CONFORMATION AND BROMINATION OF BICYCLO[3 .3.1]NONANE-2,6-DIONE AND DEHYDROBROMINATION OF THE BROMO KETONES - IMPROVED SYNTHESIS OF BICYCLO[3.3.1]NONA-3,7-DIENE-2,6-DIONE

Bromination by copper(II) bromide of bicyclo[3.3.l]nonane-2,6-dione (1 ) affords the epimeric alpha-bromo diketones exo- and ended and, event ually, the alpha,alpha'-dibromo diketones exo- and endo-7 (3:7) in hig h yield. Dehydrobromination of the mixtures of bromo diketones by calc ium carbonate in dimethylacetamide yields the unsaturated diketones 6 (30%) and 2 (72%), respectively. Bromination of 1 by molecular bromine is less selective and furnishes mixtures of brominated diketones cons isting of exo- and endo-7, 8 and the tetrabromo diketone 10. An excess of bromine yields pure 10. - The ethylene acetal 12 is brominated by pyridinium perbromide to produce the alpha,alpha'-dibromo acetal 13 wh ich is very reluctant towards 1,2-dehydrobromination by potassium tert -butoxide. Thus, with an excess of potassium tert-butoxide in tetrahyd rofuran, 1,2-dehydrobromination is achieved only on one side to afford 14. In contrast, potassium tert-butoxide in dimethyl sulphoxide induc es a transannular dehydrobromination leading to the diacetal 15 of 3-b romonoradamantane-2,6-dione. - The conformations of 1, 10, and 13 in t he solid state are determined by X-ray diffraction analyses. From tors ional angles, vicinal proton coupling constants are calculated with th e aid of the Karplus equation and compared to those determined from so lutions by high-field NMR spectroscopy. The reasonable agreement provi des the basis for the assessment of the configuration and conformation for exo- and endo-7.