The Isolation and Structure Elucidation of Secondary Metabolites in Central and South American Calea Species and Their Biochemical Systematic Implications (Phytochemistry, Natural Products).

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Fourteen species of the genus Calea (Family Compositae, Tribe Heliantheae) from Central and northern South America, including the type species for the genus, were investigated chemically to determine their secondary metabolites. The taxa studied were C. leptocephala Blake, C. megacephala Rob. and Greenm., and C. trichotoma B. Smith from Mexico, C. prunifolia Kunth (syn. C. pittieri) from Costa Rica, C. prunifolia Kunth from Panama, C. jamaicensis L. from Jamaica, and the Venezuelan species C. berteriana DC., C. divaricata Benthem, C. oliverii Rob. and Greenm., C. prunifolia Kunth, C. septuplinervia Hieron., C. solidaginea Kunth, and C. subcordata Kunth. The chemical investigation of these Calea species, undertaken as part of a biochemical systematic study, has resulted in the isolation of 83 compounds, of which 38 are new natural products. The isolated compounds were represented by a dioxin derivative, 3 benzofuranes, 5 chromenes, 12 flavones, and 62 sesquiterpene lactones. The structures of the new compounds were established by chemical and spectroscopic methods. These methods included MS, IR, UV, and CD, ('1)H NMR, ('13)C NMR, and single crystal X-ray diffraction analysis. The newly isolated sesquiterpene lactones, comprised of 19 heliangolides, 18 guaianolides, 15 eudesmanolides and 10 germacranolides, show that guaianolides are common constituents in Calea species. This skeletal type of sesquiterpene lactones, which was previously not observed in Calea, is found in C. subcordata Kunth, C. solidaginea Kunth, C. berteriana DC., C. prunifolia Kunth from Venezuela and Panama, C. jamaicensis L., and C. leptocephala Blake. Furanoheliangolides, which are common in Mexican Calea species, have a restricted distribution among the species investigated in this study. These findings, together with the presence of guaianolides in C. jamaicensis L., the type species, appear to contrast with the recently suggested transfer of the genus Calea from the subtribe Galinsoginae to the Neurolaeninae. Comparison of the chemical results with the classical taxonomic data of the genus Calea, suggests affinities somewhat different from those proposed on morphological and distributional grounds. An extension of a biogenetic scheme of all presently known Calea sesquiterpene lactones is presented.