MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2004; 42: 887–892 Published online 10 August 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1463 Note Conformational evaluation and detailed 1 H and 13 C NMR assignments of eremophilanolides Eleuterio Burgue ˜ no-Tapia, 1* Luis R. Hern ´ andez, 2 Adriana Y. Res ´ endiz-Villalobos 1 and Pedro Joseph-Nathan 3 1 Departamento de Qu´ ımica, Unidad Profesional Interdisciplinaria de Biotecnolog´ ıa, Instituto Polit ´ ecnico Nacional, Av. Acueducto s/n, La Laguna Ticom ´ an, M ´ exico D.F., 07340 Mexico 2 Instituto de Ecolog´ ıa, Universidad del Mar, Puerto Escondido, Oaxaca, 71980 Mexico 3 Departamento de Qu´ ımica, Centro de Investigaci ´ on y de Estudios Avanzados del Instituto Polit ´ ecnico Nacional, Apartado 14-740, M ´ exico D.F., 07000 Mexico Received 26 April 2004; Revised 1 June 2004; Accepted 2 June 2004 Extensive application of 1D and 2D NMR methodology, combined with molecular modeling, allowed the complete 1 H and 13 C NMR assignments of eremophilanolides from Senecio toluccanus. Comparison of the experimental 1 H, 1 H coupling constant values with those generated employing a generalized Karplus-type relationship, using dihedral angles extracted from MMX and DFT calculations, revealed that the epoxidized eremophilanolides 1 and 2 show conformational rigidity at room temperature, whereas molecules 3–6, containing an isolated double bond, are conformationally mobile. Copyright  2004 John Wiley & Sons, Ltd. KEYWORDS: NMR; 1 H NMR; 13 C NMR; 2D NMR; Senecio toluccanus; Senecionae; Asteraceae; sesquiterpenelactones; eremophilanolides; furanoeremophilanes; conformational analysis INTRODUCTION The large genus Senecio (Asteraceae) has been studied extensively for their secondary metabolites. Pyrrolizidine alkaloids, eremophilanolides and cacalolides are partic- ularly characteristic from species of this genus. 1 It is well documented that pyrrolizidine alkaloids are toxic to humans and livestock, 2 and recently it has been reported that some furanoeremophilanes and cacalolides show moderate antibacterial 3 strong insect antifeedant 4 and antihyperglycemic 5 activity. Furanoeremophilanes with an ester group at C-6 are frequently found, whereas furanoere- mophilanes with a hydroxyl group at C-6 are uncommon in nature. 1 As part of our chemical study of species of the genus Senecio, 6 we collected S. toluccanus from the mountain region of Morelia, Michoacan, Mexico. Their roots gave 1,10- epoxy-6-hydroxyeuryopsin (1), 7 6-hydroxyeuryopsin (3) 7–9 and toluccanolide A (5), 7,10 and the corresponding acetyl derivatives 2, 7,11 4 7 and 6 10 were obtained by reaction with acetic anhydride. Compounds 2 – 5 are described as natural L Correspondence to: Eleuterio Burgue ˜ no-Tapia, Departamento de Qu´ ımica, Unidad Profesional Interdisciplinaria de Biotecnolog´ ıa, Instituto Polit´ ecnico Nacional, Av. Acueducto s/n, La Laguna Ticom´ an, M´ exico D.F., 07340 Mexico. E-mail: eburgenio@acei.upibi.ipn.mx Contract/grant sponsor: CEGEPI-IPN. Contract/grant sponsor: CONACYT. products 9–11 and were also obtained by biogenetic-type conversions from the eudesmanolide alantolactone, 7 this being the first time that 1 is described as a natural product. Literature surveys show the existence of a large group of this kind of eremophilanolides but no publication gives complete 1 H NMR assignments, since the CH 2 ⊲2⊳, CH 2 ⊲3⊳ and CH(4) signals appear in a narrow chemical shift range, and in consequence there have not been studies concerning the conformation of these compounds in solutions. 13 C NMR data have been reported for only a small number of these compounds. Therefore, and in continuation of our studies of sesquiterpenes, 12,13 we decided to perform the complete assignment of the 1 H and 13 C NMR spectra as well as a conformational evaluation of 1 – 6. RESULTS AND DISCUSSION From the hexane extracts of S. toluccanus, compounds 1, 3 and 5 were obtained, which is interesting because they belong to the small group of eremophilanolides with a hydroxyl group at C-6. Copyright  2004 John Wiley & Sons, Ltd.