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