Amaryllidaceae alkaloids with new framework types from Zephyranthes candida as potent acetylcholinesterase inhibitors

Three new Amaryllidaceae alkaloids, named zephycandidines I−III (1–3), were isolated from Zephyranthes candida. The structures of 1−3 were elucidated by spectroscopic analyses including HRESIMS, 1H NMR, 13C NMR, DEPT, HSQC, 1H−1H COSY, HMBC, ROESY, and electronic circular dichroism (ECD), as well as ECD calculation. The absolute configuration of 1 was finally established by single crystal X-ray diffraction using Cu Kα radiation. Zephycandidines I (1) and III (3) with new framework types represent the first example of 7-phenyl-hexahydroindole and 5,2'-dimethyl-biphenyl-2-ylamine alkaloids, respectively, and their plausible biosynthetic pathway are proposed. Zephycandidine II (2) is the first C3a-phenyl-hexahydroindole type alkaloid isolated from the genus of Zephyranthes. These new alkaloids 1–3 were evaluated for their acetylcholinesterase (AChE) inhibitory activities, and 3 showed potent AChE inhibitory activity with an IC50 value of 8.82 μM, suggesting that the framework of 5,2'-dimethyl-biphenyl-2-ylamine in 3 may be a potential group for the AChE inhibitory activity. The docking studies of 1−3 and galanthamine with AChE revealed that interactions with W286 and Y337 are necessary for the AChE inhibitory activity.