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Determining the Absolute Stereochemistry of Secondary/Secondary Diols by 1 H NMR: Basis and Applications

2005; American Chemical Society; Volume: 70; Issue: 10 Linguagem: Inglês

10.1021/jo048643a

1520-6904

Félix Freire, José Manuel Seco, Emilio Quiñoá, Ricardo Riguera,

Axial and Atropisomeric Chirality Synthesis

[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.