Interplay of twisting and folding in overcrowded heteromerous bistricyclicaromatic enes

Fluorenylidenexanthenes 5-7 were synthesized by 2-fold extrusion diazo-thio ne couplings. 7 exhibited yellow crystals and purple (560 nm) solutions. H- 1 NMR of 5 and 7 indicated subtle equilibria twisted (t) reversible arrow a nti-folded (a) major/minor conformations. C-13 DNMR of 6 gave Delta G(c)dou ble dagger(enantiomerization/inversion) = 26.5 kJ/mol and Delta G(c)double dagger(E,Z-topomerization) = 82.0 kJ/mol. PM3 calculations of 5 revealed mi nima a, t, ts (twisted/syn-folded), Delta Delta H(f)degrees = 0.0, 14.1, 15 .6 kJ/mol, and transition states [t-ts], [a-ts], [t(perpendicular to)], [a- a*], Delta Delta H(f)degrees = 16.3, 17.4, 82.2, 99.3 kJ/mol.