Conformational preferences in diastereomeric(5S)-methyl-3-(o-aryl)-2,4-oxazolidinediones.

The thermally interconvertible diastereomers of the (5S)-methyl-3-(o-aryl)-2,4-oxazolidinediones were synthesized and their conformers studied by (1)H NMR and HPLC. The barriers to rotation about the N-C(aryl) bond were found to be very much solvent dependent. For the o-fluoro oxazolidinedione, difference in barriers to rotation in deuterated methanol and deuterated chloroform amounted to 34 kJ/mol. ortho-Bromo substitution increased the barrier to rotation up to 100 kJ/mol in ethanol, which enabled the analytical separation of the diastereomers and observation of the thermodynamic enrichment of the S-P conformer by HPLC. In CDCl(3) by (1)H NMR, on the other hand, a barrier of only 89 kJ/mol was determined. The S-M and S-P conformers of the diastereomers of o-methyl, alpha-naphthyl and o-iodo derivatives have been assigned by NOESY experiments and the kinetic and thermodynamic constants for the interconversion between the S-M and S-P conformers were determined.