Monoterpenoid indole alkaloids from Alstonia rupestris with cytotoxic, antibacterial and antifungal activities.

A chemical investigation of the 80% EtOH extract of the aerial plant of Alstonia rupestris afforded four new monoterpenoid indole alkaloids, 6,7-epoxy-8-oxo-vincadifformine (1), 11-acetyl-6,7-epoxy-8-oxo-vincadifformine (2), 11-hydroxy-14-chloro-15-hydroxy-vincadifformine (3), and perakine N1,N4-dioxide (4), together with two known compounds, 11-hydroxy-6,7-epoxy-8-oxovincadifformine (5) and vinorine N1,N4-dioxide (6). Structural elucidation of all the compounds was performed by spectral methods such as 1D- and 2D-NMR, IR, UV, and HRESIMS. Alkaloids 1, 2 and 5 showed significant cytotoxicities against all the tested tumor cell lines of the head and neck squamous cell carcinoma with IC50 value less than 20 μM and antimicrobial activities against two fungi (Alternaria alternata and Phytophthora capsici). Alkaloids 4 and 6 exhibited the activity against bacterium Staphylococcus aureus.