Abstract
Computational studies (MNDO/PM3 and MMX) were performed towards the selectivity and reactivity of the ringtransformation reactions of the 5-(1-propynyloxyethyl)pyrimidine derivatives A1 -8. Determination of the energy and dipole moments of the transition states of the different possible reaction pathways revealed that the ratio of the product formation was determined by the polarity of the transition states, as expressed by their dipole moments, in combination with the dipole moment of the reaction medium. Also the reactivity of the compounds under study appeared to depend on the polarity of the transition state of the initial Diels-Alder reaction. The differences in dipole moments of the different TS's are ascribed to the geometry of the molecules in the TS's. Furthermore, a comparison of the heats of activation of the initial Diels-Alder reaction and the subsequent retro Diels-Alder reaction showed that the first one is the rate determining step.
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