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Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones by Low-Temperature NMR Spectroscopy and Semiempirical Calculations |
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| Authors: | Milosavljević Slobodan Juranić Ivan Bulatović Vanja Macura Slobodan Juranić Nenad Limbach Hans-Heinrich Weisz Klaus Vajs Vlatka Todorović Nina |
| Affiliation: | 1.Faculty of Chemistry, University of Belgrade, 11000, Belgrade, Serbia and Montenegro ;2.Institute for Medicinal Plant Research “Dr. Josif Pan?i?, 11000, Belgrade, Serbia and Montenegro ;3.Department of Biochemistry and Molecular Biology, Mayo Foundation, Rochester, Minnesota, 55905 ;4.Institut für Chemie, Freie Universit?t Berlin, Berlin, Germany ;5.Institute for Chemistry, Technology and Metallurgy, Njego?eva 12, 11000, Belgrade, Serbia and Montenegro ; |
| Abstract: | Conformational analysis of 9  -acetoxycumambrine A 1 and 8-O-isobutiryl-9-acetoxycumambrine B 2 was carried out by low-temperature NMR studies. Results suggested that lactones 1 and 2 are mixtures of two distinctive conformers, I and II. Based on low-temperature 1H NMR spectra, in four solvents, the thermodynamic parameters of I  II exchange process were assessed. Energy of activation of I  II reaction was obtained by dynamic NMR simulations for both compounds. Results revealed that conformational exchange of lactones 1 and 2 occurs due to  chair twisted chair interconversion of a heptane ring. The same PM3 semiempirical method was applied for geometry optimization of lactones 1 and 2, as well as of 9-hydroxycumambrine A 3, 9-acetoxycumambrine B 4, and cumambrine B 5. |
| Keywords: | Guaianolides cumambrine conformations low-temperature NMR dynamic NMR PM3 semiempirical calculations |
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