Thermodynamic and geometric study of diasteroisomeric complexes formed by racemic flavanones and three cyclodextrins through NMR

Abstract

The main purpose of this work was to study the chiral recognition thermodynamics of inclusion complexes formed by flavanones and b-cyclodextrins, and its relation with the inclusion geometries, through NMR experiments. By using the racemic mixtures of (±)-flavanone (FL) and (±)-20-hydroxyflavanone (20OHFL), diasteroisomeric complexes were formed employing b-cyclodextrin (bCD), (2- hydroxypropil)-b-cyclodextrin (HPbCD) and heptakis-(2, 6-O-dimethyl)-b-cyclodextrin (DMbCD). 1H NMR experiments of the complexes showed enantiodifferentiation for FL/bCD, FL/HPbCD, FL/DMbCD, 20OHFL/HPbCD and 20OHFL/DMbCDcomplexes, so they were able to be studied by obtaining the stoichiometry (1:1 for each complex), association constants (Ka), Ka ratios, and thermodynamics (DH, DS and DG). The results show that Ka values decrease with increasing temperature and that Ka ratio values removed from 1 not always reflect better enantiodiscrimination by NMR. Thermodynamics (DH and DG) show an exothermic and spontaneous formation of the complexes. Since the results were established for each couple of diasteroisomeric complexes separately, comparison of thermodynamics between them was possible, concluding that one half of the couples of diasteroisomeric complexes present chiral recognition due to enthalpic phenomena and the other half due to entropic phenomena. Additionally, ROESY experiments were performed to estimate the inclusion geometry of the complexes, which are in good agreement with the thermodynamic and Ka results.