Chair-boat equilibrium as driving force in epimerization of 3,7-dimethylbicyclo[3.3.1]nonan-2,9-dione derivatives. Stereocontrolled synthesis of 3-exo,7-exo- and 3-endo,7-exo-3,7-dimethylbicyclo[3.3.1]nonan-9-ones

Molecular mechanics calculations and experimental 1H NMR data are in close agreement and show that the epimerization equilibrium at C3 of 3,7-dimethylbicyclo[3.3.1]nonan-2,9-diones is shifted toward the 3-endo-epimer in order to reach the lowest energy chair-boat conformation. Introduction of a 1,3-dioxolane moiety at C9 results in reversal of the equilibrium, in protic solvents, leading mainly to the 3-exo-epimer in the most stable chair-chair conformation. These results have been applied to the stereocontrolled synthesis of the title compounds.