Bioassay-Guided Isolation and Identification of Antibacterial Compounds from Invasive Tree of Heaven Stem and Trunk Bark

Flash column chromatographic fractionation of tree of heaven (<i>Ailanthus altissima</i>) stem and trunk bark extracts, guided by thin-layer chromatography (TLC)-<i>Bacillus subtilis</i> assay and TLC-heated electrospray high-resolution tandem mass spectrometry (HESI-HRMS/MS), lead to the isolation of six known compounds: (9<i>Z</i>,11<i>E</i>)-13-hydroxy-9,11-octadecadienoic acid (13-HODE, <b>A1</b>), (10<i>E</i>,12<i>Z</i>)-9-hydroxy-10,12-octadecadienoic acid (9-HODE, <b>A2</b>), hexadecanedioic acid (thapsic acid, <b>A3</b>), 16-hydroxyhexadecanoic acid (juniperic acid, <b>A4</b>), 16-feruloyloxypalmitic acid (alpinagalanate, <b>A5</b>), and canthin-6-one (<b>A6</b>). Their structures were elucidated by HESI-HRMS/MS and one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy. This is the first study identifying <b>A1</b>-<b>A5</b> in <i>A. altissima</i> tree. Except for <b>A5</b>, all isolated compounds exhibited antibacterial activity against <i>B. subtilis</i> in microdilution assays. <b>A6</b> showed the strongest effect with a minimum inhibitory concentration (MIC) value of 8.3 µg/mL. The antibacterial activity of <b>A3</b> and <b>A4</b> is newly described.