Cyano-containing Daphniphyllum alkaloids with antienteroviral activity from the leaves and stems of Daphniphyllum calycinum Benth

Enterovirus 71 (EV71), a major causative agent of hand, foot, and mouth disease (HFMD), is often associated with severe clinical outcomes. In this study, three rare cyano-containing alkaloids, cyanodaphcalycines A–C (1–3), featuring an unusual 5/6/6/7/5/5-hexacyclic scaffold, and an undescribed daphmanidin A-type alkaloid, daphmanidin G (4), were isolated from the leaves and stems of the medicinal plant Daphniphyllum calycinum Benth., along with three known Daphniphyllum alkaloids (5–7). The previously undescribed structures and their absolute configurations were determined using a combination of spectroscopic analysis, a single crystal X-ray diffraction analysis, and time-dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculations. In the biological evaluation, the antiviral activities of compounds 1–4 were tested against EV71 in human malignant embryonal rhabdomyosarcoma (RD) cells. Compounds 3 and 4 exhibited potent antiviral effects against human EV71 in RD cells, with EC₅₀ values of 3.78 ± 0.23 and 6.87 ± 0.30 μg/mL, respectively, demonstrating superior efficacy to ribavirin (EC₅₀ 65.77 ± 0.11 μg/mL). This is the first report of Daphniphyllum alkaloids as potent inhibitors of EV71.