Dsoid conformation

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

The interesting behavior own mainly by (V)-EPTM was not confirmed by terpolymers containing 5,6-dimethylene-24ioibomene (X) or 5- 2 -vinyl-3 -butenyl)-2-norbomene (XI), both being model compounds of triene (V). In fact, the first EPTM contains two conju ted double bonds in obligatory dsoid conformation which were found very reactive toward methyl free radicals. The second EPTM has one bis-allylic hydrogen atom in the unsaturated system ... 

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