The structure elucidation of 5 was complemented by densityfunctional
(DFT) calculations to define the geometry of the
Δ2,2′ olefinic bond and its hypothetical E and Z configurations
(Figure 3). The E configuration shows C2 symmetry, whereas
the Z configuration is distorted from Cs to C1 symmetry,
resulting in an energy decrease of 26.4 kJ/mol. This distortion
is probably driven by the short nonbonded distance between O-
1 and O-1″ (1.445 and 1.453 Å for Cs and C1, respectively).
The Gibbs free energy of the E configuration is lower by 247.4
kJ/mol than that of the (C1) Z configuration, which
corroborates the conclusion that E is the more stable
configuration. Also, the calculated NMR chemical shifts for
the E configuration of 5 are in better agreement with
experimental data than those for the Z configuration (