An Application of Molecular Mechanics Calculations on Thermal Reactions of Ten-membered Ring Sesquiterpenes

Abstract

Abstract Molecular mechanics calculations were successfully carried out to evaluate relative stabilities of each conformation of germacrenes in their ground states and transition states. In the calculation of such a transition state model as shown in Scheme 1, the parameters for partial bonds were taken as mean values of those for a reactant and a product. Thus, the calculation results on each transition state model indicate that the elemenes are formed from the corresponding germacrenes through the most stable transition state (CC), regardless of the most stable conformers in ground state. Calculations were also carried out on ground and transition states of acoragermacrone and isoacoragermacrone, both of which have an α,β-unsaturated carbonyl system. The calculation results on each transition state model indicate that shyobunone (5) and epishyobunone (6) both are formed from acoragermacrone through the corresponding transition states [5 from CC; 6 from C′C′ and C′T] in relative ratio (5/6=87/13) which is in good agreement with that of the experimental result (5/6=84/16). In the case of isoacoragermacrone, molecular mechanics calculations indicate that the preisocalamendiol-type transition state model is more favorable than the models corresponding to the Cope rearrangement products.