Structure Elucidation of Natural Products From Asteraceae by Modern NMR Techniques and Biomimetic Transformations of 11,13-Dihydroparthenolide.

Abstract

Chemical analysis of root cultures of Tagetes patula (Asteraceae) provided the knowns 5-acetyl-4-hydroxy-2-isopropenyl-benzofuran, 5-(but-3-en-1-ynyl)-2,2$\sp\prime$-bithiophene and 5-(4-acetoxy-1 butynyl)-2,2$\sp\prime$-bithiophene. Their structures were elucidated by spectroscopic methods. Chemical analysis by capillary gas-liquid chromatography-mass spectrocscopy (GC/MS) of Parthenium argentatum (Guayule) provided the known sesquiterpene hydrocarbons 9,10-dihydroguajene, $\alpha$-copaene, alloaromandrene, isolongifolene, germacrene A or $\beta$-elemene and $\alpha$-humulene. The known sesquiterpene esters guayulin A and B and the triterpenes argentatin A and B as well as two new cycloartenol-type triterpenes, 3$\alpha$- and 3$\beta$-hydroargentatin B, were also isolated. The relative configuration, the conformation as well as the $\sp1$H and $\sp{13}$C NMR resonance assignments of the two new triterpenes were established by 2D and multipulse NMR techniques. Hydrolysis of guayulin A yielded desacylguayulin A, a stereochemical study of which was performed using modern NMR techniques. Chemical analysis of Baccharis salicina (Asteraceae) yielded the known antitumor flavone centaureidin and two new guaianolides, bacchariolides A and B. Their structures were elucidated by spectroscopic methods. Biomimetic-type reactions of the sesquiterpene lactone 11,13-dihydroparthenolide involved the following transformations: (1) BF$\sb3$-mediated rearrangement of the germacrolide 4(5)-epoxide to produce a xanthanolide and three cis-guaianolides. (2) m-Chloroperoxybenzoic acid mediated oxidation-rearrangements of a guaianolide 1(10)-ene to produce a xanthanolide, a trans-epoxy-guaianolide, two cis-epoxy-guaianolides and a cyclobutane-type sesquiterpene lactone. (3) BF$\sb3$-catalyzed rearrangement of a guaianolide 1(10)-$\beta$-epoxide to give a cis-eudesmanolide, a trans-guaianolide, and two cis-guaianolides. The results and proposed mechanisms of these reaction sequences as well as the detailed structure elucidation of the transformation products are described.