Structural Aspects of the Enantioselectivity of Tartrates with α‐Amino‐alcohol Salts part. I. Crystal structures of eleven tartaric‐acid diesters

Abstract

AbstractEnantioselective host‐guest complexes of α‐amino‐alcohol salts with chiral tartaric‐acid esters can not be crystallised up to now. To study structural aspects of their enantioselectivity, crystal structures of the components were determined. The structures of eleven diesters with myrtanol, borneol, menthol, neomenthol, and cis‐4‐(tert‐butyl)cyclohexanol in different configurations showed a remarkable rigidity of the tartaric‐acid conformation, partly because of intramolecular H‐bonding between OH and C=O groups. The conformation of the tartaric‐acid part in these diesters is the same as the one observed in optically active tartaric acid (torsion angle OCCOH ca. 0°), The binding site for guest molecules is a parallelogram formed by two hydroxy and two carbonyl O‐atoms, all lying on the same side of a mean molecular plane. There is one exception: the dimenthyl ester, which is the most enantioselective with a norephedrine guest, has one of the ester groups turned (torsion angle OCCOH ca. 180°), forming a triangle of O‐atoms and moving the bulky menthyl group to the vicinity of the binding site.