Two New Compounds from Allii Macrostemonis Bulbus and Their In Vitro Antioxidant Activities

Two new compounds named 4,4′-bis(β-D-glucopyranosyloxy)biphenyl (1) and spirostane-25(27)-en-2α,3β-diol-3-O-β-D-xylopyranosyl(1→3)-β-D-glucopyranosyl(1→4)-β-D-galactopyranoside (2) were isolated from n-butanol extraction part of 80% ethanol extract of Allii Macrostemonis Bulbus. Alongside these, ten known compounds (3–12) were also identified, including a flavonoid glycoside (3), seven steroids (4–10), a nucleoside (11), and a phenylpropanoid glycoside (12) were found. Notably, compounds3–6were isolated from this plant for the first time. The structures of all compounds were confirmed using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), 1D, and 2D NMR spectroscopy. Some of these compounds showed strong antioxidant activity, and compound1demonstrated the most potent reduction of ferric ions (Fe3+) with an IC50value of 0.59 ± 0.18 mg/mL. Compounds2and3exhibited the highest scavenging activity against superoxide anion radicals (O2−·) with an IC50value of 0.02 ± 0.01 mg/mL. Additionally, compound3displayed substantial scavenging activity against 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with IC50values of 0.21 ± 0.17 mg/mL and 0.02 ± 0.01 mg/mL, respectively. The discovery of these two new compounds is a reference for identifying Allii Macrostemonis Bulbus quality markers. Moreover, their exceptional antioxidant activity offers a promising avenue for uncovering novel natural antioxidants.