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, α‐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. Chirality 15:242–250, 2003. © 2003 Wiley‐Liss, Inc.