Stereochemistry and conformations of natural 1,2-epoxy-guaianolides based on 1D and 2D NMR data and semiempirical calculations
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AuthorsTrifunović, Snežana S.
Milosavljević, Slobodan M.
Article (Published version)
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From detailed study of 1D and 2D NMR spectra of ten natural 1,2-epoxyguaianolides (bis-1,2:3,4-epoxyguaianolides and guaianolide-1,2-epoxychlorohydrins), we identified general spectral traits helpful for stereochemical assignment of such sesquiterpene lactones. We found that the chemical shifts of certain H-1 and 13 C nuclei are consistently dependent on the configuration of 1,2-epoxy-ring which could be used as a simple rule for establishing this configuration. Then, from 1 D and 2D (COSY, NOESY, HMQC, HMBC) NMR data, applying the observed rule, the structure and stereochemistry of two new, diastereomeric guaianolide-1,2-epoxychlorohydrins, isolated from Achillea serbica, are determined. The NMR data, namely, nuclear overhauser enhancement (NOE) correlations, pointed out two conformations of guaianolide's cycloheptane ring. The semiempirical calculations (AM1 and PM3 methods), performed in order to gain additional information regarding conformations, resulted in three geometries of in...vestigated lactones. Even so, the conformations derived from the NMR data agreed well with those calculated by semiempirical methods. Copyright (C) 2008 John Wiley & Sons, Ltd.
Keywords:H-1 NMR / C-13 NMR / 2D NMR / sesquiterpene lactones / guaianolide-1,2-epoxychlorohydrins / conformational analysis / Achillea L
Source:Magnetic Resonance in Chemistry, 2008, 46, 5, 427-431
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