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

• Autor(a) principal: Glaser,Robert
• Data de Publicação: 2005
• Outros Autores: García,Abraham, Chávez,María Isabel, Delgado,Guillermo
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Journal of the Brazilian Chemical Society (Online)
• Texto Completo: http://old.scielo.br/scielo.php?script=sci_arttext&pid=S0103-50532005000300019
• topic: tagitinin A; Horeau's rule; conformational interconversion; molecular modeling

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 1beta,4alpha,6alpha,7beta,8beta relative configuration which differed at C(1) from the 1alpha-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.