Konformation und Bromierung von Bicyclo[3.3.1]nonan‐2,6‐dion sowie Dehydrobromierung der Bromketone. – Verbesserte Synthese von Bicyclo[3.3.1]nona‐3,7‐dien‐2,6‐dion

Abstract

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‐dioneBromination by copper(II) bromide of bicyclo[3.3.1]nonane‐2,6‐dione (1) affords the epimeric α‐bromo diketones exo‐and endo‐5 and, eventually, the α, α′‐dibromo diketones exo‐ and endo‐7 (3:7) in high yield. Dehydrobromination of the mixtures of bromo diketones by calcium 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 consisting 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 α,α′‐dibromo acetal 13 which is very reluctant towards 1,2‐dehydrobromination by potassium tert‐butoxide. Thus, with an excess of potassium tert‐butoxide in tetrahydrofuran, 1,2‐dehydrobromination is achieved only on one side to afford 14. In contrast, potassium tert‐butoxide in dimethyl sulphoxide induces a transannular dehydrobromination leading to the diacetal 15 of 3‐bromonoradamantane‐2,6‐dione. – The conformations of 1,10, and 13 in the solid state are determined by X‐ray diffraction analyses. From torsional angles, vicinal proton coupling constants are calculated with the aid of the Karplus equation and compared to those determined from solutions by high‐field NMR spectroscopy. The reasonable agreement provides the basis for the assessment of the configuration and conformation for exo‐ and endo‐7. magnified image