Zephycandidine A (1), the first naturally occurring imidazo[1,2-f]phenanthridine alkaloid, was isolated from Zephyranthes candida (Amaryllidaceae). The structure of 1 was elucidated by spectroscopic analyses and NMR calculation, and a plausible biogenetic pathway for zephycandidine A (1) was proposed. Zephycandidine A (1) exhibited significant cytotoxicity against five cancer cell lines with IC50 values ranging from 1.98 to 7.03 μM with selectivity indices as high as 10 when compared to the normal Beas-2B cell. Further studies suggested that zephycandidine A (1) induces apoptosis in leukemia cells by the activation of caspase-3, upregulation of Bax, downregulation of Bcl-2, and degradation of PARP expression. In addition, zephycandidine A (1) showed acetylcholinesterase (AChE) inhibitory activity, and the docking studies of zephycandidine A (1) and galanthamine (2) with AChE revealed that interactions with W286 and Y337 are necessary.

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A new diterpene with an unprecedented carbon skeleton, micranthanone A (1), two new grayanane diterpenoids bearing an unusual 5,6-(3,4-dihydroxylbenzylidene acetal) motif, rhodomicranols A (2) and B (3), and three known grayanane diterpenoids (4-6) were isolated from Rhododendron micranthum. Their structures were elucidated by spectroscopic analyses, calculated ECD, and single-crystal X-ray diffraction. The in vitro immunomodulatory activities of 1-6 were evaluated, and a plausible biogenetic pathway for 1 is proposed.