No abstract

The 'H and 13C NMR spectra of amiodarone and several derivatives are reported and assigned. The hypothesis of an 0-trans to 0-cis conformational equilibrium in most of these molecules has been investigated by examining the 'H and I3C chemical shifts, and by NOE and LIS experiments, variable-temperature NMR and IR spectroscopy.The assumed 0-trans/O-cis equilibrium would be fast on the NMR time scale for amiodarone, which has no ortho substituents on the C-3 benzoyl substituent. However, in the 2',6'-dibromobenzoyl derivatives, separate 0-trans and 0-cis conformers are observed at room temperature in the NMR spectrum in the ratio of ca 60:40, favouring the 0-trans conformer. The coalescence temperatures and activation energies (A G$-) of the 2',6'-disubstituted compounds are obtained; the activation energy increases with the size of the ortho substituents but the conformer ratio remains unchanged. In contrast, removal of the C-2 butyl substituent on the benzofuran ring gives 100% of the 0-trans conformation.