Two New Cytotoxic Sesquiterpene-Amino Acid Conjugates and a Coumarin-Glucoside from Crossostephium chinense

The Asteraceae family is a promising source of bioactive compounds, such as the famous Asteraceae plants Tanacetum cinerariifolium (pyrethrin) and Artemisia annua (artemisinin). As a result of our series of phytochemical studies of the subtropical plants, two novel sesquiterpenes, named crossoseamines A and B in this study (1 and 2, respectively), one undescribed coumarin-glucoside (3), and eighteen known compounds (4-21) were isolated from the aerial part of Crossostephium chinense (Asteraceae). The structures of isolated compounds were elucidated by spectroscopic methods, including 1D and 2D NMR experiments (¹H, ¹³C, DEPT, COSY, HSQC, HMBC, and NOESY), IR spectrum, circular dichroism spectrum (CD), and high-resolution electrospray ionization-mass spectrometry (HR-ESI-MS). All isolated compounds were evaluated for their cytotoxic activities against Leishmania major, Plasmodium falciparum, Trypanosoma brucei (gambiense and rhodesiense), and human lung cancer cell line A549 because of the high demand for the discovery of new drug leads to overcome the present side effects and emerging drug-resistant strains. As a result, the new compounds (1 and 2) showed significant activities against A549 (IC₅₀, 1: 3.3 ± 0.3; 2: 12.3 ± 1.0 μg/mL), L. major (IC₅₀, 1: 6.9 ± 0.6; 2: 24.9 ± 2.2 μg/mL), and P. falciparum (IC₅₀, 1: 12.1 ± 1.1; 2: 15.6 ± 1.2 μg/mL).