Determination of energy barriers to rotation and absolute conformations of thermally interconvertible 5,5-dimethyl-3(o-aryl)-2,4-oxazolidinedione enantiomers

The activation barriers for the interconversion between the enantiomers of 5,5-dimethyl-3-(o-aryl)-2,4-oxazolidinediones (M = P) have been determined by temperature dependent NMR and by enantioresolution on chiral sorbents via HPLC. The activation barriers were found to increase linearly with the size of the van der Waals radii of the ortho-halogen substituents. The enantiomers of the o-iodo derivative were micropreparatively enriched on a Chiralpak AD column, leading to the determination of its barrier to rotation via thermal racemization and resulting in the assignments of conformations in the presence of the optically active chiral auxiliary (S)-(+)-1-(9-anthryl)-2,2,2-trifluoro ethanol [(S)-TFAE]. (C) 2004 Elsevier Ltd. All rights reserved.