The solid-state and solution-state reassigned structures of tagitinin A, a 3,10-epoxy-germacrolide from Tithonia diversifolia, and the interconversion of 3,10-epoxy-germacrolide conformational families via a ring-atom flip mechanism

Robert Glaser, Abraham García, María Isabel Chávez, Guillermo Delgado

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Abstract

Tagitinin A (2), a known 3,10-epoxy-germacrolide-6,7-trans-lactone isolated from Tithonia diversifolia, was investigated by single crystal X-ray diffraction analysis. It was found to have a 1β,4α,6α,7β, 8β relative configuration which differed at C(1) from the 1α-orientation originally reported in the literature which was determined by Horeau's Rule. Analysis of the 1H NMR spectrum of 2 shows the molecule to maintain its crystallographically observed twist-chair-boat (TCB) nine-membered ring conformation in acetone-d6 solution. The twist-chair-boat/skew-chair-boat type 3 conformations of saturated/unsaturated nine-membered rings within 3,10-epoxy-germacrolides can be interconverted to the skew-chair-chair (SCC) conformation by means of a C(9) ring atom flip mechanism. As a result of this conformational change, the orientation of the C(1) atom and the C(8)-oxycarbonyl moiety are transformed from diequatorial to diaxial. The reported stereochemistry of 3,10-epoxy-germacrolide lactone structures, and the DFT B3LYP/6-31g(d) modeling findings in this work indicate that tetrahedral C(1) atoms stabilize the TCB/SCB type 3 conformations, while their trigonal counterparts stabilize the SCC conformation.

Original languageEnglish
Pages (from-to)440-448
Number of pages9
JournalJournal of the Brazilian Chemical Society
Volume16
Issue number3 A
DOIs
StatePublished - 1 Jan 2005

Keywords

  • Conformational interconversion
  • Horeau's rule
  • Molecular modeling
  • Tagitinin A

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