Abstract
AbstractTreatment 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
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