Chair-boat Equilibrium as Driving Force in Epimerization of 3,7-Dimethylbicyclo[3.3.1]nonan-2,9-dione Derivatives. Stereo-controlled Synthesis of the 3-exo,7-exo- and 3-endo,7-exo-Dimethylbicyclo[3.3.1]nonan-9-ones