Bioassay-Guided Isolation of -Clerodane Diterpenoids and Monoglycerides from the Leaves of and Their Antimicrobial Activities.

A previously undescribed -clerodane diterpenoid, diangelate solidagoic acid J (), along with two known -clerodane diterpenoids, solidagoic acid C () and solidagoic acid D (), as well as two known unsaturated monoacylglycerols, 1-linoleoyl glycerol () and 1-α-linolenoyl glycerol (), were isolated and characterized from the -hexane leaf extract of (giant goldenrod). Compounds - were identified first in this species, and compounds and are reported here for the first time in the genus. The bioassay-guided isolation procedure included thin-layer chromatography (TLC) coupled with a antibacterial assay, preparative flash column chromatography, and TLC-mass spectrometry (MS). Their structures were elucidated via extensive spectroscopic and spectrometric techniques such as one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and high-resolution tandem mass spectrometry (HRMS/MS). The antimicrobial activities of the isolated compounds were evaluated by a microdilution assay. All compounds exhibited weak to moderate antibacterial activity against the Gram-positive plant pathogen , with MIC values ranging from 17 to 133 µg/mL, with compound being the most potent. Only compound was active against pv. , while compound demonstrated a weak antibacterial effect against and . Additionally, the growth of and was moderately inhibited by compounds and , respectively. None of the tested compounds showed antibacterial activity against Gram-negative pv. and pv. . No bactericidal activity was observed against the tested microorganisms. Compounds and displayed weak antifungal activity against the crop pathogens and . Our results demonstrate the efficacy of bioassay-guided strategies in facilitating the discovery of novel bioactive compounds.