Secondary metabolites from the stem bark of <i>Alstonia boonei</i> and the seeds of <i>Picralima nitida</i> with antibacterial activities

The methanol stem bark extract of <i>A. boonei</i> and methanol seed extract of <i>P. nitida</i>, were subjected to purification using chromatographic techniques. <i>A. boonei</i> yielded loganic acid (<b>1</b>), sweroside (<b>2</b>) and secoxyloganin (<b>3</b>), while <i>P. nitida</i> afforded <b>(1)</b>, akuammidine (<b>4</b>), akuammicine (<b>5</b>) and alstonine (<b>6</b>). The structures of the compounds were elucidated based on their nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS) profiles and comparison with literature data. The antibacterial activities of the compounds were evaluated using the disc diffusion assay with chloramphenicol as the positive control. Alstonine (<b>6</b>) demonstrated weak activity against <i>Pseudomonas aeruginosa</i> and <i>Streptococcus agalactiae</i> with zones of inhibition of 9.3 ± 0.6 and 10.0 ± 0.0 mm, respectively. This is the first report of sweroside (<b>2</b>) and secoxyloganin (<b>3</b>) in <i>A. boonei</i>.