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The less time-consuming calculations with the methyl esters were effected for 14, 18, 19, and 20.
The widely used AM1 method and the corresponding ZDO approximation probably lead to an underestimation of the important electrostatic repulsion forces involved in the present highly over-crowded systems, and these effects are not counterbalanced by the parametrization of the other terms of the Fock matrix. However as calculated by a referee, and we gratefully acknowledge him for this, the molecular mechanics force field PC model using VESCF charges correctly find 14b to be more stable that 14a by 7.3 kcal mol-1 and 18b to be more stable by 11 kcal mol-1 (both in ΔH); the twist angle between the two exocyclic double-bonds of 14a (torsion angle C7-C1-C2-C9, Figure 1) is about 39° with this approach.
The synthesis and thermal stability of compound 1 remains an open question for the moment; see for example: Sauter, H.; Gallen-kamp, B.; Prinzbach, H. Chem. Ber. 1977, 110, 1382; a theoretical study of this aspect will be published in a forthcoming article.
