Add time:10/01/2019         Source:infona.pl

The relatively large chemical shift differences observed in the 1H NMR spectra of the anticholinergic drugs (−)‐scopolamine 1 and (−)‐hyoscyamine 2 measured in CDCl3 are explained using a combination of systematic/molecular mechanics force field (MMFF) conformational searches and gas‐phase density functional theory (DFT) single point calculations, geometry optimizations and chemical shift calculations within the gauge including/invariant atomic orbital (GIAO) approximation. These calculations show that both molecules prefer a compact conformation in which the phenyl ring of the tropic ester is positioned under the tropane bicycle, clearly suggesting that the chemical shift differences are produced by the anisotropic effect of the aromatic ring. As the calculations fairly well predict these experimental differences, diastereotopic NMR signal assignments for the two studied molecules are proposed. In addition, a cursory inspection of the published 1H and 13C NMR spectra of different forms of 1 and 2 in solution reveals that most of them show these diastereotopic chemical shift differences, strongly suggesting a preference for the compact conformation quite independent of the organic or aqueous nature of the solvent. Copyright © 2010 John Wiley & Sons, Ltd.

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