NMR and X-ray crystallographic studies of the conformation of a 3,4,6-triphenyl-delta-lactone.

1-Oxa-3S,4S,6R-triphenyl-2-cyclohexanone and its enantiomer were synthesized, and the structure was determined by NMR and X-ray crystallography. The X-ray crystal structure showed that the delta-lactone adopts a boat conformation in the solid. The X-ray data showed a shortened C-O bond between the carbonyl carbon and the ether oxygen, consistent with delocalization involving the ester group. (1)H and (13)C NMR measurements in acetone-d(6) showed that the lactone is biased in favor of a boat conformation. In the less polar solvent chloroform-d(1), changes in the (1)H NMR coupling constants indicate a shift in the equilibrium in favor of a less rigid twist-boat conformation. The IR absorption of the lactone carbonyl at 1740 cm(-)(1) would suggest a half-chair conformation inconsistent with the dominance of the boat forms shown by NMR and X-ray.