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 Ben-Gurion University of the Negev
  • Abraham García Universidad Nacional Autónoma de México
  • María Isabel Chávez Universidad Nacional Autónoma de México
  • Guillermo Delgado Universidad Nacional Autónoma de México


Tagitinin A, Horeau’s rule, conformational interconversion, molecular modeling


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) ninemembered 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 .


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Author Biographies

Robert Glaser, Ben-Gurion University of the Negev

Department of Chemistry

Abraham García, Universidad Nacional Autónoma de México

Instituto de Química

María Isabel Chávez, Universidad Nacional Autónoma de México

Instituto de Química

Guillermo Delgado, Universidad Nacional Autónoma de México

Instituto de Química


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