title: Determining the absolute stereochemistry of secondary/secondary diols by 1H NMR: basis and applications.
abstract: [structures: see text] The absolute configuration of 1,2-, 1,3-, 1,4-, and 1,5-diols formed by two secondary (chiral) hydroxy groups can be deduced by comparison of the NMR spectra of the corresponding bis-(R)- and bis-(S)-MPA esters. The correlation between the NMR spectra of the bis-ester derivatives and the absolute stereochemistry of the diol involves the comparison of the chemical shifts of the signals for substituents R1/R2 and for the hydrogens attached to the two chiral centers [H(alpha)(R1) and H(alpha)(R2)] in the bis-(R)- and the bis-(S)-ester and is expressed as delta deltaRS. Theoretical calculations [energy minimization by semiempirical (AM1), ab initio (HF), DFT (B3LYP), and Onsager methods, and aromatic shielding effect calculations] and experimental data (NMR and CD spectroscopy) indicate that in these bis-MPA esters, the experimental delta deltaRS values are the result of the contribution of the shielding/deshielding effects produced by the two MPA units that combine according to the actual stereochemistry of the diol. The reliability of these correlations is demonstrated with a wide range of diols of known absolute configuration derivatized with MPA and 9-AMA as auxiliary reagents. A simple graphical model that allows the simultaneous assignment of the two asymmetric carbons of a 1,n-diol by comparison of the NMR spectra (delta deltaRS signs) of its bis-(R)- and bis-(S)-AMAA ester derivatives is presented.
authors.affiliation: Departamento de Química Orgánica, Facultad de Química, and Unidad de RMN de Biomoléculas Asociada al CSIC, Universidad de Santiago de Compostela, 15782, Santiago de Compostela, Spain.
authors.forename: Félix
authors.initials: F
authors.lastname: Freire
authors.forename: José Manuel
authors.initials: JM
authors.lastname: Seco
authors.forename: Emilio
authors.initials: E
authors.lastname: Quiñoá
authors.forename: Ricardo
authors.initials: R
authors.lastname: Riguera