Conformational flexibility and absolute stereochemistry of (3R)-3-hydroxy-4-aryl-β-lactams investigated by chiroptical properties and TD-DFT calculations.

The effect of conformational flexibility on the chiroptical properties of a series of synthetic (3R)-3-hydroxy-4-aryl-β-lactams of known stereochemistry (1-6) was investigated by means of electronic circular dichroism (ECD) measurements and time-dependent density functional theory (TD-DFT) calculations. The application of the β-lactam sector rules allowed a correct stereochemical characterization of these compounds, with the exception of a thienyl-substituted derivative (cis-). TD-DFT calculations yielded accurate predictions of experimental ECD spectra and [α](D) values, allowing us to assign the correct absolute configuration to all the investigated compounds. A detailed analysis of the β-lactam ring equilibrium geometry on optimized conformers identified regular patterns for the arrangement of atoms around the amide chromophore, confirming the validity of the β-lactam sector rules. However, relevant variations in theoretical chiroptical properties were found for compounds bearing a heterocyclic substituent at C4 or a phenyl substituent at C3, whose conformers deviate from these regular geometric patterns. This behavior explains the failure of the β-lactam sector rules in cis-. This study showed the importance of conformational flexibility for the determination of chiroptical properties and highlighted the strengths and weaknesses of the different methods for the stereochemical characterization of chiral molecules in solution.