Structural analysis of sterically hindered 1,4-diols from the naturally occurring lignan hydroxymatairesinol a quantum chemical study

The structures of TADDOL-like a-conidendrin-based chiral 1,4-diols (LIGNOLs) have been studied at molecular mechanics, Hartree-Fock (HF)/6-31G* and DFT/B3LYP/TZVP level of theory. The molecules included were 1,1-diphenyl, two diastereomers of 1,1,4-triphenyl, 1,1,4,4-tetraphenyl, and 1,1,4,4-tetramethyl 1,4-diol. Several conformers of each molecule were studied thorougly also including the entropy contributions. For the triphenyl 1,4-diols, which can form pi - pi interactions between phenyl rings, the DFT optimized structures differed significantly from the HF optimized ones. A property for the most stable structures, in addition to the ability to form pi - pi interactions, seemed to be the possibility to have the aliphatic six-membered ring in a boat conformation. For all of the studied LIGNOLs some conformers were found, where the two OH groups pointed almost to the same direction. By this an intramolecular hydrogen bond can be formed between them. The bridging hydrogen atom falls at the same place as a chelate-bonded metal ion would be situated, as in the case of the analogous molecules, TADDOLs, but only a few of these molecules would be able to work well as ligands for asymmetric catalysis.