Optical Rotatory Dispersion Studies. 134. Absolute Configuration and Circular Dichroism Spectrum of (R)-Cyclopentanone. Demonstration of a Conformational Isotope Effect

Article abstract of DOI:10.1021/ja00407a005

The absolute configuration of (R)-<2-2H1>cyclopentanone is established through synthesis using as the key step chiral epoxidation with the Sharpless reagent.Its circular dichroism (CD) spectrum is unusual in that it exhibits a bisignate Cotton effect being negative at 310 and positive at 275 nm.From the interpretation on the variable-temperature CD measurements, the unexpected conclusion is reached that the twist conformation with the deuterium in the quasi-equatorial position is energetically preferred by ca. 10 +/- 2 cal/mol.Evidently, even at room temperature, the contribution toward the spectrum resulting from the conformational isotope effect is of the same or larger amplitude but opposite sign compared to the difference between the partial octant contributions of an α-quasi-equatorial and α-quasi-axial deuterium substituent.