| Abstract
| - A computational approach to the proposed mechanistic rationale of the lead tetraacetate mediatedoxidative cleavage of selected unsaturated diols in the hydrindenediol (derived from the Hajos−Parrish ketone, abbreviated as HP) and octalindiol (derived from the Wieland−Miescher ketone,abbreviated as WM) series is presented. These two unexpected cascade-type transformations havebeen found to yield structurally different products despite the similarities in the starting compounds1-HP and 1-WM, which differ only by the number of carbon atoms in the cycloalkane rings. Thefinal product in the HP series is the complex ring-expanded molecule iii-6, while the final productin the WM series is the ring-expanded bisacetoxy diacetal 7. Calculations carried out at the B3LYP/6-31G* level have revealed the fact that the transformations of i-6 to ii-6 and of i-7 to ii-7 areendothermic by 19.93 and 4.20 kcal/mol, respectively. The isomerization of compounds i to ii ismore endothermic in the case of the HP series. Furthermore, calculations have shown thetransformation of i to ii to be kinetically less favored in the HP series than in its analogue WMseries. The activation barrier for the transformation of i-6 to ii-6 is calculated to be 22.09 kcal/mol,whereas the corresponding barrier in the WM series is 13.77 kcal/mol. The thermodynamic featuresof the reactions support the experimental findings as well: 7 is calculated to be more stable thaniii-7, and iii-6 is more stable than 6, as expected. The diverging behavior in the mechanism of theinterconversion of i to ii has been attributed to the stabilization of the positive charge by a carbonyloxygen in ii-7 due to the flexibility of the seven-membered ring in contrast to the rigidity of thesix-membered ring.
|
