Secondary metabolites from the stem bark of Alstonia boonei and the seeds of Picralima nitida with antibacterial activities.

The methanol stem bark extract of A. boonei and methanol seed extract of P. nitida, were subjected to purification using chromatographic techniques. A. boonei yielded loganic acid (1), sweroside (2) and secoxyloganin (3), while P. nitida afforded (1), akuammidine (4), akuammicine (5) and alstonine (6). 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 (6) demonstrated weak activity against Pseudomonas aeruginosa and Streptococcus agalactiae with zones of inhibition of 9.3 ± 0.6 and 10.0 ± 0.0 mm, respectively. This is the first report of sweroside (2) and secoxyloganin (3) in A. boonei.