Bioactive Lignan Glycosides in Stems of Marsh Rosemary (Rhododendron tomentosum): Non-Targeted Screening and Identification Using Two-Stage Analytical Strategy

<i>Rhododendron tomentosum</i> is a widespread evergreen shrub used in folk medicine due to the high biological activity of its secondary metabolites, including lignans, that has not been sufficiently studied, and overcoming this problem requires advanced analytical techniques. This study proposes a two-stage analytical strategy for non-targeted screening and identification of lignans in plant extracts that involves the detection of specific lignan-related structures by 2D NMR and the establishment of corresponding fragment ions for further mass spectrometry analysis (HPLC-ESI-MS/MS). The polyphenolic fraction of <i>R. tomentosum</i> stem extract was the object of the study. Eight secoisolariciresinol-type lignans (xylosides, glucosides, and rhamnoside), including one previously unknown compound (5-methoxysecoisolariciresinol 9-xyloside), were identified for the first time. The structures of the five compounds were additionally confirmed by preparative HPLC isolation and NMR studies. All of the obtained compounds had antioxidant activity (FRAP and DPPH) similar to that of ascorbic acid. The proposed analytical strategy can be considered an efficient tool for rapid and reliable group screening and identification of lignan derivatives in plant extracts. Its application in the study of <i>R. tomentosum</i> extracts has revealed a number of lignan glycosides that may contribute to the medicinal properties of the plant.