Cytotoxic Alkaloids from the Whole Plants of Zephyranthes candida

Seven new alkaloids, N-methylhemeanthidine chloride (1), N-methyl-5,6-dihydroplicane (5), O-methylnerinine (6), N-ethoxycarbonylethylcrinasiadine (7), N-ethoxycarbonylpropylcrinasiadine (8), N-phenethylcrinasiadine (9), and N-isopentylcrinasiadine (10), together with eight known alkaloids, hemeanthamin (2), 3-epimacronine (3), (+)-tazettine (4), N-methylcrinasiadine (11), trisphaeridine (12), 5,6-dihydrobicolorine (13), lycorine (14), and nigragillin (15), were isolated from the whole plants of Zephyranthes candida. The structures of the new compounds were established by spectroscopic data interpretation, with single-crystal X-ray diffraction analysis performed on 1. The absolute configuration of 3-epimacronine (3) was determined by single-crystal X-ray diffraction analysis with Cu Kα irradiation. Compounds 115 were evaluated for their in vitro cytotoxicity against five human cancer cell lines and the Beas-2B immortalized (noncancerous) human bronchial epithelial cell line. Compounds 1, 2, 9, and 14 exhibited cytotoxicity with IC50 values ranging from 0.81 to 13 μM with selectivity indices as high as 10 when compared to the Beas-2B cell line.