A novel methodology that allows the assignment of the absolute configuration of chiral secondary alcohols by NMR using only one derivative is presented. All that is needed is (a) the derivatization of the alcohol of unknown configuration with one enantiomer--either the (R)- or the (S)--of alpha-methoxyphenylacetic acid (MPA), (b) the recording of the 1H NMR spectrum of the resulting ester in MeCN-d3, and (c) addition of a barium(II) salt [i.e. Ba(ClO4)2] to the NMR tube till saturation and recording of a second spectrum. The assignment of the R/S configuration to the alcohol takes a few minutes and consists on the comparison of the signs of the shifts (Deltadelta(Ba)) produced by addition of the barium(II) with those predicted for the (R) and the (S) enantiomers in accordance to a simplified model that reflects the conformational changes produced by the complexation with barium and their consequences in the chemical shifts. These conformational changes are based on experimental NMR and CD results showing that the formation of a barium(II) complex with the MPA ester moves the conformational equilibrium between syn- (sp) and anti-periplanar (ap) forms toward the most stable ones (sp), and that this leads to the increase of the shielding caused by the MPA phenyl group on a certain substituent of the alcohol. In addition, ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations provide further evidence on the formation, structure, and stability of the complexes with Ba2+, Mg2+, and the influence of the solvent. The general applicability of this methodology and the reliability of the configurational assignment were assured by the study of about twenty alcohols of known configuration and diverse structural features. Its scope and limitations have also being established and other representative cations (i.e. Li+, Rb+, Cs+, Mg2+, Ca2+, Sc3+, V3+, Zn2+) were also evaluated. The procedure proposed is simple, fast, and cheap because it requires a very small amount of sample, only one derivatization, and the recording of only two 1H NMR spectra at rt. A graphical guide to facilitate the application of this new method is included at the end of the paper.

The assignment of the absolute configuration of alpha-chiral primary amines by complexation of their MPA derivatives with Ba2+ and NMR analysis of the changes generated is presented. All that is required is (a) the derivatization of the amine of unknown configuration with one enantiomer of the auxiliary reagent (MPA), either (R) or (S)-alpha-methoxyphenylacetic acid, (b) the recording of the 1H NMR spectrum of the resulting amide in MeCN-d3, (c) the addition of Ba(ClO4)2 to the NMR tube, and (d) the recording of a second spectrum after a few minutes of shaking. The above steps take a few minutes and are followed by an analysis of the shifts (measured as Deltadelta(Ba)) produced on the L1 and L2 substituents of the amine by the addition of Ba2+ and their comparison with those expected from the conformational changes produced by the complexation. The conformational changes initiated by complexation have been subjected to NMR and CD studies, which showed that the formation of the complex shifts the equilibrium from an antiperiplanar (AP) to a synperiplanar (SP) form, leading to an increase of the shielding by the phenyl group of MPA of the substituent of the amine located on the same side. In addition, theoretical calculations [density functional theory (DFT)] provide further support for the formation, structure, and stability of the complexes. The general applicability of this method and the trustworthiness of the resulting configurational assignment were guaranteed with a series of amines of known absolute configuration and varied structures, used as test compounds. The method proposed is simple, fast, and inexpensive, and it requires a very small amount of sample, only one derivatization, and the recording of just two 1H NMR spectra at room temperature. A graphical guide to simplify the application of this method is included.

The absolute configuration of ketone cyanohydrins can be assigned from analysis of the 1H NMR spectra of the corresponding (R)- and (S)-MPA ester derivatives and use of delta delta(RS) signs. This is an application of the NMR methodology for stereochemical assignment to tertiary alcohols possessing polar groups as substituents.

The sectional matrix is the best way to achieve a strong contact point in Class II restorations with composite resin in the posterior dental sector. The pre-wedging is essential to get a separation between teeth which avoid the matrix deformation during its insertion. This article describes the clinical technique for restoring Class II cavities using a sectional matrix.