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

Molecular mechanics calculations and experimental H-1 NMR data are in close agreement and show that the epimerization equilibrium at C-3 Of 3,7-dimeth ylbicyclo[3.3.1]nonan-2,9-diones is shifted toward the 3-endo-epimer in ord er to reach the lowest energy chair-boat conformation. Introduction of a 1. 3-dioxolane moiety at C-9 results in reversal of the equilibrium, in protic solvents, leading mainly to the 3-exo-epimer in the most stable chair-chai r conformation. These results have been applied to the stereocontrolled syn thesis of the title compounds. (C) 2001 Elsevier Science Ltd. All rights re served.