Stereochemical studies of bicyclo[5.3.1]undecane-8,11-dione. Conformational analysis, enantiomer separation, CD-spectroscopy and enolization

Abstract

Abstract The stereochemical properties of the title compound have been examined by various methods. Dynamic NMR spectroscopy shows the existance of two conformers or sets of conformational families separated by a barrier of 8.2±0.4 kcal/mol. At low temperatures only one conformer was appreciably populated. Chromatographic enantiomer enrichment was performed on a microcrystalline triacetylcellulose column and the circular dichroism (CD) spectrum was recorded. The conformational space was explored by MM3 calculations giving six conformers within 2 kcal/mol. The high energy barrier observed by DNMR was assigned a chair-boat inversion of the eight-membered ring. The CD spectrum of the conformational mixture given by the MM-computations was calculated by the method developed by Schellman, giving information about the absolute configuration of the first eluted enantiomer, and about the conformational mixture in solution. Formation of enol esters under equilibrium conditions gave 83% of the bridgehead enol ester.