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The onion, known as the bulb onion or common onion, is not only a key ingredient in many tasty and healthy vegetarian meals but also many traditional medicines. Nine new flavonoids [cepaflavas A, B (5, 6), cepadials A−D (7−9 and 14), and cepabiflas A−C (10−12)] and six known compounds (1−4, 13, 15) were obtained from the outer skins of Allium cepa L. Among them, compounds 5, 6, and 9 might be artificial products formed during extraction and isolation. New compounds were structurally elucidated using various spectroscopy/spectrometry techniques, including NMR and HRMS, and computational methods. Their absolute configurations were determined using time-dependent density functional theory calculations, combined with ECD spectroscopy, optical rotation calculation, and statistical procedures (CP3 and DP4 analysis). The free radical scavenging assays revealed that the new compounds 10−12 possessed considerable antioxidant activities with IC 50 values of 4.25−8.88 and 7.12−8.14 μM against DPPH and ABTS •+ , respectively. Compounds 13−15 showed substantial inhibitory activities against both αglucosidase and protein tyrosine phosphatase 1B (PTP1B), with IC 50 values of 0.89−6.80 and 1.13−6.82 μM, respectively. On the basis of molecular docking studies, 13 and 15 were predicted to have high binding capacity and strong affinity toward the active site of PTP1B.

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