Solidagoic Acids L and M: Novel Antibacterial cis-Clerodane Diterpenoids Isolated from the Inflorescences of Solidago gigantea via a Bioassay-Guided Approach

Abstract

Background/Objectives: Plant secondary metabolites remain an invaluable source of novel antibacterial phytochemicals in the fight against antibiotic resistance. The medicinal plant Solidago gigantea Ait. (giant goldenrod) is an invasive species in Europe and represents an abundant, yet largely underexplored reservoir of such bioactive compounds. The primary aim of this study was to perform a non-targeted, effect-directed screening, detection, bioassay-guided isolation, structure elucidation, and microbiological assessment of the antibacterial constituents present in the inflorescences of S. gigantea. Methods: Thin-layer chromatography coupled with direct bioautography (TLC–DB) assay using Bacillus subtilis was utilized for the non-targeted, effect-directed analysis of antibacterial components and the evaluation of in vitro antibacterial activity. Successive preparative flash column chromatography, semi-preparative reversed-phase high-performance liquid chromatography (RP-HPLC), and thin-layer chromatography–mass spectrometry (TLC–MS) were employed for the bioassay-guided fractionation and isolation. The structures of the isolated compounds were elucidated using one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and high-resolution tandem mass spectrometry (HRMS/MS). The presence of known antibacterial compounds was established via reversed-phase ultra-high-performance liquid chromatography coupled with high-resolution electrospray ionization tandem mass spectrometry (RP-UHPLC–HR-ESI-MS/MS). Results: Two previously undescribed cis-clerodane diterpenoids, the isomeric solidagoic acid L (1) and solidagoic acid M (2), were isolated, identified, and characterized from the ethyl acetate extract of S. gigantea inflorescences. Both compounds exhibited in vitro antibacterial activity against the Gram-positive B. subtilis, confirmed via TLC–DB. In addition, 23 known compounds with antibacterial activity, including 17 clerodane diterpenes, four hydroxylated polyunsaturated fatty acids, and two unsaturated monoacylglycerols, were detected. All of these are reported for the first time in the inflorescences of this plant species. Conclusions: With further optimization, the isolated compounds may represent promising leads for antibacterial drug development. Our findings demonstrate the potential of non-targeted, bioassay-guided approaches for the discovery of novel plant-derived bioactive natural products.