed minima a, t, ts (twisted/<i>syn</i>-folded), ΔΔ<i>H</i><sub>f</sub>° = 0.0, 14.1, 15.6 kJ/mol, and transition states [t-ts], [a-ts], [t<sub>⊥</sub>], [a-a*], ΔΔ<i>H</i><sub>f</sub>° = 16.3, 17.4, 82.2, 99.3 kJ/mol.