Identification of flavonoid-3-O-glycosides from leaves of Casearia arborea (Salicaceae) by UHPLC-DAD-ESI-HRMS/MS combined with molecular networking and NMR.

Introduction: Casearia is an essential source of cytotoxic highly oxidised clerodane diterpenes, in addition to phenolics, flavonoids, and glycoside derivatives. Here we identify flavonoid-3-O-glycoside derivatives in the ethyl acetate (EtOAc) fraction of the methanolic extract from leaves C. arborea leaves.
Objective: To characterise the EtOAc phase from the methanolic extract of C. arborea leaves using ultra-high-performance liquid chromatography diode array detector high-resolution tandem mass spectrometry (UHPLC-DAD-HRMS/MS) and molecular networking-based dereplication. Methodology We identified compounds not annotated in the GNPS platform by co-injection of standards in HPLC-DAD or by isolation and characterisation of the metabolites using nuclear magnetic resonance (NMR) spectroscopy. A workflow on the GNPS platform aided the organisation of spectral data and dereplication by annotations. We subjected the EtOAc phase to HPLC-DAD analysis using standard compound co-injection to corroborate the GNPS annotations. We isolated unidentified compounds with semi-preparative HPLC-DAD for structural identification using NMR.
Results: We annotated a molecular family of flavonoid-3-O-glycosides in the molecular networking created using the GNPS platform. These included avicularin, cacticin, isoquercitrin, quercitrin, rutin, and a quercetin-3-O-pentoside cluster. We confirmed the annotations with standard compounds using HPLC-DAD co-injection analysis, besides identifying quercetin-3-O-robinobioside and kaempferol. We isolated three flavonoid-3-O-pentosides and characterised them using one- and two-dimensional NMR; we identified them as reynoutrin, guaijaverin, and avicularin.
Conclusion: This work describes the isolation of kaempferol and nine known flavonoid-3-O-glycosides from the polar fraction of the methanolic extract (EtOAc) from C. arborea leaves using molecular networking to guide the chromatographic procedures. We identified eight compounds for the first time in Casearia that amplify and reinforce the genus' chemotaxonomy with the presence of glycosylated flavonoids.
(© 2021 John Wiley & Sons, Ltd.)