Anionic and photochemical behaviour of the medium-sized terpenoid ketone 8-dehydro-12-O-methyl-deacylhallerin

Treatment of the germacrane ketone 8-dehydro-12-O-methyl-deacylhallerin (2) with bases under conditions of thermodynamic enolization resulted in complete epimerization at the adjacent carbon atom C-7, with formation of 3, The conformational features of the epimeric ketones 2 and 3 were investigated by X-ray crystal structure analysis, NMR spectroscopy, molecular mechanics and dynamics simulations and ab initio calculations. While 2 showed temperature-insensitive sharp ¹H NMR signals, its epimer 3 showed only broad lines. These spectral features suggest that 2 is monorotameric, while 3 is a mixture of different conformers, as was verified by molecular mechanics and dynamics calculations. Upon UV irradiation, 2 underwent isomerization at the C1-C10 double bond, while 3 was unreactive.