Studies on the Nonalkaloidal Secondary Metabolites of Hippeastrum vittatum (L'Her.) Herb. Bulbs.

Sixteen chemically varied metabolites were isolated from the bulbs of Hippeastrum vittatum (L'Her.) Herb., including eight flavonoids [3'-methyl isoliquiritigenin (2) , 7-hydroxyflavan (8) , 7-hydroxyflavanone (9) , 7-hydroxyflavan-3-ol (10) , 7-methoxy-3',4'-methylenedioxyflavan-3-ol (11) , 7-hydroxy-3',4'-methylenedioxy flavan (12) , 2',4'-dihydroxy-3'-methyl-3,4-methylenedioxychalcone (13) , and isoliquiritigenin (14) ], four acetophenones [2,6-dimethoxy-4-hydroxyacetophenone (3) , 2,4-dihydroxyacetophenone (4) , 2,4-dihydroxy-6-methoxy-3-methylacetophenone (6) , and 2,4,6-trimethoxyacetophenone (7) ], two alkaloids [lycorine (1) and narciprimine (15) ], one phenol derivative [ p -nitrophenol (5) ], and one steroid [β-sitosterol 3- O -β-glucopyranoside (16) ]. Their structures were elucidated by combining one- and two-dimensional NMR and ESI-MS techniques and by comparison with the reported literature data and some authentic samples. Except for lycorine (1) , the isolated metabolites were obtained herein for the first time from Hippeastrum plants, among which compound 13 was identified as a new chalcone derivative. Additionally, the total phenolic and flavonoid contents of the total ethanol extract and different fractions of the bulbs were determined by the Folin-Ciocalteu and aluminum chloride colorimetric methods, respectively, whereas their antioxidant potential was compared using the phosphomolybdenum and 2,2'-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assays. Finally, the binding affinities of compounds 1-16 to some key target proteins of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), namely, main protease (M pro ), papain-like protease (PLpro), and RNA-dependent RNA polymerase (RdRp), were screened and compared using molecular docking analysis. The possible chemotaxonomic significance of the identified metabolites was also discussed.