Carbon-13 NMR Spectra of Sesquiterpene Lactones

Publisher Summary This chapter presents the carbon-13 nuclear magnetic resonance (NMR) data of sesquiterpene lactones. The structure classification and methods of structure determination are also described in the chapter. The sesquiterpene lactones are successfully used in chemotaxonomical studies and there are various biological activities of sesquiterpene lactones that form a basis for the structure–activity relation studies. Large numbers of sesquiterpene lactones are isolated from the plant material. The sesquiterpenoids are C15-compounds formed of three isoprenoid units. Based on the biogenetic assumptions, sesquiterpene lactones arise from a common precursor, farnesyl pyrophosphate, by various modes of cyclization followed in many cases by skeletal rearrangements. Their classification in general is based on the carbocyclic skeleton. The combination of various spectral methods is the most common way to determine the structure of the sesquiterpene lactones. The most important methods discussed are: mass spectra, infrared, ultraviolet, NMR, and X-ray. The 13C NMR spectra of sesquiterpene lactones are recorded with proton broad-band decoupling that eliminates the splitting from 13C-lH spin interactions and hence, the 13C signals appear as singlets. The structural assignments of the signals can be determined by chemical shifts, signal multiplicities, relaxation times, or shift reagents.

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