Antioxidant and antibacterial activities of constituents and semi-synthetic derivatives from Senna siamea (Lam.) H. S. Irwin & Barneby (Caesalpiniaceae).

Eleven compounds (1‒11) were isolated from the methanolic extract of leaves and stem bark of Senna siamea. Benzylation of compound 2 gave two new hemi-synthetic derivatives: 4'-O-benzylvitexin (12) and 7,4'-O-dibenzylvitexin (13). The structures of these compounds were established on the basis of their spectroscopic (1D and 2D NMR) and mass spectrometric (FAB-TOF-MS) data. The extracts, fractions, some isolated compounds as well as the hemisynthetic derivatives were evaluated for their antibacterial activity using the broth microdilution method and for their antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging potential and ferric reducing antioxidant power (FRAP). The ethyl acetate fraction of the stem bark showed moderate activity against E. faecalis and P. mirabilis with MIC value of 256 μg/mL. Compound 6 showed moderate activity towards P. mirabilis (MIC = 64 μg/mL). The methanol, ethyl acetate and n-BuOH soluble fractions of stem bark as well as the n-BuOH fraction of the leaves were more active against DPPH compared to vitamin C with EC50 values of 1.58 ± 0.25 μg/mL, 1.12 ± 0.67 μg/mL, and 1.02 ± 0.87 μg/mL, respectively. Additionally, compound 4 was the most active against DPPH (EC50 = 1.05 ± 0.38) μg/mL. [ABSTRACT FROM AUTHOR]