Interplay of Twisting and Folding in Overcrowded Heteromerous Bistricyclic Aromatic Enes

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