Absolute Configuration, Conformation, and Circular Dichroism of Monocyclic Arene Dihydrodiol Metabolites: It is All Due to the Heteroatom Substituents

Abstract

Absolute configurations and conformations of selected cis-1,2-dihydrodiols, isolated from bacterial enzyme-catalyzed arene dihydroxylation, have been examined by comparison of experimental and DFT-calculated CD spectra and confrontation with the results of X-ray diffraction studies in the crystalline phase. The equilibrium between the diene P and M conformers in cis-dihydrodiols is strongly dependent on the intramolecular OH-OH, OH-pi, and OH-F hydrogen bonding pattern and is crucial in determining the sign and magnitude of the long-wavelength diene pi-pi transition Cotton effect. The differences originate from a dominant contribution of either P-helical (1b, X=Me) or M-helical conformers (1d, X=F), or are due to M and P low-energy conformers, both contributing a positive rotational strength (1c, X=Br). Computations show that cis-dihydrodiol 1e (X=CF3) has only one M conformer stabilized by an intramolecular O-H...F hydrogen bond. cis-Dihydrodiol 1f (X=CN) shows a Cotton effect of the sign opposite to the sense of helicity of the dominating conformer. The results of the computations highlight the inadequacy of the Diene Helicity Rule and the Allylic Chirality Rule to correlate observed Cotton effects with dihydrodiol absolute configuration. A reliable model is presented to predict the absolute configuration of substituted benzene dihydrodiol derivatives from CD spectra, based on the confrontation of DFT-computed and experimental CD spectra. For 3-alkyl derivatives, a simple noncomputational model is offered, which is based on the contributions of the allylic hydroxy groups and the diene core in P and M conformers.