Discovery of Undescribed Clerodane Diterpenoids with Antimicrobial Activity Isolated from the Roots of Ait.

Three previously undescribed clerodane diterpenoids, including two -clerodanes, solidagolactone IX () and solidagoic acid K (), and one -clerodane, solidagodiol (), along with two known -clerodane diterpenoids, (-)-(5,8,9,10)-15,16-epoxy--cleroda-3,13,14-trien-18-ol () and solidagoic acid J (), were isolated and comprehensively characterized from the ethanolic and ethyl acetate root extract of Ait. (giant goldenrod). Compound has previously been reported from the roots of this species, whereas compound was identified from the leaves of but not from the roots. The bioassay-guided isolation involved thin-layer chromatography-direct bioautography (TLC-DB) with a antibacterial assay, preparative flash column chromatography, and TLC-mass spectrometry (MS). The chemical structures of the isolated compounds (-) were elucidated through extensive in-depth spectroscopic and spectrometric analyses, including one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy, high-resolution tandem mass spectrometry (HRMS/MS), and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. Their antimicrobial activities were evaluated using microdilution assays against and different plant pathogens. Compound was the most active against the tested Gram-positive strains, exerting particularly potent effects against with a minimal inhibitory concentration (MIC) value of 5.1 µM as well as and pv. (MIC 21 µM for both). Compound also strongly inhibited the growth of (MIC 6.3 µM). Compounds , , and displayed moderate to weak activity against and pv. with MIC values ranging from 100 to 402 µM. bacteria were moderately inhibited by compounds (MIC 41 µM) and (MIC 201 µM). Bactericidal activity was observed for compound against with a minimal bactericidal concentration (MBC) value of 83 µM. Compounds and demonstrated weak antifungal activity against . Our findings underscore the value of bioassay-guided approaches in discovering previously undescribed bioactive compounds.